Joe Lundberg

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It's Not the Heat, It's the Humidity

July 17, 2013; 9:49 AM ET

Wednesday, 11:30 a.m.

After a long weekend, I'm back at it, sweating it out like everyone else in the mid-Atlantic. And New England. The Great Lakes. The Ohio Valley. The Midwest. The Plains. It's hot all over, and it's not just the heat, it really is the humidity! This is not the same kind of heat that would tempt one to fry an egg on the sidewalk, something more indigenous to the western half the country in the spring and early summer. No, this is a tropical heat wave, one with dew point temperatures pretty much at or above 70 across the board. Here's a sampling of 10 a.m. dew points:

Pretty much every one from New England (save parts of Maine) down through the mid-Atlantic states out to the eastern Plains is suffering from this brutal humidity.

Aloft, a monster upper-level ridge is pretty much at its high point right now, centered over the Ohio Valley, but stretching from Kansas and Nebraska to the mid-Atlantic coast:

Virtually everyone away from any type of maritime influence will reach to 90 and beyond this afternoon, as well as tomorrow, and probably Friday as well. It's not so much a record-setting air mass, as it is just plain hot and steamy. Naturally, in an environment so tropical as this, you might expect to see some thunderstorms bubbling up during the heat of the afternoon. Indeed, there were some yesterday, especially over the mountains or any place where there was some sort of sea or lake breeze boundary. But in the grand scheme of things, the thunderstorm activity is fairly spotty.

The one exception to that would be across the extreme Upper Midwest in the next 36 hours. One frontal boundary came through parts of North Dakota into northern Minnesota in the past 24 hours, but it has pretty much reached its southernmost point. As it retreats slowly to the north again tonight into tomorrow, there will be some rather strong thunderstorms that will erupt. These thunderstorms will then skirt the northern Great Lakes, with anything farther south being pretty light in intensity and widely scattered in nature.

If we look at the weather map for Friday afternoon, that little feature will morph into an impressive low pressure area that will roll across Ontario to James Bay and toward western Quebec:

This feature will strengthen the low-level flow from the central Plains into the Great Lakes and Ohio Valley, and one place that will show up very nicely is in the 850mb temperature forecasts for Friday afternoon:

Note that ribbon of air at +20C being drawn across the Midwest into the Great Lakes Friday afternoon. If there's any appreciable sunshine, and there is no shower and thunderstorm activity in a given region during the morning hours, temperatures will soar into the 90s very easily and could well get within striking distance of some records.

The squeezing of heat into the mid-Atlantic and southern New England may not be quite as warm aloft Saturday afternoon, but downsloping effects may yield similar results in terms of actual high temperatures - again, assuming no appreciable shower or thunderstorm activity during the morning hours ahead of the front. And there will be no appreciable reduction in those dew points ahead of the cold front, meaning one last truly saunalike day in these areas.

Now, if you are NOT a fan of this kind of weather (which, by the way, is no exception for the latter half of July - we're at the typical 'peak' of summer heat right now), then I have good news for you! The front seen in the image above will pass through all of southern and eastern New England Saturday night, and slowly but surely clear the mid-Atlantic Sunday before stalling. That will at least knock temperatures down a good 10 degrees or so, and, in most places, will take out the brutal humidity. That may be a harder sell along the mid-Atlantic coast, particularly from the Delmarva Peninsula on south, where the front is likely to get hung up. But for areas upwind of the Appalachians, it will be a literal breath of fresh air come Sunday!

There are some places in Pennsylvania that have not seen temperatures below 60 degrees since the very day of the summer solstice! Think about that - four weeks since it has been as low as normal at night. As you would imagine, temperatures are clearly running above normal so far this summer season in the mid-Atlantic, though it's been more about the humidity keeping temperatures up at night, rather than excessive heat by day. We'll get the heat, too, for the rest of the week into the start of the weekend before both are trimmed going forward.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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Joe Lundberg
Joe Lundberg, a veteran AccuWeather.com forecaster and meteorologist, covers both short and long-term U.S. weather on this blog.