Wednesday, 11:40 a.m.
I've been on a kick of late to listen to a lot of older music, and one of the artists or bands that I have been drawn to is Count Basie. One of his most famous recordings is 'April in Paris'. I've not studied how that song came to be, or if on some trip there he was inspired to write the song. Nor have I been in Paris - in any month, let alone April! But I can imagine a scene not unlike one in much of the Ohio and western Tennessee Valleys today, complete with sunshine, a cool morning, and a pleasant afternoon. One that you can hear the birds singing, smell the flowers, hear the buzzing of the bees, and see the full array of color across the sky and landscape. That's my interpretation, anyway.
That's the nice part of April weather. It can get even better, feeling more like summer, as it did in the mid-Atlantic and southern New England on Monday. It can also be colder, as will be the case this weekend across the Ohio and Tennessee Valleys. We have seen it snow deep into April, even May, and while that's been a rare thing over the past couple of months, we saw snow in parts of the northern Plains just this past weekend. Strong thunderstorms have already caused widespread devastation in the Plains, and over the past 24 hours flooding rains have visited Mississippi.
In other words, there can be a lot of ups and downs in the month of April.
To open the week, we saw the 'up' part of this equation, in terms of a strong upper level ridge of high pressure along the Eastern Seaboard. In keeping with the saying 'what goes up must come down' (some sort of gravity thing, you know), that ridge is already much weaker, and by Sunday night, it will have been completely reversed by a deep, full-latitude trough. Here's the GFS forecast for Sunday evening:
While the cooling will be slow to get east of the mountains this weekend, it will eventually get there. At the same time, it will waste no time in moving across the Mississippi Valley into the Ohio and Tennessee valleys, taking aim at the Southeast.
Another basic property of physics is that for every action, there is an equal an opposite reaction. You remember how you were taught that when you push on a desk, there's an opposing force against your hand. In the case of the atmosphere, as that trough deepens in the East, there's a natural response in the atmosphere, and that response is to amplify the ridge - both the one offshore ahead of it, and the one behind it popping up over the Rockies.
This warming gets under way the rest of the week over the Southwest - parts of California and Nevada into Arizona, areas that have seen little warmth so far this spring. By the weekend, the warmth there will be easing, but it will expand into the Rockies. Look at the expected temperature departures from normal on Monday:
Of course, as it is warming there, it will have turned much cooler downstream. The deep trough will not only bring in the chillier air for a two to four-day stretch, but it should also bring some much needed rain to parched areas of the eastern third of the country. That's where it gets tricky, though, as the models are all over the place on who gets what, and over what time frame! There have been some model forecasts of rain changing to snow in some places! While that option cannot be ruled out, especially for the high ground of the Appalachians, it's not necessarily a very likely scenario.
Right now the best guess would be for generous one to three inch rains from portions of Alabama and Georgia up along and east of the Appalachians into New England. Some places that get convective could have more, and that could mean flash flooding. By and large, though, this should be the kind of rains that have been missing for some time, and should go a long way toward reducing the forest and brush fire danger next week.
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