317
FXUS63 KLSX 101939 CCA
AFDLSX

Area Forecast Discussion...CORRECTED
National Weather Service Saint Louis MO
239 PM CDT Thu Jul 10 2025

.KEY MESSAGES...

- Multiple rounds of showers and thunderstorms are expected late
  Friday through the weekend. A few severe thunderstorms are
  possible Friday eveninga cross northeastern MO and west-
  central IL along with locally heavy rainfall and isolated flash
  flooding.

- Hot and humid conditions are forecast on Friday with peak heat
  index values reaching the upper 90s to near 105 F, highest in
  the St. Louis metro. Temperatures will cool through the weekend.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 233 PM CDT Thu Jul 10 2025

An upper-level trough will continue passing through the Midwest this
afternoon and evening. However, with large-scale ascent passing
mainly north of the CWA, weak capping inversions, and a lack of low-
level convergence, only a stray shower/thunderstorm or two is
expected at most across northeastern MO/west-central IL this
evening. A shortwave ridge with rising mid-level heights will be
unfavorable for any showers and thunderstorms overnight/Friday
morning across IA into northern IL from advancing
south/southeastward into the CWA. Therefore, predominantly dry
conditions are expected through midday Friday. Weak low-level
southerly flow/WAA will lead to slightly warmer high temperatures on
Friday in the low to mid-90s F. With increasing moisture and
dewpoints in the 70s F with some upper 60s F during peak afternoon
heating/BL mixing, peak heat index values in the upper 90s to near
105 F are forecast, highest in the St. Louis metro.

Around 20 percent of HREF membership has isolated showers and
thunderstorms developing during the afternoon across the Ozarks and
southwestern IL with a signal for weak surface convergence (possibly
topographically enhanced) and convective temperatures to be reached.
A weak microburst with gusty winds cannot be ruled out, but warm mid-
level temperatures/weak lapse rates and weak deep-layer wind shear
may limit updraft strength. Instead, the main focus of showers and
thunderstorms will be across southern IA and northern MO during the
afternoon and evening as a shortwave trough overspreads a cold front
and other remnant outflow boundaries. It is uncertain how quickly
these showers and thunderstorms will advance south/southeastward
into northeastern MO and west-central IL with CAMs indicating an
arrival as early as 4 pm and late as 8 pm, depending on how quickly
cold pool development can occur. Deep-layer wind shear of 20 to 25
kt and MLCAPE of 2000 to 3000 J/kg will favor mainly multicell
thunderstorms or an MCS with damaging winds and marginally severe
hail. With deep warm cloud depths, PW approaching 2", and the
potential for cell training and backbuilding into a 35 kt-southwesterly
LLJ there is also a threat of locally heavy rainfall and isolated
flash flooding. All of these threats are highest across
northeastern MO and west-central IL with showers and thunderstorms
expected to gradually weaken with time and southward extent
Friday night as they become increasingly outflow dominant.

Pfahler

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 233 PM CDT Thu Jul 10 2025

On Saturday, the cold front will reach the CWA before it slows and
possibly wavers across the region over the weekend. There should be
an overall southeastward progression of the front with time, but
this progression is uncertain due to the impact of multiple passing
mid-level shortwave troughs/MCVs within a variable period of
southwesterly flow and associated showers and thunderstorms.
Therefore, ensemble membership has a broad-brushed area of 50 to 70
percent probabilities of measurable rainfall that encompass most of
the CWA on Saturday and then shift to southeastern MO and
southwestern IL on Sunday. With the slow front, slow or training
cell motions, and PW straddling 2", the threat of locally heavy
rainfall will exist but whether or not flash flooding occurs will
hinge on mesoscale details that are still uncertain. With weak deep-
layer wind shear, confidence is low in any organized severe
thunderstorms over the weekend. With a gradual passage of the front
over the weekend and increasing clouds and precipitation, a cooling
trend will take place over the weekend with near/slightly below
average high temperatures in the 80s F nearly CWA-wide by Sunday.

A broad upper-level ridge will briefly dominate the Mid-Mississippi
River Valley Monday into Tuesday, allowing the aforementioned front
to eventually lift back northward along with the most favorable
corridor of showers and thunderstorms. However, there are little
signs of any appreciable large-scale forcing with mainly scattered
diurnal showers and thunderstorms expected. Global model guidance
thereafter is in general agreement in another relatively active/wet
period during the middle of next week with upper-level quasi-zonal
to southwesterly flow and multiple passing shortwave
troughs/perturbations. The NBM high temperature distribution is also
reasonably clustered (3 to 5 F IQR) on a warming trend to near or
slightly above average through the first half of the week along with
another increase in moisture/dewpoints.

Pfahler

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1225 PM CDT Thu Jul 10 2025

Dry and VFR flight conditions will persist through the TAF period,
although there are low chances (20 percent or less) of isolated
showers and thunderstorms this afternoon/evening at KUIN and Friday
afternoon at KSTL. Since these probabilities/confidence is low, no
explicit mention has been included at this point. Southerly winds
today will become more southwesterly late Friday morning.

Pfahler

&&

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

$$

WFO LSX