Author: Pam Knox

Linda’s post last week about “drought-resistant” plants made me think about drought and how different types of drought affect gardeners in different ways. In her article, she defined drought as “an unusual lack of rainfall”. This is one of four different kinds of drought that climatologists talk about, and I thought it might be interesting for you to hear about how the four (or maybe five) types of drought differ and how they affect gardeners in diverse ways. A great source of drought information across the U.S. is https://www.drought.gov/.

Meteorological drought

The first type of drought, the one Linda described last week, is what climatologists consider a meteorological drought. A meteorological drought is related to how much rain you get compared to usual conditions at your location. I like to think of it as “too many days of nice weather in a row”, since in these dry conditions, the sun is shining and it is a great time to garden, play golf, or do construction. Of course, if you don’t get rain for a long time, you start to see impacts on plants, water bodies, and wells, but meteorological drought is usually not identified in terms of impacts, just on the amount of precipitation measured over weeks, months or years. Meteorological droughts look different depending on where you are. It is possible to have drought even in a desert if rain does not fall over an unusually long time. Droughts in the Pacific Northwest might look quite different since the frequency and amount of rain looks a lot different there. In the Southeast, drought can be hard to identify by looks since even when rain does not fall for a long time, things tend to stay relatively green because in our worst droughts we still get 35 inches of rain a year. Most gardeners can cope with meteorological drought by watering their plants at appropriate intervals and reducing impacts of the dry conditions by mulching to help keep moisture in the soil.

Agricultural drought

I spend a lot of time talking about agricultural drought to the farmers and extension agents I work with, because agricultural drought is always on their mind. Agricultural drought is defined by a negative water balance that can be related to both lack of rainfall and/or high temperatures that increase evaporative water stress on growing plants. It occurs mainly in the growing season because that is when the crops are actively growing and impacts are most noticeable. A 3-week dry spell may not be a problem for most gardeners that water their plots, but if you are a dryland farmer without irrigation, you can lose an entire crop of corn for the year if the dry spell occurs when the corn is pollinating and the silk dries out before the pollen can stick to it. Often agricultural drought can occur even when there are no other impacts to us because it is subtle; most people don’t see the impacts until months later during harvest. If you have limited access to water for irrigation or very sandy soil in your garden, then you are more likely to be affected by agricultural drought since it will be harder to maintain plant health when the soil is dry.

Agricultural droughts are often related to flash droughts. Flash droughts are characterized by very rapid development or intensification over a short time period, and crops are often the first things affected because of their need for frequent watering. Flash droughts are often characterized by a lengthy dry spell coupled with very high temperatures, something that is common when you have a persistent area of high pressure right over your location.

Hydrological drought

Where agricultural drought is related to a shortage of water over time periods as short as a week to a month, hydrological drought is related to a shortage of water over months or years. Climatologists measure hydrological drought as precipitation deficits over periods that range from three months to multiple years. You can see hydrological drought in dropping stream, lake, and reservoir levels and in dropping groundwater levels if the deficit lasts long enough. A hydrological drought can occur even if no agricultural drought is observed when you get rain at frequent intervals but it is less than normal over a long time period, as long as the rainfall is enough to sustain the crops (or if it is winter, when there are not many crops growing).

Hydrological drought tends to affect gardeners’ access to water for irrigation because the long-term water deficits lead communities to enact water conservation measures to protect drinking water supplies. Most local and state governments have tiered conservation measures that get more strict as the water supplies get lower and lower. They may start by merely providing educational materials on water conservation and then progress to even-odd watering by dates or watering during overnight hours only (since there is less loss of water due to evaporation in cooler night-time temperatures). In the worst droughts, they may cut off the use of water for establishing new lawns and gardens (often with an exception for gardens that are used for food production). If a drought lasts for many years or even decades, then it is considered a megadrought, such as the one that is occurring now in the Southwest U.S. Megadroughts are related to long-period shifts in global atmospheric patterns and can lead to the abandonment of cities because of the loss of water to keep their citizens alive over time.

Socio-economic drought

Socio-economic drought is a little different than the other kinds of drought mentioned above. It is drought caused by a lack of water due to overuse, hoarding, or war. An example of a socio-economic drought might be one caused by one country damming a major river in their country to create a reservoir, keeping the river water from flowing downstream to other countries that depend on the water for agriculture or water supply. In the United States, disagreements between who is allowed to use available water often end up in court as cases like the Georgia-Florida “water war” that was recently adjudicated in the U.S. Supreme Court. Locally, disagreements about who is allowed to use the water sometimes result in tiered water pricing, where the more water you use, the higher the price. This affects gardeners who have plots that use a lot of irrigation because of the use of water features, plants with significant water needs, or lack of mulching or other methods of protecting soil moisture.

Recently, a fifth type of drought called ecological drought has been identified, since a lack of rainfall can affect natural ecosystems in ways that are distinct from gardens, farms, or watersheds. I won’t address it further here, but if you are interested in how natural ecosystems are affected by dry conditions, you will no doubt read about ecological drought in publications in the future.

Drought is a naturally occurring part of the climate across the world, and gardeners must understand the nature of drought in their area to recognize how it affects the weather and climate where they live. Linda’s article last week gives some good guidelines for how to make your garden work in your climate.

I got a Facebook message early this week from a friend in Sacramento CA that said after over 200 days with no rain, she got 4.83 inches in a 24-hour period from the latest extreme rainfall that occurred over northern California. Others have reported up to a foot of rain in three days. If you follow the news, you may have heard the term “atmospheric river” used to describe the torrential rains and flooding that have occurred this week in San Francisco and other parts of Northern California. In this post, I want to explain what atmospheric rivers are and how they affect rain climatology in the Western U.S. as well as other parts of the United States and the world.

Tahquamenon Falls–Autumn. Source: Wfgc, Commons Wikimedia.

What is an “atmospheric river”?

The term “atmospheric river” first appeared in the modern scientific literature in the early 1990s. Since it was first used, there has been a lot of discussion about what the term actually means. Commonly, it is seen as a band of very moist air flowing into a coastal area, bringing the potential for a lot of rain to the region that is at the downwind end of the flow. In some respects, it is like being on the receiving end of a firehose streaming high-intensity water right towards you! After a lot of discussion by meteorologists (described in this Bulletin of the American Meteorological Society article) the official definition in the Glossary of Meteorology became:

Atmospheric river – A long, narrow, and transient corridor of strong horizontal water vapor transport that is typically associated with a low-level jet stream ahead of the cold front of an extratropical cyclone. The water vapor in atmospheric rivers is supplied by tropical and/or extratropical moisture sources. Atmospheric rivers frequently lead to heavy precipitation where they are forced upward—for example, by mountains or by ascent in the warm conveyor belt. Horizontal water vapor transport in the mid-latitudes occurs primarily in atmospheric rivers and is focused in the lower troposphere. Atmospheric rivers are the largest “rivers” of fresh water on Earth, transporting on average more than double the flow of the Amazon River.

Source: NASA Earth Observatory

Why do atmospheric rivers produce so much rain?

The strong flow of moisture into a region provides an excellent source of water vapor for the development of heavy rain, especially if it is moving into an area with flow up mountain slopes that can help storms develop vertically. That enhances the rain-producing process. The West Coast of the United States provides a perfect location for the occurrence of atmospheric rivers since there is a broad expanse of ocean to provide the water vapor, dynamic storms that concentrate the flow into bands that can stretch all the way from the Hawaiian Islands to California (which explains an alternate name, “Pineapple Express”), and mountains near the coast to provide lifting for the moist air once it comes onshore. Cliff Mass of the University of Washington often discusses them in his blog on the weather of the Pacific Northwest.

Do atmospheric rivers occur in other places?

The short answer is Yes! While historically they are discussed most often when talking about weather on the West Coast, atmospheric rivers (ARs) can and do occur in other places as well. Anywhere that has a good source of moisture plus dynamic storms with strong airflow can experience ARs. In the Southeastern U.S., we get them when strong flow from the Gulf of Mexico or the Atlantic Ocean feeds into our region, usually ahead of a strong low pressure center that provides the necessary dynamics to create a narrow band of moisture feeding into the region. According to research by University of Georgia researchers, they occur most often in the cold months of November through March but can occur in any month of the year. In the Southeast, we get about 40 events per year that are classified as ARs. I was surprised to read that there are slightly more events on the East Coast than along the Gulf of Mexico, but anywhere along the Southeast coast can be affected. No trend towards more or fewer events was seen in the 1979-2014 period.

NOAA’s Physical Sciences Laboratory’s page describing ARs says that on average, about 30-50% of annual precipitation in the west coast states occurs in just a few AR events, thus contributing to water supply. ARs move with the weather and are present somewhere on the Earth at any given time. This site has some great resources for tracking and forecasting ARs around the world.

Of course, atmospheric rivers are not the only source of heavy rain events, but they are one of the primary sources for the West Coast. In other areas, tropical systems like slow-moving hurricanes or stalled fronts can also drop a lot of rain. You can also get very heavy rains from small local systems of thunderstorms if conditions are right, especially if the storms “train” or move one after another over the same area like cars on a freight train. We saw this in the Nashville area a few weeks ago, where the heavy rains resulted in significant flooding over a few counties.

Rain garden in the Allen Centennial Gardens on the campus of the University of Wisconsin-Madison. Source: James Steakley, Commons Wikimedia.

How do atmospheric rivers and other heavy rain events affect gardeners?

If you are a gardener in the Western United States, you are already well aware of the long dry season over the summer followed by bouts of rain that can occur over the winter months. The timing of the switchover from dry to wet conditions depends on how far north you are on the coast, with the summer dry spell coming earliest in southern California and moving northward with the position of the jet stream as the summer progresses. Dealing with the effects of an AR is like any other attempt to protect your garden against heavy rainfall, and can mean proactive action to make sure that water-sensitive plants and trees are not located in low-lying areas where rain collects. This 2013 article from the Garden Professors blog on-site assessment is still good advice for planning ahead for soggy conditions by walking through your property in the rain. Designing for erosion control, such as rain gardens, can also help divert water in high-intensity rainfall.

In spite of the heavy rain that fell in this last atmospheric river event, the rainfall barely made a dent in the long-term drought that is present across a lot of the Western U.S. Drought will continue to be a part of the hydrologic cycle that affects gardeners, farmers, and water managers across that region and across the world.

Do you wish you had a crystal ball that could tell you what the climate will be next year when you plan your garden? So do many other gardeners (and climatologists). But while there is no magic answer, we do know that in many parts of the United States and other countries, year-to-year climate variability is strongly dominated by what is going on in the eastern tropical Pacific Ocean. This is through a phenomenon called “El Niño Southern Oscillation” or ENSO for short.

What is ENSO and how does it affect climate?

ENSO has three phases—a cold phase with unusually cold water in the equatorial Eastern Pacific Ocean (EPO) called “La Niña”, a warm phase with unusually warm water in the EPO, and the neutral phase that occurs between the two extreme phases. The ocean see-saws back and forth between the two opposite phases on a semi-regular pattern that usually lasts between two and five years from one El Niño to the next. Sometimes you can have two La Niña years (or even three) back-to-back (the end of 2021 is expected to be a second La Niña in a row), but you almost never have two consecutive years of El Niño.

In many parts of the world, the phase of the ENSO is highly correlated with the climate. Scientists can use that relationship to predict what the climate might be like in the coming months. That is helpful for gardeners who need to know what to expect both next season and next year for planning purposes. Not all parts of the world have a climate that is well correlated with ENSO, however, and so folks in those areas will have to depend on other methods to look ahead to next growing season. Winter has the best correlation between ENSO phase and climate, while summer is much less predictable. And every El Niño and La Niña is distinct, leading to variations from the statistical pattern we expect.

How does the temperature of the tropical Pacific Ocean affect climate in other parts of the world?

You might think that unusually warm or cold water in the equatorial Pacific Ocean would not have much impact in other parts of the world because of the distances involved, but it does. Since the atmosphere flows like a river, putting unusually warm water (El Niño) into the EPO acts like putting a rock into a stream. The flow of water (or air) shifts around the rock, changing the pattern of atmospheric winds that blow weather systems around. When we are in a warm El Niño phase, the storm track shifts south and covers the southern US, leaving the northern US warmer and drier than usual. When I lived in Wisconsin, we noted that lake ice cover in El Niño winters did not last as long as other years, which made ice fishermen like my dad unhappy. La Niña shifts the storm track in the opposite direction. Because of that, La Niña winters are colder and wetter than average in the northern US since the storm track shifts north into the Ohio River Valley and sometimes even farther. This leads to cold, damp winters in the northern US. Similar correlations, called teleconnections, are seen statistically in climate records at many places on earth.

If we know what the phase of ENSO is likely to be, that tells us what climate conditions are expected in areas where there is a teleconnection between the EPO and that region. While every El Niño and La Niña is unique, statistically they do provide guidance on what to expect in that region, and most years they are correct, although once in a while a wildcard like a Sudden Stratospheric Warming will occur and give us an occasional busted forecast, as it did in February 2021.

What do we expect this year?

Right now, we are in neutral conditions following last winter’s La Niña, but we are headed back towards another La Niña in the next couple of months (almost an 80% chance in the November through January period). That phase should last for most of the winter but is expected to return to neutral by spring.  After that, it is too far out to make a believable prediction. The Global ENSO Temperature and Precipitation Linear Regressions website provides global correlations between the ENSO phase and what kind of temperature and precipitation anomalies to expect. In it, each three-month period shows the relationship between the temperature anomaly of the EPO and other parts of the world (regression) and how strong that relationship is (correlation).

In the map below for December-February (DJF) temperature, it shows that if the EPO is unusually warm (+) in an El Niño, then the northern part of the US will also be unusually warm (+) while the southern states are cooler than normal (-). The storm track over the southern US in an El Niño year brings rain and clouds to that region, keeping conditions wet and cool due to lack of sunshine. A La Niña year is just the opposite. The strong correlation in both southern and northern states shows that it happens most of the time, but in areas with little correlation, you can’t use ENSO reliably to predict seasonal conditions. If you have a hard time interpreting these maps, the website has a tab that explains it in more detail.

The bottom line

For this coming winter, I expect warmer and drier conditions than usual in the southern tier of US states as the storm track shifts north. That means more overwintering of insect pests and diseases; an early start to the growing season is also likely. The northern US is expected to see colder and wetter conditions than usual, which means a later start to the 2022 growing season but less chance of drought next year, although fungal diseases could be bad if the damp conditions continue into spring and summer. Western Europe could see warmer conditions than usual but the correlation is weak so that is not a strong forecast. Australia is likely to be colder than normal, with a fairly high probability because the correlations are high, at least near the coasts. This should last until spring, when the La Niña ends, and we swing back into neutral conditions when other climate factors become more important. In the Southeast, the summer after a La Niña ends is also a hot and dry summer due to the lack of recharging rain over the winter, so I think we have the potential for drought in the Southeast next summer.

In the past week, two new major climate reports have been released. One is the latest (6^th) report from the Intergovernmental Panel on Climate Change (IPCC) and the other is the State of the Climate 2020 report. Of the two, the IPCC report has garnered a lot more press, but both are compilations of work by hundreds of scientists looking at recent weather and climate patterns and how they are affecting us here on earth. The IPCC report also provides projections of what the future climate might be like, using a number of assumptions about how the earth behaves, which can be difficult, and how humans respond, which is arguably even tougher to determine. In this post, there is no way that I can cover both sets of reports in meaningful detail and I won’t address how we need to address the rapidly changing climate here, but I do want to try to pull out some things that you can use as gardeners now. {Note, the pictures are ones I have taken myself on recent trips to use as eye candy!}

What do the new reports tell us?

The State of the Climate 2020 report, published jointly by NOAA and the American Meteorological Society, focuses on global climate events that happened in 2020. You can read some of the notable findings from the report at my blog. The report also discusses many of the “big” climate events of 2020 and puts them into historical context, including how frequently these extreme events occur and how the changing climate is making them more likely.

The United Nations’ IPCC 6^th Assessment Report presents similar information but also makes more explicit the cause of the warming, which scientists have known for well over 100 years has been caused primarily by human emissions of greenhouse gases in the atmosphere that trap heat near the earth’s surface. The IPCC report makes it clear that the rapid pace of the warming will cause severe changes to the earth’s climate that will be difficult for humans and ecosystems to adapt to.

What do the conclusions of these reports mean for gardeners?

Here are some of the changes that we will have to adapt to in the future:

• Rising temperatures across the globe—Temperatures are rising across nearly all the globe, both on land and in the oceans. Warmer temperatures mean warmer winters, hotter summers, and longer growing seasons. They also mean more increases in both evaporation from water surfaces and more evapotranspiration from plants, resulting in increases in water stress. That means you may need to water more often or use other techniques like mulch to preserve soil moisture. You may also need to switch to more heat-tolerant species as the USDA plant hardiness zones shift north (in the Northern Hemisphere). It may become harder to work in the middle of the day when it is the hottest.
• Rising temperature leads to rising humidity levels, at least where there is a source of water vapor nearby. The higher humidity is contributing to higher night-time temperatures, which puts stress on animals living outdoors (pets, livestock, and wildlife) and also stresses some plant species. It can also lead to more clouds, which reduce direct sunlight and cool the air but also reduce solar radiation available for plants, slowing their development. You may have to manage your gardens for more diseases that are related to the high humidity levels.
• Some areas like the northern US may see more rain, while others like the Southwest become increasingly dry. Year-to-year variability in precipitation is also likely to increase, with both more floods and more droughts. In both cases, water management of your gardens will become increasingly important, with the heavy rain events causing more erosion and the potential for loss of plants and trees from too much water and not enough air in the soil, and the longer dry spells making gardens more dependent on either drought-tolerant species or more frequent irrigation. You may have to put in rain gardens to help slow the movement of water through your gardens in heavy rain.
• With the rising temperatures, frost and snow will become less likely but will still occur (there will still be winter!). This will allow you plant earlier than in previous decades but will still make the plants vulnerable to late-season frosts.
• Increases in carbon dioxide may provide some fertilization of some plants, but only if there is enough water available for growth. Since some weedy species are more efficient at using carbon dioxide than other plants, you may need to deal with more weeds and invasive species in the future than you do now.
• Strong storms like hurricanes and derechos may occur more often and be more damaging than the ones we are already seeing now. The research in this area is less definitive than that for rising temperatures because there are many different factors that go into storm development, but scientists generally agree that the number of hurricanes seems to be climbing upward and that the seasons are getting longer. In addition, the storms appear to be moving slower, and that is likely to lead to more rain from the storms over a specific area and more likelihood of rapid storm development. If you live in an area that is prone to strong thunderstorms or tropical cyclones, you may see them more often and the season may start earlier in the year. Rains and winds are likely to increase, leading to more tree damage and flattened plants.

Will we be able to see these changes over the next few years?

Year-to-year variations in climate will continue to plague gardeners, since whatever happened last year is unlikely to occur again this year. The climate naturally varies over time and space as well as exhibits these long-term changes. That means it can be hard to see the creeping trends in temperature and precipitation in the noise of yearly climate swings. If you are only worried about next year’s garden, what is happening in 50 years may not be of much interest. But if you care about your children’s gardens and their future on a warmer earth, than it is something these two reports make clear we have to think about and do something about.

Personal note: This week I was also invited to participate as an author on another upcoming large climate report, this one the 5^th National Climate Assessment (NCA) that focuses on changing climate in the United States. I will be one of a number of authors contributing to the chapter on the Southeast US. If you are interested in what the content of that report includes, you can view the 4^th National Climate Assessment, released in November 2018. There are chapters for each section of the country, but also chapters that deal with economic sectors like water and agriculture. The 5^th NCA will update the information in the previous version as well as add additional information based on scientific studies completed since then.

References:

The State of the Climate report in a peer-reviewed series published annually as a special supplement to the Bulletin of the American Meteorological Society. The journal makes the full report openly available online, here. NCEI’s high-level overview report is also available online, here.

Sixth Assessment Report, Climate Change 2021: The Physical Science Basis is now out.  The report addresses the most up-to-date physical understanding of the climate system and climate change, bringing together the latest advances in climate science, and combining multiple lines of evidence from paleoclimate, observations, process understanding, and global and regional climate simulations. Get more information including links to the press release and some videos here.

Do you have a favorite kind of weather that you love to experience? For me, it’s the first warm evening of spring, when the air is just warm enough and the wind just strong enough for the air to feel as though it is dripping off my fingers. On nights like that, I can tell that the air really is a fluid, the topic of this week’s blog.

What causes atmospheric weather patterns?

You may have noticed this year that the weather patterns across both the United States and Europe have been very persistent. That has led to the occurrence of record-setting high temperatures in some locations like the Pacific Northwest and the central US and southern Europe as well as day after day of rain in the Southeastern US. Both of these weather patterns have caused no end of grief for gardeners and farmers, since weather is seldom stuck on day after day of “perfect” conditions (even if you could define what those are).

To understand how weather patterns get stuck, it helps to know how the air moves through the atmosphere. Wind is driven by differences in heating between two areas. That contrast leads to differences in density and pressure between the areas, and the air flows from higher pressure to lower pressure to try to equalize the amount of air between those areas. The large-scale weather patterns across the globe are caused by differences in the sun heating the spherical earth at the equator and at the poles at different angles due to latitude. There are also smaller-scale wind circulations due to differences in heating between land and water (oceans or lakes) that cause the same movement of air molecules. The earth is not uniform, with continents in some areas and oceans in others and has mountain ranges that also divert the flow of air. The earth is also rotating, and that makes the wind appear to be turning towards the right in the Northern Hemisphere (left in the Southern Hemisphere), which we call the “Coriolis force.” Friction can also slow down the wind and cause it to change direction near the earth’s surface, which adds to the calculation.

Atmospheric waves

The net result of all these forces acting on the air are a series of atmospheric waves of high and low pressure around the earth that control where the weather goes (these are known as Rossby waves after the great meteorologist Carl-Gustav Rossby). A great website to view these waves is at https://earth.nullschool.net/, with a dynamic view of the air flowing around the earth. You can move the earth around with your mouse by clicking and dragging it, and you can change the size by pinching and dragging on your touchscreen if you don’t have a mouse with a wheel. If you click on “earth” on the bottom left of their map, it pulls up a menu that allows you to pick different heights in the atmosphere (1000 mb is closest to the surface, 500 mb is about halfway up in the atmosphere, and 250 mb (shown in this figure from July 29, 2021) is roughly the height that jets fly.

The image shows areas of strong winds (in red) and weak winds (in blue). The strong winds at 250 mb are called “jet streams”, and they push weather systems around. The strong jet stream over the upper Great Lakes in this image helps explain why the severe weather that moved through Wisconsin on July 29 moved from northwest to southeast. The jet streams outline the atmospheric waves that control the large-scale weather patterns. In the Northern Hemisphere, winds blow clockwise around areas of high pressure (“ridges”) and counterclockwise around low pressure (“troughs”). It is much easier to see on the website than on this static image, although traditional weather maps usually show the wind direction using arrows to help. As the image shows, often a ridge in one half of the continent is accompanied by a trough in the other half just like an ocean wave.

The large-scale weather patterns that are connected to atmospheric waves are constantly shifting as the air in the atmosphere tries to balance out all the forces that are pushing it around. (This 30-second video shows how the waves typically evolve over time.) Often the waves move, usually from west to east in the mid-latitudes, and so the weather at those latitudes also tends to move from west to east. But sometimes the weather patterns get stuck in one spot, and cause day after day of the same weather. We call these “blocking” patterns because they block the natural movement of the waves, locking the weather pattern in place for long periods. That is what we have seen this summer, with a very strong high-pressure center locked over the western U. S. and a trough of low pressure draped over the eastern U. S. for much of the last few months. In summer, high pressure usually causes sinking air, lack of clouds, warm temperatures, light winds, and no rain. In contrast, low pressure leads to rising air that forms clouds and rain and cooler temperatures due to the clouds.

What is happening this summer?

The blocking this summer has been more persistent than usual, with strong high pressure in the west causing a very hot and dry area to form (sometimes called a “heat dome”). Once you get a strong high like that to form, it can get anchored in place by the dry conditions and high temperatures at the surface, making it very hard to move. The length of time that this summer’s high has lasted has contributed to the string of record-setting temperatures and lack of rain and subsequent drought that region has experienced this year. By contrast, in the Southeast, low pressure has led to a stubborn low-pressure trough that has brought rain to the region almost every day, preventing farmers from doing field work or drying hay and increasing the occurrence of fungal diseases on their crops. If you know you are in a blocking pattern, that means you should prepare to experience the same weather for protracted periods of time and adjust your gardening schedule to accommodate the weather you are likely to see for the next week.

How will atmospheric patterns change in the future?

Some but not all atmospheric scientists think that as the earth gets warmer, the temperature difference between the equator and the poles will decrease and that this will lead to atmospheric wave patterns that are loopier, like winding rivers in an almost flat coastal plain. This could lead to both more extreme weather and potentially, more frequent blocking patterns. Gardeners need to prepare by designing their gardens to handle both more extremes of weather, including heat waves and floods, and periods of more persistent weather, which could lead to more frequent and longer droughts. That means making sure that you need to have good drainage for high-intensity rainfall events but also need to use methods like mulching with arborist chips to preserve the moisture in the soil for those times when no rain is in the forecast for extended periods.

How do you plan your work in your garden? One of the things that is most likely to affect what you do is rainfall. But how do you know when and how much rain is likely to fall? One way to get an idea of the possibility of rain is to look at something called “Probability of Precipitation”, or as we call it, “PoP”. How often have you heard someone say that the weatherman (or woman) was wrong because they predicted 30 percent chance of rain and they did not get anything? Or someone else says there was only a 10 percent chance of rain and they got flooded? If you understand how these forecasts are made, it might help you plan your outdoor activities, including your garden work and when you water.

How is “PoP” defined?

According to the National Weather Service (NWS):

PoP = C x A where “C” = the confidence that precipitation will occur somewhere in the forecast area, and where “A” = the percent of the area that will receive measurable precipitation, if it occurs at all. The forecast is what we call a “conditional” forecast—that means it depends on two different things, one of which requires the other to occur. It’s important to keep in mind that these forecasts are made for a particular period of time (often 12 hours) and for a particular area (the forecast zone). The first part of the calculation is whether or not it will rain at all anywhere in the forecast zone during the time that the forecast covers. The second is how much of the forecast zone will be hit by precipitation sometime during the forecast period.

How likely is it that precipitation will occur?

The first part of the equation above, “C”, is whether rain will occur or not in the forecast zone during the forecast period. Sometimes that is easy to determine if a big high-pressure center is over the area and no rain is expected anywhere in the region. That means that the first part of the equation is zero, and so the PoP forecast would be zero. If a strong front is moving through your area or a tropical storm is headed your way, the probability of rain somewhere in the area is probably close to 100%. But often, the likelihood of rain is not so clear. What if you are not sure about the timing of the front or the tropical storm? If it moves slower than expected, it might not make it to the forecast area before the clock ends for that forecast period. Or if you are not sure the conditions are going to be right for a rain shower to occur, then you might or not get precipitation, depending on the actual conditions. Then C becomes something between 0 percent and 100 percent, depending on how much you trust the computer models that produce the forecasts.

How much area will be covered by the storms if it does rain?

As I am writing this, it is raining at my house just southeast of Athens, GA (you can see the tiny yellow splotch on the radar map above). Sometimes it is clear that rain will cover the entire forecast area during the forecast period. But most of the time, we think it will rain in parts of the forecast area, but it will be “hit or miss” rainfall from discrete storms, not widespread coverage. The second part of the equation, A, is the forecaster’s estimate of how much of the area will be hit by rain sometime during the forecast period. It’s not as easy as you think, because those storms are moving, and they cover more of the region than you might expect. In my research I have found that often the PoP forecast is too low, and that rain as estimated by radar covers a wider area than you might expect based on the forecast. Fortunately, the NWS does provide radar estimates of precipitation that are calibrated by actual ground-truth rainfall data. The map below shows the 24-hour rainfall ending on the morning of June 27, 2021, including the rain you see in the maps above. For my CoCoRaHS rain gauge, the 0.05 inches I got correspond quite well to the light blue on the map.

Where do you get PoP forecasts?

So where do you get PoP forecasts and how do you use them? Most meteorologists provide PoP forecasts on their broadcasts or written forecasts. I tend to use the similar Precipitation Potential from the NWS hourly forecasts because they go six days out, which allows a longer planning horizon. You can see a simplified example of a forecast graph below. The “Rain” categories of Slight Chance, Chance, Likely, and Occasional correspond to Precipitation Potentials of roughly 5-25%, 25-50%, 50-75%, and 75-100%. The amount of rain in inches is shown superimposed on the bars, so you get an estimate of how wet it will be in that time period. You can find instructions for how to get your own hourly forecasts at my blog.

The hourly graph allows me to plan well ahead of whatever outdoor activity I am doing. The forecasts show hour by hour how likely precipitation is, including the chance of thunderstorms, and an estimate of how much rain will occur. Keep in mind that the forecasts six days out are not likely to be as accurate as the ones for tomorrow, but they are still useful for planning purposes. Using this information tells me when and how much rain to expect.

How do you use PoP forecasts in your planning?

These forecasts are especially useful for determining when to irrigate or apply insecticides or fertilizers that require specific wet or dry conditions to work properly. If you can see rain will be starting soon, you might choose not to water your garden unless the rain amounts are likely to be small or the chance low. Or maybe you want to mow your lawn now before it gets wet. If you are planning to fertilize and if it needs to be watered in, now might be a great time! If you need to apply a treatment that requires wet leaves to be effective, then you might wait until after the rain is over rather than applying now and seeing the chemical wash away in the storm. These precipitation forecasts can help you make the best use of your time by providing targeted, timely information on when rain will occur and how much is likely to fall when it does.

For more information on PoP forecasts, check out ” Do You (Or Your Meteorologist) Understand What 40% Chance of Rain Means?” by Dr. Marshall Shepherd, my colleague at the University of Georgia.

You may have read in the news earlier in May that NOAA has updated their “normals” for temperature and precipitation at stations around the country. In climatology, normals are the calculated averages over a specified time period. Usually, we use a 30-year period to capture what the average weather is like in a time period that is about the length of a generation, but now NOAA is also calculating normals based on other time periods like 15 years. Utility companies often use 10-year normals because electricity-generating technology and energy demand is changing so quickly that 30 years is considered too long.

Why do they update the normals every 10 years?

Normals are updated every ten years, so the new period of 1991-2020 is replacing the older normal period of 1981-2010. They only do it every ten years because a lot of work goes into quality control of the data as well as adjusting for station moves, missing data, and changes in observation time. All of those events can introduce artificial “climate change” into the record, leading to averages that don’t really represent the current climate at the location of the station being described. Climatologists follow rigorous methods of making these corrections, and even scientists who are skeptical about their techniques by and large end up with nearly the same corrections if they follow scientifically and statistically accurate methods. NOAA has provided some FAQs that explain more about the process of creating the new normals if you are interested.

How are the normals changing?

Determining what a “normal” temperature is when the temperatures are relatively stable is easy, because you can use any long-term average to describe the expected temperature. But when the climate is not stable but is changing over time, what you think of as “normal” weather changes as cooler decades get replaced by warmer decades. For example, here is a graph of the annual average temperature for the Midwest with 30-year normals plotted on it for 1961-1990 (green), 1971-2000 (blue), 1981-2010 (violet), and 1991-2020 (yellow). Early in the record, the 30-year averages (not shown for the early time periods) did not change all that much from one decade to the next because there was no trend towards warmer conditions. But now, every new set of normals gets warmer. We are not living in the climate that our parents or grandparents grew up in! This Washington Post article by Bob Henson and Jason Samenow provide an excellent overview of all the changes that we are seeing and why those changes are occurring. We can expect the next set of normals to be even higher as the temperature continues to rise.

How are the normals changing across the country?

The annual average temperature is not changing by the same amount everywhere. The map below shows that even though most of the lower 48 states are getting warmer, the upper Great Plains got cooler when the latest normals were calculated. Western Texas and parts of New Mexico had the largest increases in temperature. NOAA also has these maps for select months.

Of course, it is not just the annual average temperature that is changing. The minimum temperatures are increasing at almost twice the rate that the maximum temperatures are rising. Most but not all monthly temperatures are rising at many stations. The precipitation is changing in northern and western high-elevation areas from snow to more rain. Most parts of the US are getting wetter, but the Southwest is getting drier. And the rain is coming in higher intensity bursts, with longer dry spells between precipitation events in many areas.

As temperature and precipitation change, other variables that are related to heat and moisture are also changing. The length of the growing season is increasing in most of the country, allowing gardeners to plant new varieties of heat-loving plants but stressing plants that prefer colder temperatures. This is a concern for peach farmers in Georgia, for example, since peach trees need a certain number of hours below 45 F to set a good crop of fruit. As the temperature rises, it becomes harder for the trees to get the cold weather they need to produce enough blooms. Other plants like lilac, which I enjoyed every spring when I was growing up in Michigan, do not grow in Georgia because of the heat and may someday be scarce even in the Midwest. Growing degree days (a measure of the amount of time above a base temperature, commonly 50 F, used to track plant development) are increasing, affecting the growth patterns of commercial crops as well as garden plants. Humidity is also rising, leading to more fungal diseases and more oppressive working conditions for gardeners and farm workers who are affected by both the higher moisture levels and more frequent extremely hot days. At the same time, higher evapotranspiration from plants accelerates the water cycle, making droughts (and floods) more likely.

Where do you find your local normal weather?

If you are interested in finding your new “normal” temperature and precipitation and comparing it to the old values at your location, you can find instructions at my daily blog. Of course, there are many other places to find it as well—just do a search online and several sites should pop up. If you want to do an average over a different set of years, you can use the Custom Climatology Tool from the University of Nebraska-Lincoln to do those calculations.

Ultimately, the changes in the climate reflected in the new normals will show up in other garden-related values such as the USDA Plant Hardiness Zone, although it’s hard to know exactly when those values will be updated. Even without knowing exactly what zone you are likely to be in over the next decade, with the continuation of rising temperatures that we expect, you can try out plants that are just on the warm side of your current zone to see how they do. Of course, your local microclimate will also affect their ability to thrive, so don’t forget to consider that too.

Should we just get rid of “normals” since they keep changing? I don’t think so, since they do provide useful information about what we expect over a number of years. You can use normals to determine what clothes to have in your closets, how much heating and cooling you need for your homes, and what to plant in your garden. Just be aware– “normal” is no longer normal in a changing climate.

If you’ve been around as long as I have, you will no doubt remember the Creedence Clearwater Revival song “Have You Ever Seen the Rain”. This week I want to talk about sensing the rain using radar and how you can use it to provide you with local rainfall information if you don’t have a rain gauge of your own.

How does radar work?

Radar is what scientists call an active sensor, because it sends out a beam of electromagnetic radiation that is reflected back to the radar if it hits something reflective like raindrops or hail (it also works on birds, insects, and cars traveling along interstates, but that’s another story). By detecting how much of the original beam is returned and how long it takes to get back, the radar can determine how much precipitation there is and how far away it is falling. The radar emitter usually rotates around a circle to provide a 2-dimensional picture of the precipitation in the area around the radar instrument. They can make it 3-dimensional by tilting the radar up at different angles to see different levels in the atmosphere. Now, the newest doppler radars used by the National Weather Service can also sense the size of the falling particles and how fast they are moving towards or away from the sensor. The radar displays that are usually used on television or online show a color-coded map with the brightest colors corresponding to the highest radar returns and thus the heaviest rain rates.

Radars can be used to estimate rainfall, but some assumptions must be made about the rain to get a good estimate. The major estimate that is needed is what size or sizes are the raindrops and how many of them are present. That will allow the radar software to calculate the volume of water that is falling and relate it to the strength of the return “echo” of the radar beam.

But how do they know the distribution of raindrop sizes in a rainstorm?

I learned this week in a video on raindrop shapes that the first person to measure rainfall size distributions was William Bentley, a citizen scientist in Vermont who is best known for his spectacular photographs of snowflakes. Bentley used a tray filled with a shallow layer of flour and exposed it to falling rain. The drops landed on the flour and dried into balls that provided a measure of how the size of the drops varied in the storm. Of course, now there are more sophisticated ways of determining this using optical sensors and other devices, but this was surprisingly good for its time.

Today, by measuring the amount of radar emissions returned to the sensor and calibrating it to rain gauge measurements at the surface, atmospheric scientists have been able to provide good estimates of the rain falling across the region that the radar is able to sense. That is usually within about 120 miles before the radar beam overshoots most of the rain clouds due to the earth’s curvature. Fortunately, with a network of radars across the country, we can get a pretty good estimate of rainfall that is spatially much more detailed than we can get with a network of surface observers from the National Weather Service, state networks like the agricultural weather network I manage at the University of Georgia, or the volunteer corps of observers in CoCoRaHS (for more on this network, see https://gardenprofessors.com/the-weather-where-you-are/). That allows us to have a pretty good sense of how the rain is varying across fairly short distances and provides a reasonable estimate of the rain at your house if you don’t have a rain gauge available.

Radar-estimated rain where you are

To find the rainfall estimates for your location, the easiest way to do it is to use the National Weather Service’s Advanced Hydrologic Prediction Service. This website provides a daily rainfall amount based on radar estimates for the period currently from 8 AM EDT on the previous day to 8 AM on the day of the map. They are usually available an hour or two after that time period ends so they can receive the data and perform quality control before releasing the maps. You can zoom in on the maps to your location and add county outlines or other backgrounds to help pin it to your exact location. The site also allows you to look at 7-day, 14-day, and longer accumulation periods and to compare those to normal or expected precipitation. The map below is one I created for a heavy rain event in Georgia this past week on April 25, 2021, where a few locations in southern Georgia got up to 10 inches in just a few hours, causing problems for farmers there due to standing water, erosion due to runoff, and scattered loss of seed and fertilizer.

The radar maps are not perfect. You can only zoom down so far, and the smallest unit is still at least a few kilometers or miles on a side, so you will never be able to distinguish the exact edge of a summer thunderstorm that drops rain on one side of the road and leaves the other side dry. The estimates also tend to be too low in high-intensity rainfall because the relationships that the radar software uses to estimate the volume of water don’t work very well when it is raining harder than normal. But by calibrating the rainfall to observers’ reports, they are usually pretty reasonable. If you are not in the United States, you will need to check with your own nation’s weather service to see what radar information is available.

Coming in May…

Speaking of “normal”, in May NOAA is expected to update the normals for temperature and precipitation for the US from the 1981-2010 values to the 1991-2020 values. The new temperature values will be higher than the previous ones due to the upward trend in temperature in the US and the globe over time. Rainfall will also change but it will go up in some places and down in others due to wet and dry spells in different parts of the country over time. I will talk about the new normals and how they are created in my blog post in late May.

Most gardeners this time of year are thinking about the last frost dates for their locations and how soon they can get out into their garden plots. Here in the Southeast, many areas have already passed their last frost or will soon, while in other parts of the country, it may be many weeks before the threat of frost is over. In this week’s column, I want to describe a way to get frost dates for your location and discuss the mystery of why the date of the last spring frost is getting later in the Southeast in spite of temperatures that are rising across the country.

Resources for finding your frost date

There are many places that you can go to find information on the average date of the last spring frost. Many gardening guides publish them, and John Porter had an excellent discussion of last frost and planting dates a year ago, including a number of sources of information and a map for the continental United States.

You can also look at frost dates for individual locations using xmACIS, an online free database that allows you to list yearly last spring and first fall frost dates and the growing season length. This database contains observations taken by National Weather Service cooperative observers and is incorporated into the NOAA 30-year averages (normals) that John mentioned in his posting. You might find it helpful to see not only the average but also the variability from one year to the next at whatever station is closest to you. (Here is a quick reference sheet for xmACIS.) Of course, there are other places to get this data in a variety of formats, but xmACIS is quick and easy and works for the whole country, which is an advantage for all our readers.

To access data near you,

1. Go to the top under Single Station and choose “First/Last Dates.”
2. Under Options Selection choose:
   1. your preferred output, (Graph, table or CSV)
   1. Year range (POR is period of record, which will vary depending on which station you choose)
   1. Under Criteria set minimum temperature at less than or equal to 32 F (or another threshold for a special crop)
   1. Period beginning (for spring frost dates, usually July or August)
   1. Pair results (for spring frost dates, usually by Calendar year)
3. Under Station Selection, you can find a station by ID if you know it, by choosing from the list or searching by zip code. Or change your CWA (National Weather Service County Warning Area) to your local region and available stations in that area will be listed. A map of the CWAs is shown below. Pick the station that is closest to you to get the best data for your location.
4.  Hit “Go” and you will get a list of the yearly last and first frosts of the growing season. The average date is at the bottom.

Climate change and frost dates

With increasing temperatures due to global warming, you might wonder how these frost dates are changing over time. As temperatures get warmer, you might expect that the average date of last spring frost would be getting earlier in the year over time and the average date of first fall frost would be getting later. And this is generally true in most of the US, with the exception I will discuss in a minute.

I did some work with Melissa Griffin of the South Carolina State Climate Office in the past, and we determined that a 1-degree F rise in average temperature over time corresponded roughly to a 1-week increase in the length of the growing season. That is an important statistic for farmers, who plan what to plant depending in part on how long the growing season is. If the temperature in the US goes up 4 F by the year 2100, then we can expect that the growing season would increase by generally four weeks or one month, although that will vary from place to place.

Southeast frost date mystery

In most places in the US, the date of last spring frost is getting earlier in the year, as expected. But there is one regional exception, and that is the Southeast, especially in Georgia and to a lesser extent, Alabama. You can see this in the once-again public EPA climate change page.

It is not clear why this trend towards a later spring frost date is occurring in the Southeast. One theory is that perhaps a local weather phenomenon we call “the wedge” is changing due to alterations in weather patterns across the region as the global temperature increases. “The Wedge” is a thin, dense layer of cold air which moves southeast along the eastern edge of the Appalachian Mountains, bringing cold air and cloudy conditions to that region.

A group of University of Georgia students and I looked at this “wedge theory” in 2020. We tried to identify where the wedge of cold air was most likely to be occurring in the Spring and correlate those areas with changes in frost date. So far, the results have been inconclusive. More research will be needed to figure out why this odd pattern is occurring now and whether it will continue in the future. 

Implications for home gardeners

Knowing your average spring frost date can be an important brake on most gardeners’ eagerness to get back out in the garden in spring. Who hasn’t wanted to start planting on the first warm and sunny day? But if you know that more frosts are likely based on the local climate, you may be willing to wait to get started until your plants are safe from cold damage. Then the real growing season can begin!