Today was a “three-peat” of stormy weather for much of the WAFB viewing area. However, you likely noticed that today's storms were not as intense as those on Tuesday and Wednesday.
So what was the difference? The reduced severity of today's storms was largely a result of the difference in the storm trajectories. Today's action arrived from the southwest and south; the storms on Tuesday and Wednesday came in from the east and northeast.
Storms that move into our area from the north and northeast (and east, at times) have the potential to become stronger and more “electrified” than storms coming in from the south. We emphasize “have the potential” since this does not always prove to be the case.
What's the explanation? There are a string of factors at play here, but a key one has to do with the steering pattern for the storms.
During a “normal” South Louisiana summer day, our afternoon showers and storms drift inland off the Gulf, fueled by the heating of moist Gulf air over the land. The inland movement is referred to as the “sea-breeze effect.” The sea-breeze is set-up by faster heating of the air over land compared to air over the Gulf waters. As the air over the land rises from the faster heating, air off the Gulf flows inland to fill the void. The effect is to create something that often looks like a broken line of showers and storms advancing inland through the afternoon.
But during the previous two days, our storms came into the area from the east and northeast, steered around the eastern flank of the upper-level high pressure ridge. This ridge-driven flow pattern can also bring slightly-cooler air into our region at the upper levels. With the temperatures aloft dropping as a result of the “import” of high-level cooler air from the north and northeast, we set up a larger-than-normal vertical temperature gradient (in other words, we end up with a larger temperature difference between the ground and the air a couple of miles above the surface).
In both the sea-breeze and ridge-steered situations, temperatures near the ground are about the same, typically climbing into the 90°s before the clouds and rains arrive.
But with the flow out of the east and northeast around the upper-air ridge, the temperatures aloft gets cooler-than-normal. That cooler air aloft enhances the potential for rapidly-intensifying t-storms, frequent lightning and hail because (1) the greater vertical temperature gradient makes the warm surface air rise even faster than normal and (2) the cooler air above creates a lower freezing level in the upper atmosphere. A lower freezing level means more opportunity for moisture to freeze at height and that increases the opportunity for hail and lightning. (A leading theory for lightning formation is that it is generated by static electrical charges produced as ice and water pass and collide in the cumulus cloud. More ice means more lightning.)
On Tuesday, we saw some of the bigger t-storms topping out at 60,000 feet or more. Wednesday's storms weren't quite that “tall” yet still climbed to 45,000 to 50,000 feet at their peak. Today's storms, by comparison, were peaking around 30,000 feet or so. Lower tops: less energy … taller storms: greater intensity.
The ridge is being pushed out of the way as a trough builds over the eastern states. That means little or no chance for storms out of the east and northeast any time soon.
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