Friday, 11:20 a.m.
The weather pattern is a pretty active one right now, and much of it can be tied to the undulating nature of the jet stream. Granted, there are always some troughs and ridges present around the globe, but at times those features are exaggerated. At this time of the year, the natural contraction of the circumpolar vortex can often leave behind 'puddles' of cold air aloft, and you can see several of them on the hemispheric 500mb chart this morning:
These closed lows are often left to drift along rather aimlessly for days at time, and woe to anyone who is stuck underneath one! One of these is quite evident off the coast of Nova Scotia, and another is near Spain. There's still another very strong feature near the North Sea, arguably another over western Asia, still another east of the Korean Peninsula and yet another over the Bering Straits, not to mention the features over the Arctic Ocean and Hudson Bay!
Not only do closed lows form, but also sometimes upper-level ridges become so amplified that an upper-level high closes off, such as we're seeing now near Greenland. Note also the very strong ridges over east-central Asia and off the Kamchatka Peninsula into the northwest Pacific.
When you see such strong, high amplitude features, quite often there are extremes in the weather. In conjunction with the ridges, very warm to record warm air masses, and, at this time of the year, you're probably also talking about relatively high humidity, too. Yet not far away with these strong upper-level troughs are some very cool air masses - just ask the folks in the mid-Atlantic and New England that are still waiting to see the sun, while ares just to their west have had enough to send temperatures into the 80s!
What does this have to do with the actual weather? Plenty. In the short term, like through the weekend, there will be plenty of warmth and humidity still in play from the southern Plains and Gulf Coast region up into the Ohio and Tennessee valleys. At the same time, a cold front is trying to come down from the Midwest and Great Lakes, and that's triggering some strong to severe thunderstorms in the vicinity of this front.
The forcing for this front and the associated strong thunderstorms will fade this weekend, only for a new upper-level trough to come over the Rockies and stir up the pot again over the northern Plains later tomorrow and tomorrow night as a new low pressure area begins to take shape. These thunderstorms will move into the Midwest later Sunday and into Sunday night and head for the Great Lakes and Ohio Valley Monday and Monday night.
Look at the upper-level pattern by Tuesday:
That's Tuesday evening, and by then, a front will be pushed over the Appalachians, though it will be slowing down at this point. However, when you combine it with the warmth still holding its ground ahead of the feature, it will likely trigger some sever weather yet again.
By the same token, the passage of this trough will be followed by a large high pressure area for Wednesday. That should lead to a drier air mass, and any place that clears out at night and with little wind should see a couple of very cool nights during the middle of next week.
Of course, with this cooler air mass coming in with and behind the trough running into a very warm to downright hot and humid air mass ahead of it, chances are good there will be some rain and strong thunderstorms to contend with yet again as the front progresses toward the Appalachians and the East Coast.
If there is any break in this active weather pattern, it might come after the front clears the East Coast, as it will erase most of the large contrasts across the country for a couple of days. Then again, it is early May, and it won't be long before the heat and humidity build somewhere some place yet again. And that will almost assuredly lead to return of 'active' weather. My hunch is it can come in two places. One would be back in the central and northern Plains and Midwest as some sort of front comes along in the flow. The other would be the southern plains over to the lower Mississippi Valley as the upper-level low comes rolling along from southern California.
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