Summer Weather Forecasting
What are "Pop-Up" Thunderstorms?
One of the challenges of forecasting weather in Alabama during the summer-time is the inability to determine the exact location where afternoon storms will develop. In the summer months, as the afternoon heat builds from sunshine, the atmosphere destabilizes. Air rises forming clouds, which eventually build to thunderstorms. They form at rather random locations, but sometimes form on micro-scale boundaries, much like "mini cool fronts". But since the conditions that determine where storms will develop occur on the micro-scale and the computer models we use to make forecasts are on a larger scale, the "synoptic-scale", they do a poor job of forecasting these features very far in advance.
As a result of these issues, on most summer days you will see a 10-30% chance of "widely scattered and afternoon showers and thunderstorms". This literally means that about only 10 to 30% of locations in North Alabama will see rainfall that day. These storms are very sporadic and typically do not move very much. So in many cases one place may see a heavy downpour for up to an hour, while someone just a few miles down the road could see nothing but sunshine. This is something to keep in mind when you look at a weather forecast and automatically assume the weather forecast is wrong just because it rained on a day with a 10% chance of rain forecasted for that day.
There are ways to determine the areas that have the best chance of seeing the pop-up storms, if it is possible to detect the boundaries they are developing along. Also by looking at visible satellite images you can detect building cumulus which could later form thunderstorms. But for the most part pinpointing exactly where these "pop-up" (or "air mass") thunderstorms will develop during the afternoon is very difficult, if not impossible.
The typical pop-up thunderstorms that occur during the afternoon hours in the summer-time are not associated with any larger weather system (another reason it is nearly impossible to predict exactly where they will develop). As a result, they typically are not severe, but instead "pulse" up quickly and may briefly produce strong winds and small hail - before quickly dissipating. However, they sometimes can produce a damaging wind gust as the storm collapses.
The key point to remember is that on a summer day in Alabama, a 20% chance of rain does NOT mean there is 20% chance that all of North Alabama will get rain, it means that about 20% of North Alabama will get rain. Hopefully this information will help you to better understand the weather forecasts you see each day during the summer-time here in Alabama.