Joe Lundberg

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Easy Transition Into Fall

September 03, 2013; 9:58 AM

Tuesday, 11:30 A.M.

Now that the Labor Day weekend is officially in our rear view mirror, we can effectively declare summer over. Oh, there will be no doubt be some heat and humidity to deal with in the coming weeks, but with the sun angle getting steadily lower each and every day, the swing toward much cooler weather is inevitable. The astronomical end of summer is 19 days away with the autumnal equinox on Sunday, September 22 at 4:44 P.M. EDT.

The cold front moving to the New England and mid-Atlantic Coasts today is helping aid in this transition to a cooler and less humid regime. The dew points are coming down in the wake of the front, and that alone will be a big change considering how tropical it was over the long holiday weekend. In reality, though, the largest departure are being left behind in the Midwest yesterday into this morning. The air mass is already modifying, and the departures tomorrow a full day behind the front in New England and the mid-Atlantic won't be as large as we will have seen in the Midwest. Here's the 0z September 3 GFS ensemble temperature departure forecast for tomorrow:

A more significant cool down will follow a second that will dart across the Great Lakes into New England tomorrow. Here's the 12z September 3 NAM surface forecast for tomorrow afternoon:

The cooler air will manifest itself across the Midwest tomorrow night into Thursday morning, but across the entire Northeast Thursday night into Friday. Look at the expected departures on Friday:

Frost is a real possibility across upstate New York and northern New England before the air mass quickly moderates this weekend.

And that's the beauty of these early fall air masses! They have the clearness and crispness of autumn, but yet they will moderate easily to provide some late season warmth, typically without the tropical humidity and mid summer heat of June, July, and August. The air masses tend to be pretty dry, and when fronts move through, they often have limited moisture to work with, thus little in the way of severe weather with most, and sometimes hardly any precipitation.

The easy transition to fall is not widespread. Heat is far from done in the West thanks to the presence of an upper level ridge over the Rockies. This upper level ridge really isn't going anywhere, either. Look at the latest NAM model 500mb forecast for Friday evening:

This will drive temperatures to 15 to as much as 20 degrees above normal in the northern Rockies and northern Plains late this week into the start of the weekend before the upper level low currently off the Northwest Coast rolls inland Thursday night and Friday, then over the top of the ridge Friday night and Saturday. This will trim the heat away later in the weekend and early next week, but it won't be replaced by a chilly air mass, just something more akin to normal for the second week of September.

I would be remiss if I failed to mention the concern in the tropics. There is a wave east of the Leeward Islands right now that is not terribly well organized. The Canadian model has insisted for a week now this feature develops into a tropical storm, then hurricane, if not a major hurricane, as it heads northwestward, then turns northward to approach the East coast, but generally steer clear of it. While the chance of that happening are not very high right now, it's not non-zero, either. The NAM model now latching on to this idea, at least through Friday evening, and the GFS is at least hinting at it. I would not at all rule out such a possibility, and it might be one thing that would make for a harder transition to autumn in the next two weeks.

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.