Monday, 11:55 a.m.
The changes in the weather at any given location will be comparatively slow in the coming days, but that doesn't mean they won't be rather dramatic! There's still plenty of warmth in place from the eastern Plains all the way to the East Coast this morning, and from parts of the Midwest and Great Lakes on south. Contrast that to the cold and snow in the eastern Rockies, and there's plenty of room for very active weather in the next few days. However, the changes won't necessarily take place very quickly from west to east, as would normally be the case.
Let's start by looking at the 12z May 12 NAM 500mb forecast for tomorrow morning:
What you might pick out are several things. For one, there's a pretty strong upper-level ridge in place from South Carolina to James Bay, Canada. A similar strong ridge is building just off the West Coast, which will foster a growing heat wave in the West. In between there's a broad-based upper-level trough, but you'll note it is in two pieces. The first of these is tied to a storm in the Midwest that will slowly lift out to the northeast and across the western Great Lakes into Ontario later tonight and tomorrow. While quite chilly behind this low, the vast majority of the precipitation tied to this wave will be in the form of rain.
Severe thunderstorms are also likely across parts of Missouri up into southern Wisconsin and southern Michigan this afternoon and tonight ahead of the cold front tied to this storm. The front will move across the Great Lakes tomorrow, but as it tries to get much farther east, it will run into increasing resistance, eventually coming to a grinding halt. Here's the 850mb temperature forecast for Wednesday morning:
This will provide the pathway for the second wave of low pressure to travel along. If you refer back to the first image, you can see the feature rolling through the southern Rockies toward the southern Plains. Then, in the image above, you can see where the new low will be by Wednesday morning, over Arkansas. As this second upper-level disturbance comes out of the southern Rockies tonight and tomorrow, it will generate strong to severe thunderstorms as well:
There will be some drying across the Ohio Valley tomorrow night for a time between these two features, but by Wednesday, the northeastward movement of the second one will cause rain and thunderstorms to spread out again from the Mississippi Valley into the Ohio and Tennessee valleys, with some of the rain stretching toward Chicago. So, instead of a front coming through and clearing out the moisture, it will instead linger and, in many cases, be renewed!
As we head into Thursday, a third disturbance will become involved, but from an entirely different source region - this time the tropics. No real development of any tropical wave appears likely at this point, which is good news, but a tropical wave is likely to bypass the northern Caribbean Islands and be drawn across the Bahamas toward the East Coast of Florida. That may well increase the rainfall potential for Florida and into the Southeast Wednesday night and Thursday into Thursday night.
That means the real cold front coming from the west may take until late Friday or even Friday night before finally clearing the East Coast! That says nothing of the back door cold front that will be back in from off the Atlantic and New England into New Jersey, Delaware and eastern Pennsylvania. The cooling will take place across New England tonight and back into New Jersey tomorrow, reaching the rest of the mid-Atlantic tomorrow night into Wednesday. The marine-chilled air will hold on for a couple of days before warming again on Thursday.
Once we get to the weekend, the weather should steadily improve in most areas from the Plains to the East Coast. By then much of the amplification will be gone from the flow. Look at the Sunday evening GFS 500mb forecast:
Only then will the weather change at a more normal pace from west to east across the county.
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