Although the ‘official’ start to summer by the calendars is June 21st, we typically think of the summer months as June, July, and August around these parts. We also typically see a shift in our weather pattern that can leave people with a lot of questions as we struggle to explain the almost daily forecast of ‘isolated to scattered afternoon and evening showers and thunderstorms’. Over the course of thisblog, we’ll break down some things that might help folks get a better grasp of forecasting summer-time storms here in the Deep South!
PATTERN CHANGE: At some point during the month of May, we typically transition out of a pattern than brings us the risk for major severe weather events, and to a pattern that begins to bring us summer-like temperatures and these daily showers and storms. One of the major players in this pattern change is the jet stream – a river of fast moving air high in the atmosphere that helps to ‘steer’ our storm systems and areas of cool and warm air. When the jet stream retreats farther to the north, we see less and less major storm systems and intrusions of colder air – this also, in turn, shifts the severe weather threat during May and the summer months farther to the north and west. Another big forecast player is the Bermuda high – a semi-permanent area of high pressure off the Atlantic coast. The position will change from time to time over the summer, but that helps to bring a steady supply of warm, moist air in our neck of the woods, along with warm temperatures.
THE STORMS: So, with warm and moist air in place almost every day during the summer months, you might be wondering why it just doesn’t rain all the time. The answer to that question is this – even though we may have moisture, we need something to lift it up to cause showers and storms. The problem in the summer is that this form of lift can come in many different forms. Sometimes it’s a difference in temperature across an area, or a difference in moisture. Sometimes it’s an ‘outflow boundary’ – an area of cooler air rushing away from ongoing storms that acts as a mini cold front, lifting warm and moist air out ahead of it, setting off more storms. And sometimes it may be more of a larger scale front or something like that. Either way, the main problem is this: we can’t determine the exact places where these boundaries will set up very far in advance. It’s just a limitation to our science. And it’s not just us – no meteorologist can tell you that with certainty! That means if you catch a late newscast, we’re still not going to be able to tell you exactly where it will rain the following afternoon.
The best that we can do is determine the best area of storm initiation – where they will most likely start to fire up during the day. We will try to convey this as a geographic area – East Alabama vs. West Georgia, north of Highway 80 versus south, east of Highway 19 versus west, etc. It’s this way we will be able to convey the areas most likely to see the highest risk of being in an environment where there may be a storm, but of course even then it doesn’t guarantee a storm will pop up where you are. Because these storms gain their energy from the heating of the day, the best chances for these storms typically are during the afternoon and evening hours - we'll say between 2 PM and 10 PM on average. They usually dissipate during the nighttime hours as they lose that energy from the sun.
The good news when it comes to these kinds of storms is that we typically don’t see major severe weather events in the summer months. Wind shear is usually minimal, so the tornado threat is very low overall. However, these storms have plenty of atmospheric energy to work with, so they can pack quite a punch. Overall, the biggest threat is lightning, especially since so many folks are out and about during the summer months. If you see lightning, or hear thunder, you need to seek shelter indoors until the storm passes by your location! High winds are another threat, and sometimes can come in the form of a microburst – either way, severe wind gusts can cause damage just like a tornado can, so it is necessary to take these storms seriously! Usually, the hail that may come from a summer storm isn’t large enough to be damaging, but hail to the size of quarters would still be possible with the strongest summer storms. Flash flooding is also possible since these summer storms don’t like to move very much – in fact, the storms can move very erratically since typically there isn’t a lot of ‘steering’ when it comes to these storms. Sometimes they can build back over the same areas, causing a few inches of rain to fall pretty quickly. Their erratic motion also means that 'storm tracks' when you often see us use during severe weather events are almost useless with these storms.
PROBABILITY OF PRECIPITATION: So, now that you have a little background on how these storms form and the uncertainty with which we have when dealing with where and when they will pop up, it’s important to understand how rain chances this time of year are presented to you! You will likely hear of percent-based rain chances, also called the ‘probability of precipitation’. When the folks at the National Weather Service use these, they are conveying their confidence that it will rain and the coverage of that rain with one number. Personally, I don’t think this is a very intuitive thing for most folks to grasp. For example – if a forecaster is 50% confident that it is going to rain – and they expect it to do so over 80% of a particular area – the PoP , or probability of precipitation, is 40% {( .5 x .8) x 100 = 40%}. You can see where that’s a little confusing!
For our viewing area, and on our forecasts, I like to do it a little different. Most of the time we’ll be fairly confident that it will rain SOMEWHERE across the viewing area during the summer months, and when I use a percentage on one of our weather graphics, that will convey the COVERAGE of rain across the area. Most of the time in the summer months, this will mean the chances will be in the 0-30% range depending on the particular situation. A 10% rain chance for our area would mean I’m expecting about 10% of the viewing area to pick up at least some rain. Also keep in mind that our coverage area stretches from LaGrange to Eufaula and Tuskegee to Butler!
HOW WE BREAK DOWN THE FORECAST: When we present the forecast to you, we also try to use descriptive words to describe the coverage of rain as well. From least to greatest, those words are:
slight --> isolated --> chance --> scattered --> likely or numerous
This, combined with our thoughts on the percent coverage of any possible showers and storms, along with descriptive areas that will have the best chances are about the best we can do when it comes to conveying the chances of summer-time convection to the folks who consume our forecasts. Inevitably, this is the time of year when someone talks about ‘how wrong we always are’ and how ‘we’re the only profession where we can be wrong most of the time and still keep a job’. A lot of that stems from the difficulty of forecasting summer storms, and some of those unkind words are misguided! When the rain chance is just 10%, and a thunderstorm rains out someone’s outdoor plans, I understand the frustration; however, if you listen to a forecast closely, a lot of times we’re also giving you a lot more information to go along with that number!
UNDERSTANDING WEATHER APPS: This also brings us to the discussion of how that kind of weather information is presented on your weather apps on your phone. When you see an ‘hour by hour’ breakdown of the forecast during a summer afternoon, some weather apps might generate a thunderstorm cloud each hour, trying to convey the chance of rain. That doesn’t mean that it is going to rain all day, or will rain at that specific hour; but rather, it’s trying to convey what I’ve spent this blog trying to explain – the ‘pop-up’ showers and thunderstorms which we often get this time of year, with the best chances in the afternoon and evening. Use those weather apps wisely, and don’t automatically assume a thunderstorm on those hour by hour breakdowns means it WILL rain.
The same holds true with our weather graphics we show on television every night. On our seven day planner, if you see a sun, cloud, and a tiny thunderstorm on a particular day (which you might in the summer – if the chances are 20% or less, we usually leave off the thunderstorm), you won’t just be able to look at that and know what will happen without listening for those descriptive words about where and when the storms will pop up, what kind of rain chance there will be, etc. Inevitably, some folks look at those and will say “you said it was going to rain, and it didn’t!” or “you said it wasn’t going to rain, and it did!”. We just want you all to make sure you have all of the information available to help you plan out your outdoor summer activities – just glancing at a forecast graphic without getting the whole story won’t help you do that!
For example… suppose you wake up to some low clouds or fog one summer morning, and those clear out in the mid-morning. It stays sunny, hot, and humid for a few hours, but then you notice clouds building, and at 4:53 PM, a storm moves over your house. It rains for about ten minutes, and that storm falls apart. It has cooled down quite a bit, but the sun comes back out and it feels more humid than ever! Temperatures rise again, and the rest of your evening is dry – but then, at 8:14, another storm moves over, and just dumps heavy rain, lightning and thunder, and gusty winds on your location until about 11 PM. Tell me how we can display that kind of forecast on a graphic!
TROPICAL STORMS AND HURRICANES: Something that can provide us with a bit of a break from the typical summer-time pattern is tropical disturbances. Tropical waves, tropical depressions, tropical storms, and hurricanes can throw all of this thinking out the window. Tropical systems, depending on their track, can affect the forecast in a big way, and if we find ourselves on the east side of the track, we could be in line for heavy rain, flooding, and tornadoes that spin up in the spiral bands around these systems.
As always, if you folks have any questions, comments, or concerns about our weather or how we present it to you, let us know! Ultimately, we’re here to serve you and your interests, so let us know what you think!
STAY CONNECTED: Don't forget, you can get the latest forecast updates on Twitter & Facebook. The weather tab at WTVM.com has great weather information as well (including LIVE Doppler 9, a link to sign up for daily weather emails, and our ALFA Skycams). My personal Facebook page is also updated with the latest weather info and various thoughts and musings of mine!
Chief Meteorologist Derek Kinkade, WTVM / WXTX
dkinkade@wtvm.com
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