As part of our examination of the hazards of summer storms in the Tampa Bay region this week, we sought the wisdom of an expert behind one of the best sources of weather-related intel out there, The Weather Underground's Bob Henson.
For commuters like myself, and for anyone interested in the latest big blow (like Tropical Storm Bill, above), Weather Underground (wunderground.com) is a wealth of info. WU meterologist/blogger Bob Henson answered some of my burning questions.
What are the factors that result in (almost) literally daily afternoon thunderstorms here in Florida in summer?
The starting ingredients are plenty of low-level moisture from the Gulf of Mexico and the Atlantic, combined with the heat of summer. These factors combine to make the air very unstable. Then you stir in the regular sea breezes that move in from the coastlines every afternoon, pushing air upward. Thunderstorms often form along the edge of the Gulf or Atlantic sea breeze. And once in a while there’s a “sea breeze collision,” when the Gulf and Atlantic sea breezes run into each other and their two lines of storms merge and intensify. There are also upper-level centers of low pressure that can bring more widespread storminess in the summer.
We get some pretty cool-looking (but menacing) clouds. If I’m about to get on the road, what, if any, kinds of clouds should make me want to turn around?
As you might expect, darker clouds are a good indication of heavy rain, since the density of the precipitation relates to how much sunlight is being blocked out. A line of fast-moving low clouds is often produced by strong winds near the surface, blowing outward from a thunderstorm. Sometimes a shredded-looking, low-hanging cloud called “scud” is confused with a tornado. But a more reliable sign of a potential twister is when you see a large area of clouds in or near a thunderstorm that are rotating rapidly around a central point. If this happens, be ready to take shelter right away!
Using your app, what color storms are the ones to avoid?
The precipitation intensity on the WU app starts out at green (the lightest) and works its way into the warmer colors, from yellow and orange onward to red and purple (the heaviest). A very thin line of red may bring intense rain and strong winds for a short period, whereas a large area of red is more likely to cause prolonged heavy rain and potential flooding.
Sometimes what looks like a little squall will apparently get sucked into a bigger storm. Why is that?
Larger thunderstorm complexes have their own centers of low pressure. They can actually rotate and pull air into themselves, a bit like a hurricane. Sometimes this evolution means a smaller cell will get pulled into the orbit of the main complex.
This article appears in Jun 18-24, 2015.