023
FXUS63 KLSX 120535
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1235 AM CDT Sat Jul 12 2025

.KEY MESSAGES...

- A few thunderstorms could be severe this evening, mainly across
  northeastern MO and west-central IL, along with locally heavy
  rainfall and isolated flash flooding.

- A relatively active pattern will exist through next week with
  varying daily chances of showers and thunderstorms, although
  most of the period will be dry versus wet.


&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 242 PM CDT Fri Jul 11 2025

A hot and humid day is underway with temperatures in the 90s F and
dewpoints in the 70s F supporting heat index values in the upper 90s
to near 105 F as well as a very unstable airmass with MLCAPE of 2500
to 3500 J/kg. A subtle mid-level perturbation and responding low-
level confluent flow will continue to result in isolated showers and
thunderstorms across southeastern/east-central MO and southwestern
IL this afternoon/early evening. Weak microbursts with gusty winds
are possible with these thunderstorms but overall strength will be
limited by poor mid-level lapse rates and weak deep-layer wind shear.

The main focus of showers and thunderstorms as well as severe
thunderstorms is across northeastern MO into IA this afternoon,
reaching northeastern MO and west-central IL this evening. As an
upper-level shortwave trough overspreads a surface low and quasi-
stationary front straddling the MO-IA border surface-based
initiation of thunderstorms is forecast. Initial thunderstorms could
exhibit transient supercellular characteristics with 20 to 30+ kt of
deep-layer wind shear in those area, capable of all hazards
including a tornado. However, most CAMs indicate that this activity
will remain just northwest of northeastern MO and west-central IL
until upscale growth into an MCS takes place with cold pool
development through the 23 to 02z timeframe, leading to
predominantly a damaging wind threat. Locally heavy rainfall and
isolated flash flooding is also possible with PW approaching 2" and
deep warm cloud depths mainly before the strengthening cold pool
would accelerate forward motion. As the MCS continues to track
southeastward, all model guidance indicate that it will be weakening
and increasingly outflow dominant as it encounters weaker low-level
and deep-layer wind shear before it reaches the I-70 corridor.

Along with a minimum in shower and thunderstorm coverage Saturday
morning, HREF membership indicates that the outflow from this
evening`s MCS will fully clear the CWA. Subsequent convective
inhibition, lingering clouds, and a lack of discernible large-scale
ascent limits confidence in where and how much redevelopment of
showers and thunderstorms will occur during the afternoon, with 20
to 30 percent of membership having development along the cold front
reaching the I-44 and I-70 corridors. A greater amount of membership
depicting development along remnant outflow largely to the
east/southeast of the CWA. Broken clouds and frontal passage in the
northwestern half of the CWA will lead to a much cooler day on
Saturday with high temperatures in the mid-80s to near 90 F.

Pfahler

&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 242 PM CDT Fri Jul 11 2025

On Sunday, model guidance has varying solutions of convectively
augmented mid-level shortwave trough or MCV tracking northeastward
through the CWA sometime during the morning or afternoon,
accompanied by a greater chance of showers and thunderstorms (50 to
70 percent chance of measurable rainfall) mainly south of I-70 or
along/south of the front`s position at that time. This feature will
also be accompanied by a conditional threat of locally heavy
rainfall and isolated flash flooding (efficient rainfall favored
with potential slow cell motions) along with microbursts containing
gusty winds. Sometimes MCVs can create an enhanced environment for
severe thunderstorms within a less favorable background environment,
which will need to be monitored, although there is currently no
strong support for this scenario. Otherwise, NBM high temperature
distributions are very similar both Saturday and Sunday, supporting
little change between days outside of more numerous
showers/thunderstorms.

A broad upper-level ridge and weakness of flow will dominate the Mid-
Mississippi River Valley Monday through Tuesday allowing both a
northward advancement of the front that will have been wavering
across the region with warming temperatures from weak low-level
southerly WAA and continued opportunities of showers and
thunderstorms. Showers and thunderstorms should be mostly diurnally
driven but model guidance indicates the potential for a few slow-
moving mid-level perturbations/MCVs meandering with the weak flow.
During the second half of the week, upper/mid-level quasi-zonal flow
will overlay more of the Mid-Mississippi River Valley, leading to
greater ensemble membership containing measurable rainfall with
stronger bouts of ascent associated with more robust passing
shortwave troughs. The details of these patterns are very difficult
to decipher at this juncture including the timing and amplitude of
these features, ultimately determining the the timing, coverage, and
placement of showers and thunderstorms. That said, more of this
period will be dry than wet. NBM probabilities of 90+ F high
temperatures increase each day Monday through Wednesday, especially
in the St. Louis metro and southwestern IL, which would be
accompanied by increasing moisture/dewpoints. There are mixed
signals for a cold frontal passage around Thursday or Friday next
week with uncertainty largely in whether or not the front fully
clears the region or stalls. This leads to increasingly uncertain
temperatures (NBM IQR 7 to 10 F) and the summit of ensemble model-
based probabilities of showers and thunderstorms (40 to 60 percent).

Pfahler

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1235 AM CDT Sat Jul 12 2025

A decaying line of showers and thunderstorms is moving through the
region, bringing impacts to all local terminals through the first
few hours of the TAF period. Given the decaying nature of the line,
the threat of impacts will rapidly decrease, and could end earlier
than forecast. The cold front responsible for the convection will
sag southward overnight through Saturday, with low stratus along it
bringing a brief bought of MVFR ceilings in the morning, thinning and
lifting around mid-day. The front is expected to have sunk far enough
south so that confidence is high that additional convection will
form south of the local terminals Saturday afternoon.

Elmore

&&

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

$$

WFO LSX