Author: Pam Knox

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!

Greetings from Athens, GA! I am happy to join the group of contributors to the Garden Professors blog. My name is Pam Knox, and I am an agricultural climatologist in Extension at the University of Georgia as well as the Director of the UGA Weather Network and a former State Climatologist from Wisconsin. While I don’t claim to be an expert in gardening, I do know a thing or two about how weather and climate affect plants and hope to share some of that expertise with you over time. You can learn a little more about me from my bio on the blog page.

If you really like learning more about weather, climate, and agriculture, you are welcome to visit my own blog page, “On the CASE—Climate and Agriculture in the SouthEast” at https://site.extension.uga.edu/climate/, where I post almost daily about stories that have caught my eye as well as climate summaries and outlooks for the southeastern US. I plan to post on the Garden Professors blog here about once a month and am happy to answer questions at any time at pknox@uga.edu.

A simple way to compare temperatures around your yard

For my first post, I thought I would talk a little bit more about the weather in your yard and how you can learn more about it. As gardeners, you probably spend more time in your yards than I usually do, and so you have noticed that the climate of your yard or field can vary quite a bit from one spot to another. We call that “microclimate” and if you search this blog for that term, you will find several articles about microclimates in previous years, so I won’t spend a lot of time on that here.

One easy and inexpensive way to measure how temperature varies across your domain is to use an infrared thermometer to spot-check the temperature at a variety of locations. These thermometers are used a lot now to check forehead temperatures in the age of COVID, but they are also used by HVAC technicians to check heating and air conditioning, for example. You can find inexpensive ones selling for less than $20 online, and many hardware stores have them, too. You will be amazed how much difference there is in temperature between sunny and shady locations! Don’t forget to try it at night too to see how much tree canopy can affect night-time temperatures. Of course, if you want a more systematic and scientific approach, you can follow Linda Chalker-Scott’s experience using multiple min-max thermometers as described in http://gardenprofessors.com/microclimate-follow-up/.

CoCoRaHS: Precipitation measurements by citizen scientists

One of the many things I do is to serve as a regional coordinator for CoCoRaHS, short for Community Collaborative Rain, Hail, and Snow network. This is a group of dedicated citizen scientists who take daily rainfall measurements and report them online via computer or smartphone as part of a nationwide (and now international) network of precipitation observers. Theses observations are used by the National Weather Service, drought monitors, water supply managers, and others to document local variations in rainfall at a much denser scale than other available observing networks. I am sure that some of the readers of this blog are already contributing! You can learn more about the network and how to sign up at https://www.cocorahs.org/. Please keep in mind that they do require the use of a particular scientific rain gauge, so a hardware store gauge is not likely to have the degree of accuracy that is needed to participate. A list of inexpensive vendors (costs start around $40 plus shipping) can be found on their site in the right column. By measuring precipitation at your house, you are not only monitoring your own conditions but contributing to our knowledge of water availability around the US and beyond.

I am looking forward to interacting with you all in the months ahead, and please feel free to contact me if you have specific weather or climate questions.