424
FXUS63 KLSX 051926
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
226 PM CDT Fri Jun 5 2026

.KEY MESSAGES...

- Daily, varying chances of showers and thunderstorms will exist
  through Tuesday. There is a threat of locally heavy rainfall
  Sunday and Monday.

- Temperatures and humidity will increase after Monday with peak
  afternoon/evening heat index values potentially reaching 100 F.


&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 225 PM CDT Fri Jun 5 2026

With this morning`s upper-level trough departing and the next trough
passing north of the Mid-Mississippi River Valley, any redevelopment
of showers and thunderstorms this evening and overnight is uncertain
owing to a lack of clear low-level forcing mechanism. A few CAMs
have inconsistently indicated potential showers and thunderstorms in
northeastern MO/west-central IL this evening as well as clusters/MCS
of showers and thunderstorms across IA and northern IL along a quasi-
stationary front sagging southward into the LLJ overnight. However,
latest HREF probabilities of measurable rainfall are 20 percent or
less, further diminishing confidence. IF thunderstorms occur this
evening, 2000 to 3000 J/kg MLCAPE and modest 15 to 25 kt of deep-
layer wind shear, a briefly severe thunderstorm with damaging winds
and marginally severe hail cannot be ruled out. But this
conditionality and an already marginal environment keeps confidence
too low to advertise the threat.

On Saturday, the CWA will be sandwiched between the aforementioned
upper-level trough passing to the northeast and another southern
stream low/trough approaching from the south. Since the quasi-
stationary front is also expected to stay northeast of the CWA, low-
level forcing mechanisms are once again not clear. With a weak
capping inversion, at least isolated showers and thunderstorms are
expected during peak afternoon heating of the warm and moist
airmass. There are timing differences in the arrival of large-scale
ascent leading the approaching low/tough with the earliest being
late Saturday afternoon, which would support increasing coverage of
showers and thunderstorms if there is an overlap with peak heating.
The exact location, coverage, and timing of thunderstorms is
unclear, with CAMs also advertising notably different solutions. An
isolated microburst with gusty winds is possible, but sub-15 kt deep-
layer wind shear will limit the threat of severe weather. High
temperatures will generally be similar to today, if not a couple of
degrees warmer where clouds are less prevalent during the afternoon
between early day stratocumulus and increasing upper-level clouds.

Pfahler

&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 225 PM CDT Fri Jun 5 2026

The upper-level low/trough will propagate northward over the Central
Plains and Mid-Mississippi River Valley Sunday and Monday,
accompanied by the most favorable period of more widespread showers
and thunderstorms with 60 to 90 percent of ensemble membership
containing measurable rainfall. Probabilities are highest during
each afternoon and evening but there are indications that nocturnal
LLJs could also support some overnight showers and thunderstorms.
There will be a locally heavy rainfall threat during this time
period with precipitable water nearing 2" (99th climatological
percentile) and very deep warm cloud depths combined with kinematic
profiles suggesting potential backbuilding or training of some
thunderstorms. However, flash flooding would conceptually require
locations to receive multiple rounds of heavy rainfall and/or
thunderstorms to loosely organize into MCSs or clusters, which
cannot be said with much confidence at this range. The majority of
the NBM high temperature distribution is cooler (upper 70s to 80s F)
both Sunday and Monday, but how cool temperatures actually remain
will come down to the amount of precipitation and cloud cover.

Following the departure of the upper-level low/trough late Monday,
global model guidance continues to advertise a blossoming ridge
dominating the Mid-Mississippi River Valley within varying
depictions of an Omega Block Tuesday through late next week. In this
pattern, the warm and moist airmass will remain in place and even be
reinforced by weak, but persistent low-level southerly flow. As 850-
hPa temperatures rise to near the 99th climatological percentile and
the ridge inhibits daily shower and thunderstorm development, high
temperatures will warm to the upper 80s to mid-90s F. With dewpoints
in the 70s F, peak afternoon and evening heat index values are
projected to reach 100 F, but values will depend on the balance of
diurnal boundary layer mixing allowing warmer temperatures with
lowering dewpoints.

Pfahler

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1225 PM CDT Fri Jun 5 2026

Showers and thunderstorms continue to gradually wane as they reach
east-central MO and south-central IL early this afternoon, likely
before reaching St. Louis metro terminals. Otherwise, MVFR ceilings
will also lift early this afternoon with areawide VFR and
predominantly dry flight conditions through most of the rest of the
TAF period. That being said, there are low chances (20 percent or
less) of showers and thunderstorms at KUIN this evening through
overnight and then Saturday afternoon across much of the region.
However, confidence is very low in any impacts at the terminals.
Additionally, a period of MVFR ceilings is favored at KCOU, KJEF,
and KUIN Saturday morning. Winds will vary between southwesterly and
southerly with gusts diminishing this evening.

Pfahler

&&

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

$$

WFO LSX