Joe Lundberg

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A September Look in the Middle of Summer

July 30, 2013; 9:45 AM ET

Tuesday, 11:20 a.m.

I am quite fascinated by the weather pattern entering the month of August. If you were to physically remove some of the upper-level lows on the map, there really would be a lot of heat across North America. Take a look at the 12z July 30 NAM 500mb forecast for Thursday afternoon:

A couple of things stand out right away. One is the upper-level ridge over New Mexico and West Texas. That will promote one of the few areas of relatively hot and uneventful weather across the country. You should also pick out the upper-level low along the Northwest coast, a feature that might actually bring measurable rain to western sections of Oregon and especially Washington to open up the month of August. Granted, it won't be much, and a lot of places will still miss out on the rain. Considering the fact the region has had NO measurable rain since late June, it will be noteworthy.

It should also be noted that there is really a split in the jet stream across North America. The one coming in underneath that upper-level low along the Northwest coast goes up and around the upper-level ridge over the eastern Rockies then heads southeastward across the northern Plains and Mississippi Valley into the East. Near that jet stream, the weather is active, and any time a defined feature rolls through the flow, there will be an enhancement in shower and thunderstorm activity.

If you look farther north, though, you can also follow the 558dm line. By and large, it passes north of Alaska and north of an upper-level low over Hudson Bay. For all practical purposes, that's where the northern branch of the jet stream is. Look at what that looks like when you compare the heights to normal Sunday evening:

Effectively you have an large pool of cool air that is surrounded by warmth. Most of that warmth may be locked up in the high latitudes of Canada and Alaska, but it is there. However, much as we saw at times during the spring season, the low heights and closed lows underneath will dictate the temperature patterns into the first week or two of August.

With this last air mass coming through the northern Plains into the Mississippi Valley, the Midwest and the Ohio Valley, a slew of record lows were established. The air mass coming across the Midwest into the Ohio Valley later this week may not be cool enough to promote record lows, but it will certainly be cooler than normal. Still, a few records may fall this weekend across the Midwest into the Great Lakes and Ohio Valley.

And that's with the first intrusion of cooler air late this week. If you look at that image above, another upper-level low forming over Hudson Bay this weekend rolls southward early next week. Compare the GFS temperature anomaly forecast for Sunday to that of next Tuesday:

As the first cool air mass fades next week in the East, here comes another one poised to drill southward across the Plains and Midwest. Don't be surprised if record lows are set with that air mass, and it wouldn't completely shock me if someone in the northern Plains or Upper Midwest gets a touch of frost! Yes, it would take the perfect timing of high pressure over a given area with light winds at night to do it, but that's not out of the question, and is certainly worth watching.

It may still be late July, but the pattern over the next week to 10 days will have a September look and feel to it.

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

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About This Blog

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