Winter pattern repeating in summer: Fall-like weather may loom next week for eastern U.S.

Forecast model predictions a week into the future are advertising a second major shot of cooler than normal temperatures in the eastern U.S. this July.

Upper level weather pattern simulated by GFS model shows big dip in the jet stream over eastern U.S. and much cooler than normal air at high altitudes (WeatherBell.com)

Upper level weather pattern simulated by GFS model shows big dip in the jet stream over eastern U.S. and much cooler than normal air at high altitudes (WeatherBell.com)

The American GFS, European and Canadian models all call for a big dip in the jet stream in the eastern Northern Hemisphere that will allow temperatures 10 to 20 degrees below average to feed from the Northern Plains to the East Coast early-to-mid next week.

The early autumn-like air should enter the Northern Plains next Sunday and reach the East Coast some time Tuesday – timed almost exactly two weeks after the arrival of the last cool weather episode.

Temperature difference from normal next Tuesday morning from GFS model (WeatherBell.com)

Temperature difference from normal next Tuesday morning from GFS model (WeatherBell.com)

This infusion of cooler than normal air is unlikely to be as strong as last week’s which set records from northern Minnesota to the Gulf and East Coasts.

But it’s a case of a pattern we’ve witnessed on a number of occasions since winter – sometimes described as a “Rex Block” (named after scientist Daniel F. Rex, who discovered the pattern) in which high pressure and low pressure join forces to promote airflow from the Arctic down towards the eastern U.S. In this case a high pressure center or ridge will set up near the Hudson Bay while low pressure establishes itself near the Great Lakes.

European model forecast of upper level pattern next Tuesday night, July 30 (WeatherBell.com)

European model forecast of upper level pattern next Tuesday night, July 30 (WeatherBell.com)

“Strong ridging [high pressure] north of the low causes the airflow to move from high latitudes to low latitudes with little comparable west-east movement of the air,” describes the weather-explainer Web site Haby’s Hints. “The airflow pattern follows a backward “S” trajectory.”

Explainer link: What is a Rex Block?

The backward “S” Haby’s Hints refers to is unmistakable in the the European model projection above (and in the Tweet below, referring to the Rex block) portraying airflow from north of the Hudson Bay smack down into the eastern U.S.

While most models and their ensemble simulations suggest such a cooler than normal pattern next week, I should point out the specifics of its timing and intensity are subject to change.

How unusual would it be to see this kind of “winter-like” pattern repeating in summer? Such a pattern has been rare in recent years, but more common a decade ago and further back in time.  The National Weather Service’s Weather Prediction discussed the frequency of these big East Coast troughs or jet stream dips in July-August in its long-range discussion today:

THE PRONOUNCED WRN RIDGE/ERN TROUGH PATTERN HAS MADE FEW APPEARANCES DURING THE JULY/AUG TIME FRAME IN RECENT YEARS BEFORE 2014. HOWEVER COMPOSITE ANALOGS BASED ON SOME OF THE D+8 MULTI-DAY MEANS VALID AT THE START OF NEXT WEEK REVEAL MULTIPLE INSTANCES OF A SIMILAR PATTERN IN JULY AND/OR AUG OF EARLIER YEARS. IN REVERSE CHRONOLOGICAL ORDER THESE INCLUDE 2003..2000..1996..1994..1993..1990 AND 1989. AFTER A LENGTHY GAP THE YEARS 1969..1967..1962 AND 1956 ALSO FEATURED THIS TYPE OF PATTERN AT SOME POINT IN JULY AND/OR AUG.

National Weather Service Climate Prediction Center forecasts below average temperatures 6-10 days from now over the northeastern third of the U.S. (NWS)

National Weather Service Climate Prediction Center forecasts below average temperatures 6-10 days from now over the northeastern third of the U.S. (NWS)

Those of you appreciating these bouts of early fall-like conditions in mid-summer will also appreciate this tweet from WSIEnergy,  which notes model forecasts for temperatures in August have trended cooler for the East over the last week:

Big time heat just can’t seem to get its footing this summer. I know many of you are so disappointed…

Related: Capital Weather Gang summer outlook | Extreme Great Lakes ice extent could portend cool summer for eastern U.S. | Poor man’s polar vortex to make shocking summer return in eastern U.S. next week (from July 10) | Polar vortex deniers are wrong: Here’s why…

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