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September in July: Big cold front to bring stellar weather next week

National Weather Service 6-10 day temperature outlook

Following in the footsteps of the record-setting cool air mass in mid-July, a second blast of air with polar origins will invade the eastern U.S. to close July. Like the original, the sequel will be glorious.

This taste of September may not be quite as strong mid-July’s version, but should produce temperatures  10-20 degrees below normal across a large swath of the eastern U.S. by early Tuesday.

Temperature difference from average Tuesday morning in GFS model (

The jet stream will take a big southeastward dive on Sunday, with a cold front along its leading edge.  Thunderstorms  – some severe – are likely to erupt out ahead of it in the Ohio Valley and East Coast Sunday into Monday.

Animation of upper level weather pattern simulated by GFS model Sunday to Friday next week. Blue and green shades roughly approximate high altitude cold pools (or troughs) while orange shades approximate warm pools (or ridges) (

Once the storms are through, the gorgeous weather stretch commences and below average temperatures take hold for much of the work week. In Washington, D.C., this means highs mostly in the 80-85 degree range.

GFS model ensemble forecast highs and lows Monday to Friday next week (
GFS model ensemble forecast highs and lows Monday to Friday next week for D.C. (

Tuesday may be D.C.’s coolest day, in which highs could struggle to reach 80.

European model forecast highs Tuesday (

Wednesday morning will likely be the coolest in D.C., in the 50s in the suburbs to near 60 downtown. Yes – that’s 40s you see in the mountains.

Wednesday morning low temperature forecast from European model (

Humidity levels will also drop to low levels – so this deep summertime trough, on balance, will deliver quite delightful weather.

Like the mid-July cool episode, the origins of the air mass can be traced inside the Arctic circle and within the summer (mid-tropospheric circum)polar vortex.  However, it’s a less well-defined case so I’ll refrain from calling it a polar vortex “event.”

On Monday, when I first discussed this upcoming weather pattern, I mentioned models showing it taking on a Rex Block configuration.  Models have since gently backed off  from this idea, but haven’t backed off on the idea of an extended period of well-timed cooler than normal weather.

Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10.
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