WEATHERAmerica Newsletter, Medium Range Discussion, Saturday, September 30, 2023

[Larry Cosgrove]

MEDIUM RANGE OUTLOOK

(Four To Ten Days From Now)

 

Cooler, Refreshing, But Not Really Cold; Beware Of Thunderstorms In The Change!

ECMWF (4)

PivotalWeather.Com (4)

College Of DuPage Weather Laboratory

With a modest +PNA configuration (shortwave ridge western North America) and no blocking signatures, the polar air pass that advects into the U.S. will be a cool type, and not present any major dangers for frost outside of the border with Canada in the north central states. The introduction of a 500MB shortwave from the "Four Corner" on Tuesday and Wednesday may prove problematic, however. With very unstable values in the southern Great Plains, Texas and the lower Mississippi Valley, there is a very real danger of a severe weather outbreak that starts west of the Interstate 35 corridor and proceeds into sections of Dixie. The push of energy across the Midwest and Northeast early in this period may also set up a convective threat. Note that while the locations to the right of the Continental Divide tend to cool off, hotter air is likely to rebuild in the Desert Southwest in the medium range.

Cool North, Warm South Alignment; Unstable Air, Hurricane Remnants May Target Mexico And Texas Late

UQAM Meteocentre (4)

TwisterData.Com (4)

TrueWx.Com (4)

There is no Arctic air in your immediate future. The ridge complex in western Canada will weaken substantially in the second week of October. Canada and the northern fringe of the USA should remain cool, while much of the lower 48 states outside of the Pacific Northwest will warm up. In fact, I would not be surprised to see an area from Texas to Nebraska have some cases of maximum temperatures exceeding 90 deg F. But there is a problem lurking in the mix, which is the expected development of a tropical cyclone along or offshore of the Mexican Riviera in the 6-10 day time frame. A scenario might emerge where a southern branch shortwave picks up the disturbance and its huge moisture axis (note the lifted index field at 240 hours), and turns the low pressure center inland through Mexico and into Texas. There is still a lot of time to watch of this feature materializes, but there is decent support from model guidnace and analogs.