Summer Weather Forecasting In Alabama

With a summer weather pattern setting in a little early this year, there is no doubt it is time to write my annual article on summer weather forecasting in Alabama. From now until early September, the primary belt of strong upper air winds across North America shifts to the north, near the Canadian border, keeping the big rain-producing storm systems well to the north of Alabama. Light winds and warm temperatures aloft mean a number of things; some good, some bad. We rarely have organized severe weather events here in the summer, and widespread flash floods are rare as well (with the exception of tropical weather, which is another subject for another day). On the negative side, we are usually left in a maritime tropical airmass, with very high humidity levels which means oppressive afternoons during the middle of the summer.

High temperatures rise into the 90s, and parcels of moist air become buoyant and rise by mid to late morning. Those columns of rising air often develop into showers and thunderstorms during the afternoon and evening hours (generally between 2:00 and 8:00 p.m.) in random spots that are widely spaced. It is almost impossible for us to tell you on a summer morning exactly when and where it will rain later in the day, and you wind up with a forecast that includes “scattered afternoon thunderstorms” on most days.

Coverage and intensity of storms on a summer afternoon depend on small scale low level boundaries (often generated from storms on the previous day), temperatures aloft (about 10,000 to 20,000 feet off the ground), and morning cloud locations (differential heating). The National Weather Service uses “probabilities of precipitation” in their forecasts, and we don’t. They are not understood by the public, and really get you in trouble on a summer day.

For example, on a day with only about four good showers around here, somebody is driving through one of the showers. They hear on the radio the "probability of precipitation" is 20 percent. That is a horrible forecast to them since the rain is pouring down and the sky is dark from clouds. They have no way of knowing the shower is isolated. To any one person at any one place at any one time, to be prefectly correct the chance of rain should be zero or 100 percent.

We prefer to use descriptive terms in summer forecasting. Like "a couple of spots will see a passing shower or storm between 2:00 and 5:00 p.m., with the best chance of a shower east of I-65". That is more useful than the blanket "partly sunny with scattered afternoon thunderstorms; probability of precipitation is 30 percent". I avoid percentages at all cost.

But, for all of us, the main problem is the simple fact we don't know at 6:00 a.m. where it will be raining at 3:00 p.m. I don't care how smart you are, or how long you have been in the meteorology business. We struggle with afternoon convection on summer days.

Here are some suggestions on getting the most out of a summer forecast:

*Read the discussion that goes with the seven day grid on this page:

http://beta.abc3340.com/weather/7day.hrb

That discussion will take you beyond the numbers and parameters and can be very helpful during the heat of summer.

*Read the numbers in the seven day grid, like "hours of rain", "rain potential", and "sun percentage". Those do change on a daily basis and can give you clues about the subtle weather changes we have in summer.

*Watch the radar. That is really the best way to figure out your rain situation; on most summer days showers and storms begin to fire sometime between 1:00 and 3:00 p.m. Of course, unlike the colder months, they don't move much, and can often fall apart quickly as new ones develop. Our radar images refresh every minute here:

http://www.abc3340.com/weather/doppler.html

*Read the blog and watch the video updates. If you are reading this now, you know we often go pretty deep here and on the video. Once again, more clues for you on how to plan a summer day.

Most of our major weather events in summer are related to tropical systems coming out of the Gulf of Mexico. Even disorganized tropical waves can bring copious amounts of moisture as far north as Birmingham. And, of course, tropical storms and hurricanes can mean big trouble. We will discuss tropical weather here on a daily basis during the season, which begins on June 1.

While it might seem that summer weather forecasts are all the same (hot and humid with scattered afternoon storms), we can have some very interesting days in the months ahead. You can know be we will be watching!