WEATHERAmerica Newsletter; EXTENDED PERIOD FORECAST; Saturday, August 16, 2025

[Larry Cosgrove]

EXTENDED PERIOD FORECAST

(Between Day 11 And Day 15)

 

Amplified Pattern Across Pacific Basin Has Stormy Implications For North America

CIMSS (3)

GRAPES/WMO Beijing

TropicalTidbits.Com (Dr. Levi Cowan) (3)

NOAA/PMEL

NOAA/CPC

HPRCC/University Of Nebraska (2)

Environment Canada

TropicalTidbits.Com (Dr. Levi Cowan) (5)

WeatherBELL (3)

NOAA/CPC

This particular weak La Nina episode has been characterized by two conflicting forces. One is the usual pulsation of the Sonoran and Bermudan heat ridges (this year mostly separate from each other). The second is some prominent storms or troughs in the polar westerlies, which occasionally have reached as far south as the Interstate 10 and 20 corridors. This is why in the first two weeks, the Mid-Atlantic, Southeast and Appalachia have averaged well below seasonal normals for temperature. You may have noticed that nationally this is one of the most humid summers in recorded history, with contributions from the Mexican monsoonal fetch, 500MB weaknesses, and tropical disturbances. I will admit that this moisture component was largely not forecast, and it played havoc with predictions in Texas as well as the Eastern Seaboard.

The issue that the nation is now facing is that the vigorous, early presentation of full-latitude westerlies across the Pacific Basin starts to show influence in the numerical model forecasts as early as the first week of September. A strong cold front and 500MB trough appears in the Midwest/Great Lakes, and supplies a cP regime in lower Appalachia and Atlantic Coastal Plain. Some of the predictive schemes see the very warm waters and elongated lower latitude heat ridging, and show chances for tropical cyclones to ride out of the African ITCZ into Florida and the Gulf of Mexico. Since weaknesses are likely to be present in the flow aloft, recurvature into higher population zones is a strong possibility.

Will the Sonoran heat ridge section be dominant like the European model family suggests? Perhaps, but numerical model outlooks have constantly overdone the position and strength of the positive 500MB height anomaly. I would not be surprised to see September turn into a cool West/Canada/North Central vs. hot elsewhere alignment. Which of course would allow for more warm-core cyclogenesis cases to reach the Greater Antilles, lower 48 states and Mexico. Many interesting turns in the forecast for North America are likely, and some may be extreme.

 

Prepared by Meteorologist LARRY COSGROVE on 
Saturday, August 16, 2025 at 11:40 P.M. CT

Disclaimer: 
The previous statements are my opinions only, and should not be construed as definitive fact. Links provided on this newsletter are not affiliated with WEATHERAmerica and the publisher is not responsible for content posted or associated with those sites.

Copyright 2025 by Larry Cosgrove

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