Any storms today?

April 26, 3:30 PMDC Weather ExaminerScott Sumner

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After "Mother Nature" lit up the sky with a few different types of lightning Saturday night, can we expect another round this evening? With the expectation of another day of hot weather and little change in the surface weather pattern, some isolated and scattered thunderstorms could form. The thunderstorms that developed further north and in greater coverage last night ,were a result of two factors:

1.) Pressure trough ( can also be thought of as a line of convergence )
2.) Bay Breeze

Pressure troughs are near and created by cold fronts and sometimes warm fronts ( on a surface map the pressure trough is seen as a black dashed line ), but because the average cold front has a stronger temperature gradient than the average warm front, a pressure trough is more defined and usually stronger around a cold front. Since fronts force the air to rise , the rising air counteracts the downward force of gravity, lowering the atmospheric pressure. It's just as if someone started pushing up on you when standing on a scale; your weight would decrease. The atmosphere works in the same way and this causes the surface pressure to decrease in the vicinity of the front.

The other element that helped in creating the thunderstorms was the bay breeze which is created when the difference between the temperature over land and over water (check out diagram below) are usually large enough that it develops a boundary (i.e. cold front, trough). The resulting temperature gradient can drop temperatures up to 20F degrees. Yesterday was a perfect example of that,as the temperature at Annapolis was in the upper 60s, while about 10 miles away, Bowie was in the upper 80s!

The graphic above shows hot air ( red arrows) rises and then moves over the cooler water. To replace the hot air, cooler air off the ocean ( purple arrow) moves over land. A continual cycle develops creating the cold front.

Both factors played a role in thunderstorm development ,along with a few other factors, but the same weather story will be felt today, so certainly keep your eye to the sky just in case a strong enough thunderstorm forms. Check out the latest local radar from the National Weather Service:
http://radar.weather.gov/radar.php?rid=lwx&product=N0R&overlay=11101111&loop=no

 Photo Credit: Rafael Ketelhohn