Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
349 AM EDT Wed Sep 20 2023

Valid 12Z Wed Sep 20 2023 - 12Z Fri Sep 22 2023

...Much cooler, Fall-like temperatures and locally heavy rainfall for the
West...

...Above average heat forecast from the Plains to the Great Lakes through
Thursday...

...Chances for heavy rain and severe thunderstorms continue for the
Southern Plains...

A deep upper-level low dropping southward over the West from Canada will
provide quite the cool down over the next couple of days. The low will
help to drive a quasi-stationary front lingering over the Intermountain
West further southward, bringing much below average, Fall-like
temperatures. Highs in the upper 50s and low 60s in the Pacific Northwest
and Northern Rockies on Wednesday will spread into the Great Basin by
Thursday. Highs along the Pacific Coast will be much below average too,
with 60s into Southern California. Conditions will remain a bit more
seasonable in the Desert Southwest, with mostly 90s expected. In addition
to the cooler temperatures, enhanced lift from the approaching upper-low
as well as moisture pooling along and behind the frontal boundary will
lead to increasing shower and thunderstorm chances across the
Intermountain West Wednesday into Thursday. Some more widespread, locally
heavy rainfall and increasingly wet antecedent conditions may lead to a
higher risk for an isolated chance of flash flooding for portions of the
Great Basin into the Northern Rockies on Thursday.

To the east, much above average temperatures will continue for the Plains
and spread into the Great Lakes as an amplifying pattern helps to
reinvigorate upper-level ridging over the central U.S. Forecast highs in
the upper 70s to mid-80s will be 10-20 degrees above average for portions
of the Northern Plains/Upper Great Lakes. Highs in the mid- to upper 90s
in Texas are similarly above average for late Summer. While not quite as
anomalous, highs in the mid- to upper 80s for locations in the Central and
Southern Plains are still quite warm. Meanwhile, thunderstorm chances will
continue along a lingering quasi-stationary front draped from the Southern
Plains into the Lower Mississippi Valley Wednesday into Thursday with both
locally heavy rainfall and some instances of damaging winds/large hail
possible. An upper-level shortwave approaching the region later Thursday
should help to enhance storm development/coverage, leading to a higher
risk of more widespread heavy rainfall and some scattered instances of
flash flooding for portions of southeastern Oklahoma into northeastern
Texas, with a Slight Risk of Excessive Rainfall (level 2/4) in place.
Increasing lift ahead of the deepening upper-low over the West should lead
to increased shower and thunderstorm chances along a quasi-stationary
front over the Northern Plains Thursday with some locally heavy rainfall
possible here as well.

Elsewhere, conditions will remain relatively dry with seasonable
temperatures for most of the eastern U.S. Forecast highs Wednesday and
Thursday will be in the 60s and 70s for New England, 70s and 80s for the
Mid-Atlantic, and 80s for the Southeast into Florida. A lingering frontal
boundary over the Florida Peninsula will continue to lead to daily
thunderstorm chances, with some locally heavy downpours and isolated flash
flooding possible. Rain chances may begin to increase late Thursday
northward along the Georgia and Carolina coasts as an area of low
pressure/frontal system begins to organize just off the coast.

Putnam

Graphics available at
https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php