It’s been a hot and humid week across Kentucky and Southern Indiana with hit and miss storms firing up during the heating of the afternoon and early evening hours. But do you ever wonder why it’s raining cats and dogs at your house, yet a few blocks down the street from you it is sunny and dry? Welcome to the “pulse” thunderstorm!
These typically develop during the summer months here in Kentuckiana but unlike clusters or lines of thunderstorms that move across and impact a larger area, these are typically isolated in nature and their impacts are very localized.
You need the right ingredients for these storms and this week was the perfect set-up. Plenty of heat, tropical moisture and the key factor, the lack of any upper level wind.
A frontal system or boundary normally “triggers” storm development and the upper level flow drive storms through the area quickly but the last few days the air mass over Kentuckiana has been totally stagnant with no trigger or wind so speak of. So how do these storms get going?
As the surface heats up and temperatures climb, the warm moist air rises, and storm fuel helps the thunderstorm to intensify. Usually you can see the direct vertical development of the storm from a distance since the storms are isolated and there is no wind aloft to blow the clouds at the top of the storm downstream.
The term “pulse” is used because these storms gain strength very quickly, can produce torrential rain that causes localized flooding as the storm sits over the same area with no wind to move it along. As a result, a small area may pick up 1” of rain or more while nearby spots remain totally dry. Gusty and even damaging winds are possible as the thunderstorm collapses and the winds within the storm rush out and make it to the surface. The life cycle of the typical pulse thunderstorm is generally short…anywhere from 15 minutes to around 1 hour before the storm rains itself out.
As the storms collapse and the “cooler” air rushes out and them and bumps up against the surrounding warm moist air, the cooler air acts as a wedge or boundary that many times helps additional thunderstorms to develop. Once the sun sets and there is a loss of the daytime heat, the storms diminish but the process is repeated the following day until this pattern breaks. A change in our weather pattern should happen this weekend as a few weak boundaries drop through and lower our temperatures and humidity levels briefly.
Meteorologist T.G. Shuck