Wednesday, 11:30 a.m.
Another day, and another set of records went down for the count on Tuesday. The core of the heat has maintained itself over the Midwest and the northern and central Plains. Look at the actual highs from yesterday:
The overall pattern from the past few days is relatively unchanged. Here's the evening 500mb forecast:
The lethal humidity that is accompanying the heat surged into the Great Lakes and across the Ohio Valley into Pennsylvania yesterday, with dew points surpassing the 70-degree mark over a wide area. For this late in the summer, it is very impressive to say the least! This surge, though, has helped to spawn thunderstorms over the Upper Midwest and northern and central Great Lakes over the past 24 hours that have since been moving southeastward. The relatively light flow aloft is allowing some of these showers and thunderstorms to move slowly enough to cause excessive rains in some areas. One of those has been the Lehigh Valley in Pennsylvania, and another has been from extreme northeastern Ohio and northwestern Pennsylvania into the panhandle of Maryland and even parts of northern Virginia.
This disturbance does has have a relatively feeble cold front associated with it. That shows up more clearly if you at a regional pressure analysis:
The front is south of Chicago now, and the north to northeast winds behind it has sent a deck of low clouds into the city to help take out the near-record heat of Tuesday. This front will progress southeastward this afternoon and tonight behind that weak low over Lake Erie, enough to pull somewhat drier (meaning lower dew point) air southward across the Great Lakes into the mid-Atlantic states by tomorrow. There may still be a couple of instability showers and thunderstorms scattered around New England tomorrow, but they will be exactly that - scattered, not concentrated.
As has been pointed out already this week, it will help to keep the brutal heat and humidity out of the northern mid-Atlantic and New England the rest of the week.
The ridge, though, is going to change in the coming days, and you can rest assure that as it does, the heat will move around as well. Look at the 12z Aug. 28 NAM 500mb forecast valid Friday evening:
The ridge is starting to retrograde back to the Rockies at this point. The upper-level low well off the Northwest coast this morning opens up and moves inland to southern British Columbia in order to make room for another big upper-level low upstream. This helps to flatten the ridge a bit and gives that ridge room to migrate in some way, or at least begin a metamorphosis. Downstream in the East, one disturbance is long gone well off the coast, while another is entering the upper Great Lakes.
The flatter flow will allow another attempt at the really intense heat and humidity to flow toward the East, but I think that lead disturbance will likely be enough to trigger scattered showers and thunderstorms, enough to keep the heat from really getting in, at least through Saturday.
As this happens, the core of the heat starts to shift southward. Look at the GFS 6-hour, 2-meter maximum temperature forecast for Saturday afternoon:
The front is just reaching the Dakotas, so it'll be the last day of heat there. Compare that to the same chart for Monday afternoon:
It's pretty easy to pick out where that front will be by then, as there's a surge of 90-degree heat through Virginia ahead of it, but on the other side of the Appalachians, it's considerably cooler! And look at the projected 100-degree heat over the lower Mississippi Valley and the southern Plains. At the same time, you might also note the pocket of 90s in Montana. Look at the 500mb forecast from the GFS for that time:
The ridge is strong again, but farther west, and that means the core of the heat will then be shifting more to the west at that time. If I were to project it out even more, there's evidence of a second front that will dart from the northern Plains to the East Coast later next week to bring another shot of cool air through the pattern.
I'm hoping that it takes any rain away from southern New England by next Friday. I'm bring a posse up to my home area for the weekend, and I'm hoping the weather is perfect to let the region shine for them. While the center piece of the trip is a ride out of the University of Massachusetts-Dartmouth campus, The Flattest Century in the East (TFCE for short, and the first ever 100-mile bike ride for someone I've been helping train this summer), the sights and smells of my home turf of the greater Mystic area will undoubtedly be a hit - and more so IF the weather cooperates!
One thing looks fairly certain - it won't be hot. The shifting nature of the heat will only bring it to the Northeast ever so briefly over the Labor Day weekend. Instead, the heat will be on the move to the south and west over the course of the next week to 10 days.
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