Friday, 11:55 A.M.
The Northwest has suffered through some intense heat for the first half of the month. That heat is now being trimmed as an upper trough races into south-central Canada this afternoon, and another one follows later tomorrow. The end result will be temperatures going below normal for the first time all month. That heat has to go somewhere, and it will resurface over the eastern Rockies and western Plains this weekend.
In recent days, the heights were much above average over the Northwest and up into British Columbia, even up into the Northwest Territories. That is now gone. Here's the Friday evening 500 mb forecast from the 12z July 18 NAM model:
You'll note the broad west-northwest flow over the Rockies out onto the Plains. By the time we exit the weekend, the heights will have risen dramatically over the central and eastern Rockies into the central and southern Plains:
Temperatures are already on the upswing today, and will easily jump into the 90s in many places tomorrow. Sunday will be hot from eastern Montana and North Dakota southward to eastern New Mexico and Texas, with most places into the 90s. Some will even exceed 100 in parts of Kansas and Nebraska. The northern end of this heat may be clipped a bit Monday in Montana and North Dakota as an upper-level disturbance crosses the region. But to the south in the central Plains, it will stay up there in the 90s to 100 or so.
That upper-level ridge isn't likely to move too much next week. If anything, it will reorient itself a little farther west by the end of next week, bringing some of the heat back to the interior West.
While all of that is taking place, the eastern half of the country will see the warming take place at a much slower pace. The surface high over Ohio and Pennsylvania this morning will slide eastward, while at the same time the upper-level trough that is helping to bring flooding rains to parts of southeast Texas, Louisiana and Mississippi right now will bring moisture up into the Ohio Valley and the Appalachians. The clouds and precipitation will slow the warming through the weekend. Farther east, the onshore flow will mean ocean moisture being added into the mix, and not really allowing for much mixing.
This broad eastern trough weakens even more early next week, but the heights still won't be all that high. Thus the heat moving across the northern Plains into the Upper Midwest will slide over the northern Lakes toward the Northeast. By Tuesday and Wednesday, some places will be nearing the 90-degree mark, if not inching past it - hot, but not excessively so. In the end, there will be about two or three days with temperatures getting above normal in the Northeast, namely Tuesday and Wednesday, perhaps even Thursday in the mid-Atlantic, before the first of two cold fronts trims temperatures back by next weekend.
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