546
FXUS63 KLSX 172333
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
633 PM CDT Thu Jul 17 2025

.KEY MESSAGES...

- Scattered to numerous showers and thunderstorms will continue
  this afternoon into evening with a few severe thunderstorms
  capable of isolated damaging winds as well as isolated flash
  flooding.

- Increasing heat and humidity will lead to a heat wave beginning
  sometime over weekend into next week. This heat wave has the
  potential to dangerous from at least a longevity perspective.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 249 PM CDT Thu Jul 17 2025

The combination of an approaching MCV with upper-level quasi-zonal
flow and effective front draped just south of the I-44 and I-70
corridors will lead to increasing development of showers and
thunderstorms this afternoon near around and ahead of the front. A
few thunderstorms could be severe this afternoon with MLCAPE of 1500
to 3000 J/kg and modest deep-layer wind shear of 15 to 25 kt. This
shear will limit the overall severe threat and organization of
thunderstorms into multicell clusters with families of wet
microbursts containing isolated damaging winds. Anomalous PW of over
2" and deep warm cloud depths (10+ kft) will also be favorable for
efficient heavy rainfall with isolated flash flooding where
thunderstorms train or backbuild into 10 to 15 kt southwesterly flow
at 850 hPa. The latest HREF LPMM illustrates the potential of very
localized 3 to 5" of rainfall. The greatest coverage of showers and
thunderstorms will be this afternoon and evening, but at least
isolated showers and thunderstorms are expected overnight across
portions of the area with a weak 20-kt west-southwesterly LLJ
overrunning the front stalling across southern MO providing the only
discernible source of forcing. Patchy fog may also develop where
clouds do not form overnight/Friday morning with very weak flow and
abundant moisture in the BL.

On Friday, the aforementioned front will lift back northward into
the CWA, providing a general focus for another round of showers and
thunderstorms Friday afternoon along/south of the I-70 corridor.
However, the exact location of the front and lack of appreciable
large-scale ascent decreases confidence in the location and coverage
of showers and thunderstorms which illustrated by differences across
CAMs. Model guidance point to instability and deep-layer wind shear
on Friday being similar to today, which suggests that a strong to
severe thunderstorm or two cannot be ruled out. Strengthening low-
level southerly flow/WAA will also help high temperatures warm a few
degrees from today and in the low-80s to near 90 F, but what will be
arguably more noticeable is another surge in dewpoints well into the
70s F.

Pfahler

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 249 PM CDT Thu Jul 17 2025

Upper-level quasi-zonal flow will very gradually transition to
broadly anticyclonic flow as a broad ridge across most of the
southern CONUS begins its initial amplification. This pattern will
be favorable for both increasing heat and locked-in moisture and
competitively frequent "ridge-running" shortwave troughs and MCVs
skirting the Mid-Mississippi River Valley or just northeast.
Ensemble model guidance indicates that the most favorable track for
rounds of showers and thunderstorms, organized into MCSs in some
cases, will clip the northern and eastern portions of the CWA
through Saturday which does include the potential for severe
thunderstorms and locally heavy rainfall if showers and
thunderstorms backbuild into a nocturnal LLJ in place. That being
said there is large amount of uncertainty regarding any impacts to
the CWA due to the influence of mesoscale features and evolutions.
With the favored track of showers and thunderstorms/MCSs lifting
toward the northeast by Sunday, confidence in heat and humidity also
increases through the weekend as probabilities of 100+ F heat index
values increase from 20 to 50 percent on Saturday to 50 to 80
percent on Sunday, mainly in the St. Louis metro and adjacent
central MO and southwestern IL.

Early next week, confidence further increases in widespread heat and
humidity as the upper-level ridge is progged to amplify more
aggressively with its axis near the Mid-Mississippi River Valley.
Accordingly, less the 30 percent of ensemble model membership have
any measurable rainfall Monday through mid-week along with high
temperatures in the 90s F to approaching 100 F and dewpoints in the
70s F supporting 100+ F heat index values (60 to 90+ percent
probabilities) nearly CWA-wide. Only around 25 percent of ensemble
model guidance allow the ridge to weaken and allow a return of
showers and thunderstorms along with the arrival/approach of a cold
front, which is really the only foreseeable support for a break in
the heat and humidity through next week. Confidence remains high
that this heat wave will be at least dangerous from a longevity
perspective and heat headline decisions will need to be made in the
coming days.

Pfahler

&&

.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 629 PM CDT Thu Jul 17 2025

This afternoon`s showers and thunderstorms will continue to
diminish tonight. After that, MVFR cloud cover is forecast to
develop across much of the area. Model guidance is keen on
producing IFR clouds amongst this deck, but confidence in this
impacting any terminal is too low to include at this forecast
issuance. Lower ceilings will lift and scatter by early tomorrow
afternoon.

Jaja

&&

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

$$

WFO LSX