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FXUS63 KLSX 051132
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
632 AM CDT Tue May 5 2026

.KEY MESSAGES...

- Periods of showers and thunderstorms will continue across the
  area through this evening. Severe thunderstorms and flooding
  are not anticipated.

- Below normal temperatures are expected today through Thursday.


&&

.SHORT TERM... (Through Wednesday Night)
Issued at 300 AM CDT Tue May 5 2026

Convection from yesterday evening has produced an outflow boundary
that is sinking southward through southeastern Missouri and
southwestern Illinois this morning. This makes the synoptic front in
initially responsible for the convection hard to pick out in surface
observations, but it is most likely currently just south of Highway
36/I-72.

Through the day, southwesterly flow aloft will continue will feed
moisture along and north of the synoptic front and the outflow, with
subtle disturbances aloft providing sufficient forcing for light
rainfall through much of the day. A more potent disturbance
approaches the CWA this afternoon and evening, which will lead to an
uptick in rainfall coverage and intensity, as well as weak
thunderstorms. Guidance consensus is that the front will be just
south of the CWA by peak heating when more robust convection is
expected. A majority of hi-res models have sufficient instability
for strong to severe storms remaining south of the CWA, with worst-
case scenario outliers struggling to reach 1,000 J/kg of SBCAPE
across far southern Reynolds County during the afternoon. Therefore,
confidence in strong to severe storms this afternoon is very low,
and we are currently not planning to publicly message the SPC Day 1
Marginal Risk.

Tomorrow, the post-frontal airmass becomes further entrenched across
the area, with temperatures topping out around 60 degrees beneath
mostly cloudy skies. These below normal temperatures carry into
Wednesday night, with a chilly night in store for the area. A
majority of guidance favors temperatures remaining above the
threshold for frost concerns (36 degrees), and the center of the
post-frontal high is expected to remain well west of the area. This
decreases the chances of winds becoming calm and skies completely
clearing, which will mitigate radiational cooling. The most recent
run of the NBM has a 20-30% chance of overnight lows reaching or
dropping below 36 degrees across portions of northeastern and
central Missouri, and even if these parts of the area can drop into
the mid-30s, weak winds and at least partly cloud skies will still
make it difficult for widespread frost. Therefore, confidence in
this hazard occurring at this time remains very low.

Elmore

&&

.LONG TERM... (Thursday through Monday)
Issued at 300 AM CDT Tue May 5 2026

The axis of an upper-level trough is set to pass through the Middle
Mississippi Valley on Thursday per guidance consensus. A minority of
global ensemble members (15% or less) show this trough axis being
strong enough to force light rainfall across the CWA during the day.
Given the preponderance of guidance is dry, a dry forecast has been
maintained. While temperatures are still expected to be below
normal, low to mid-level ridging quickly building into the region
through the day and clearing skies support a relative warm up
compared to Wednesday.

While guidance has converged on another FROPA Friday, temperatures
among global ensemble members are unfazed. The associated shortwave
does not notably amplify until it is east of the region, and the
post-frontal airmass is moreso Pacific vs. Canadian. The greatest
impact from this front will be a chance of rainfall, and while the
NBM only has 30-40% probability of precipitation on Friday, global
ensembles suggest that it could be as high as 75%, with the
discrepancy being due to timing differences among the blended
guidance. Low to mid-level ridging quickly builds back into the
Middle Mississippi Valley on Saturday, supporting temperatures
continuing to warm.

Yet another shortwave-cold front combo swing through the Midwest on
Sunday, and this system is expected to be relatively more potent
than the first. That being said, the main impact from this FROPA is
expected to be another shot of showers and thunderstorms as well as
cooler temperatures compared to what is expected on Saturday;
though, ensemble means still hover right around climatological
normals. Currently, the confidence is very low in strong to severe
storms accompanying this front, as guidance consensus is that
sufficient instability for such storms will remain well south of the
CWA. This is supported by recent runs of ML/AI-based severe
probabilistic guidance, which keeps very low probabilities south of
our area.

Elmore

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 625 AM CDT Tue May 5 2026

Impacts are expected at all local terminals through the period,
mainly due to showers and thunderstorms. The first round of this
convection is moving through the region as of the start of the 12z
TAFs, impacting KUIN, KSUS, KSTL, and KCPS. This activity should
wind down by 13-14z, at which time our attention turns to an
additional round moving in from the west. During the afternoon,
there is a low chance of occasional thunderstorms, though KUIN is
expected to stay north of this threat. As rain begins to end,
lower ceilings will stream northeastward into the region, again
impacting all local terminals except for KUIN. KSUS, KSTL, and
KCPS will be on the northwestern periphery of the worst of the
ceilings, and if the front moving through the area is faster than
currently expected, ceilings may end up being a couple hundred
feet higher than forecast. Once these ceilings move over the
terminals, they are expected to remain through the end of the TAF
period and possibly just beyond.

Elmore

&&

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

$$

WFO LSX