WEATHERAmerica Newsletter, Sunday, August 31, 2025; MEDIUM RANGE OUTLOOK

[Larry Cosgrove]

MEDIUM RANGE OUTLOOK

(Four To Ten Days From Now)

 

Are You Kidding Me? A James Bay Vortex? Chilly Nights In The Midwest/Northeast!

ECMWF (4)

PivotalWeather.Com (4)

No, what you see is real, a numerical model forecast of a deep upper low forming and digging through James Bay. The cPk cold pool (not Arctic, but the 534dcm core would qualify as a vortex as opposed to an ordinary low) will succeed in bringing record overnight lows across eastern Canada, the Midwest/Great Lakes and the Northeast for much of the medium range. Curiously, the small strip of heat across Texas and the Deep South will be broken up by the progression of a monsoon-related weakness from the "Four Corners" into the Interstate 35 corridor below Kansas City MO. A slow advance of the tropical air mass over the Gulf of Mexico should create opportunities for diurnal and sea breeze convection. All of this means that any harsh summer heat will be hard to come by for the next ten days or so.

The Inevitable Warm-Up (With Thunderstorm And Tropical Threats) Afterwards

UQAM (4)

TrueWx.Com (4)

TwisterData.Com (4)

College Of DuPage Weather Laboratory

If you like the thought of turning off the air conditioner, you will be pleased to know that excessive warmth and humidity will largely go missing. But this early in the season, the kind of cool intrusion that is showing up will not last forever. Around September 10 onward, the 500MB weakness over the south central states will likely break up, and the Sonoran and Bermudan heat ridge complexes will return. The disappearance of the extensive ridge along the West Coast looks to cut off the supply of cP values in the lower 48 states. Tropical systems in the Atlantic Basin should be a factor in this time frame, as the Saharan air layer becomes smaller and mostly situated east of the Azores Range. So even though we are progressing toward the end of calendar summer, the chance to see some cooling demand will return.