Thursday 8 a.m.
Today's video includes a weekend forecast.
The greater the horizontal temperature difference, the faster the winds increase with height. In the summer, when temperatures are rather uniform, the main upper-air currents tend to be weak. However, in autumn, the Arctic turns cold while it can remain warm farther south (as we are seeing in the Midwest and East right now). Since the horizontal temperature difference is now greater than it was in midsummer, the winds increase with height more rapidly. If you follow along with this idea, you can see why fronts and their attendant low pressure areas are located near the jet stream. Fronts are boundaries between cold and warm air masses, so that's where we expect to find the greatest horizontal temperature differences... and hence the strongest winds aloft. The belt of strongest wind aloft is called the jet stream.
So what? If we have warm, humid air in place down here where we live, then a cold front approaches along with high winds aloft, we have a setup for strong or severe thunderstorms. Those preconditions are similar to what makes springtime so active in terms of severe weather threats. As a cold front reaches the Chicago area late Saturday or early Sunday, then the Pittsburgh area late Sunday, then the I-95 corridor later Monday, there may be a band of strong thunderstorms. Cooler, drier air will follow the cold front, but computer models suggest it will warm up again just a few days later.
One wild card in this situation is a tropical system causing heavy rain in the southern Gulf of Mexico. In the video, you could see where the US GFS model takes this system (it had not been named Karen as of early this morning). The main issue for the Middle and North Atlantic states is whether and when any moisture from that storm is injected into the cold frontal zone. The answers will help determine how much rain to expect with the front.
Yesterday and last night, thunderstorms were most concentrated near the Plains cold front that should make it to the East Coast early next week. This map shows the lightning strikes between 7 a.m. EDT Wednesday (yesterday) and the same time today. More than 50,000 strikes are represented here.
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