Area Forecast Discussion

553
FXUS63 KLSX 030349
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1049 PM CDT Wed Jul 2 2025

.KEY MESSAGES...

- Seasonable temperatures will give way to warm and gradually
  more humid conditions on Friday/4th of July, into the holiday
  weekend.

- Largely dry conditions are expected into Saturday, including
  the 4th of July, but opportunities for showers and thunderstorms
  will exist late Saturday into next week.


&&

.SHORT TERM...  (Through Late Thursday Night)
Issued at 230 PM CDT Wed Jul 2 2025

Upper/mid-level northwesterly flow above a very slowly departing
surface anticyclone will remain the dominant features of the Mid-
Mississippi River Valley tonight through Thursday, followed by an
upper-level ridge arriving Friday. The remnants of a small
thunderstorm cluster continues to dissipate along the IA-MO border
before reaching the CWA this afternoon. A similar case will exist
Thursday and Friday as well, with greatest forcing from passing mid-
level perturbations and nocturnal LLJs focused incrementally
northeastward into the Upper Midwest as the ridge arrives. This
notion is supported by 90 percent of ensemble model guidance keeping
the CWA dry through Friday/4th of July with a capping inversion also
lowering the threat of airmass diurnal thunderstorms. As low-level
flow/WAA gradually increases coupled with strong insolation through
mainly just diurnal cumulus, a warming trend will also exist with
high temperatures around 90 to the mid-90s F expected across most of
the CWA by Friday. Modest moisture transport will also increase
dewpoints, but it will be a slow increase that competes with deep BL
mixing each afternoon, generally limiting peak heat index values to
the mid to upper 90s F.

Pfahler

&&

.LONG TERM...  (Friday through Next Wednesday)
Issued at 230 PM CDT Wed Jul 2 2025

Over the holiday weekend, the upper-level ridge will progress
eastward into the Great Lakes as a weakening trough arrives sometime
late Saturday into Sunday accompanied by a cold front on Sunday.
Much of Saturday is expected to be dry but 30 to 40 percent of
ensemble model membership have showers and thunderstorms reaching
northeastern and central MO late in the day. A greater chance of
showers and thunderstorms will exist on Sunday, supported by 40 to
60 percent of membership, with relatively greater low-level forcing
associated with the cold front and less convective inhibition
evident in forecast soundings. Large-scale ascent/forcing is not
expected to be particularly strong, which leaves coverage of showers
and thunderstorms unclear. Confidence is high in high temperatures
again in the 90s F on Saturday, but the spread in the NBM increases
to between remaining steady and cooling closer to average on Sunday
with showers/thunderstorms and the cold front. Peak heat index
values are forecast to be in the mid-90s to near 100 F with some
additional increase in dewpoints.

Global model guidance is in loose agreement that time-mean upper-
level flow will become quasi-zonal early next week and navigated by
multiple shortwave troughs above Sunday`s cold front wavering across
the Mid-Mississippi River. In this pattern, details are difficult to
discern at this point including the front`s position at any given
time and timing of individual shortwave troughs. As a reflection,
ensemble model guidance has broad-brushed 30 to 50 percent
probabilities of measurable rainfall through early next week. These
probabilities are lower at night, supporting a nocturnal minimum in
coverage with little indication of nocturnal LLJs being present to
maintain/develop showers and thunderstorms. Unsurprisingly, the NBM
interquartile high temperature range is also 4 to 7 F next week,
ranging between near average and further into the 90s F.

Pfahler

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1047 PM CDT Wed Jul 2 2025

High pressure is keeping winds light or calm overnight, with a
return to light southerly winds tomorrow. VFR conditions are
expected to continue overnight and on Thursday. There`s some
potential for valley fog by morning, but after a warmer start to
the evening and lower dewpoints, combined with some passing mid
level cloud cover, we are unlikely to see strong enough cooling
overnight to produce significant valley fog.


Kimble

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX

NWS LSX Office Area Forecast Discussion