Area Forecast Discussion

647
FXUS63 KLSX 301116
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
616 AM CDT Wed Jul 30 2025

.KEY MESSAGES...

- A cold front will push through the region today bringing heat relief
  to areas generally north of I-70. The heatwave will end
  entirely by tonight.

- The cold front will also bring a marginal threat for strong to
  severe thunderstorms to the region this afternoon into evening.
  Damaging winds and isolated flash flooding will be the primary
  hazards.


&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 337 AM CDT Wed Jul 30 2025

An MCS currently traversing Iowa will sink southeast through this
morning, entering northeast Missouri and west-central Illinois
shortly after sunrise. As it passes through these areas, the MCS is
expected to lay out an outflow boundary which may act as a catalyst
for convection this afternoon depending on how strong surface
convergence is along it. Hi-res guidance varies with this, with each
model showing differing amounts of coverage if it has convective
initiation at all. The presence of thunderstorms is more certain
along a cold front that will be dipping south through the area late
this afternoon through tonight. These thunderstorms will be entering
an environment characterized by 1500-2000 J/kg of SBCAPE and 25-35
kts of 0-6 km bulk shear. These conditions will be sufficient for
thunderstorm organization, with a few strong to severe thunderstorms
expected. Damaging winds will be the primary threat in any
thunderstorms that do become severe, and heavy rain is expected in
any thunderstorms period. Bulk shear vectors nearly parallel to the
cold front and PWATs approaching 2" bring the potential for flash
flooding into question. It is possible given the potential for
training and the high rain rates. However, thunderstorms are
expected to be progressive, so they shouldn`t sit over a single
location for too long. The absence of a nocturnal LLJ tonight will
also be unsupportive for continuous convective development along the
boundary, which lowers the potential for training and widespread
flash flooding impacts. I believe flash flooding will be localized
to areas that 1) experience multiple thunderstorms and/or 2) have
had an abundance of rainfall recently and are primed for flooding.
Thunderstorms will slowly diminish overnight. Redevelopment is
likely south of I-70 tomorrow as the last of the mid-level forcing
interacts with the exiting front. Severe weather is not expected.

For locations generally south of I-70, today`s temperature forecast
will follow the status quo. Dangerous heat index values of up to 110
degrees are expected yet again, thankfully for the final time during
this hot stretch. To the north, the arrival of the decaying MCS this
morning and cold front later today will keep the heat dampened
compared to days past with heat index values landing in the 90s.
Tomorrow will be our first day of widespread heat relief as much
cooler air floods into the region. Locations north of I-70 are
forecast to remain in the 70s for highs, which is very close to what
their lows have been for the past week. Highs in the 80s are
forecast south of I-70 where the cooler air will not have fully
infiltrated.

Jaja

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 337 AM CDT Wed Jul 30 2025

The influence of surface high pressure will be the dominant force
for our sensible weather from Friday through Sunday. These days will
be dry as a result with highs in the 70s to low 80s. Morning lows
will be brisk compared to what we`ve become used to. Portions of
northeast Missouri will fall into the mid-50s each night, with areas
to the south ranging from the low to mid-60s for lows.

The pattern changes early next week as the surface high exits off to
the eastern US. Southerly flow will return, and temperatures will
gradually rise through the week. Aloft, a shortwave is forecast to
slide through the Mid-Mississippi Valley sometime in the early week,
bringing rain chances back into the forecast. The only similarity
among guidance concerning the shortwave is that they all show one,
so any details remain uncertain at this time.

Jaja

&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 607 AM CDT Wed Jul 30 2025

A thunderstorm complex will move into northeast MO and west-
central IL at the start of the TAF period. Gusty winds of around
30 kts and IFR conditions are expected on the front edge of the
storm, with flight conditions improving after the front line of
storms passes. This afternoon, thunderstorms will develop across
much of the area as a cold front passes. These thunderstorms will
be capable of gusty winds and IFR flight conditions as well.
Exact timing on impacts at each terminal is still uncertain and
will need to be refined with later updates, but the general
forecast is on track.

Outside of any thunderstorms, winds will remain light and
variable before slowly shifting to northerly late in the period.
Widespread low MVFR to IFR stratus will fill in behind the front
overnight and last at least through the late morning.

Jaja

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 7 PM CDT this evening for Boone MO-Callaway
     MO-Cole MO-Crawford MO-Gasconade MO-Iron MO-Lincoln MO-
     Madison MO-Moniteau MO-Montgomery MO-Osage MO-Reynolds MO-
     Saint Francois MO-Warren MO-Washington MO.

     Extreme Heat Warning until 7 PM CDT this evening for Franklin MO-
     Jefferson MO-Saint Charles MO-Saint Louis City MO-Saint
     Louis MO-Sainte Genevieve MO.

IL...Heat Advisory until 7 PM CDT this evening for Calhoun IL-Fayette
     IL-Greene IL-Jersey IL-Macoupin IL-Montgomery IL.

     Extreme Heat Warning until 7 PM CDT this evening for Bond IL-
     Clinton IL-Madison IL-Marion IL-Monroe IL-Randolph IL-Saint
     Clair IL-Washington IL.

&&

$$

WFO LSX

NWS LSX Office Area Forecast Discussion