518
FXUS63 KLSX 271939
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
239 PM CDT Wed May 27 2026

.KEY MESSAGES...

- The next, higher chance (30 to 60 percent) of showers and
  thunderstorms is Friday across central/southeastern MO. Locally
  heavy rainfall is possible.

- Seasonable temperatures will persist into next week, along with
  mainly dry conditions aside from Friday.


&&

.SHORT TERM...  (Through Late Thursday Night)
Issued at 239 PM CDT Wed May 27 2026

Isolated to scattered showers and weak thunderstorms along/south of
I-44 (MO) and I-70 (IL) will decrease in coverage through the
evening as a mid-level lobe of vorticity departs to the east, a weak
cold front advances to the south, and diurnal instability also
wanes. Clouds will also decrease this evening, aside from some upper-
level clouds of varying thickness returning overnight. With the
assumption that these clouds will mainly be thin permitting
efficient radiational cooling, residual moisture and light winds
will favor the development of at least patchy fog overnight in river
valleys and more broadly across southwestern IL where clouds have
limited heating/boundary-layer mixing and there has been more
numerous showers. Dense fog cannot be ruled out in southwestern IL,
but there is not currently a strong signal in model guidance.

On Thursday, the Mid-Mississippi River Valley will be largely under
the influence of an upper-level ridge connected to an Omega Block
spanning much of North America. This feature will contribute to
predominantly dry conditions with a lack of large-scale forcing and
weak, subsidence-induced capping inversion. However, there will be
an upper-level trough navigating and a plume of deeper moisture at
the western flank of the ridge that could be just enough to promote
a couple showers and/or weak thunderstorms in southeastern MO,
primarily during peak heating in the afternoon. With little change
in airmass, high temperatures will once again reach the 80s F.

Pfahler

&&

.LONG TERM...  (Friday through Next Wednesday)
Issued at 239 PM CDT Wed May 27 2026

Overnight Thursday night into Friday, the aforementioned upper-level
trough and associated plume of moisture will lift northward into the
CWA providing another opportunity for showers and thunderstorms.
Global ensemble model probabilities of measurable rainfall are
greatest across central and southeastern MO Friday morning through
the afternoon, indicating when and where confidence is highest in
showers and thunderstorms. High PW nearing the 90th climatological
percentile, deep warm cloud depths, and slow-moving
showers/thunderstorms lead to a threat of locally heavy rainfall as
well. Further to the north, confidence in precipitation decreases as
a result of the trough weakening and fragmenting as it moves further
northward and encounters unfavorable confluent flow near the ridge
axis decreasing forcing and northward advancement of deeper
moisture. With the precipitation and more extensive cloud cover
spanning the CWA, high temperatures will be cooler and in the mid-
70s to low-80s F, although a relatively larger interquartile range
in the NBM leads to lower confidence in exact values.

Through the weekend and into Monday, the large-scale pattern will
change very little across the Mid-Mississippi River Valley with
lobes of mid-level vorticity continuing to decay overhead and
stagnant moisture leading to diurnal, isolated to scattered showers
and a few thunderstorms, still mainly across central and
southeastern MO. It is worth noting that the vast majority of the
time and area will be dry during this period, accompanied by similar
high temperatures in the upper 70s to mid-80s F.

Early next week, there are variations of a slight westward
retrogression of the Omega Block providing an exchange of influence
from the ridge to northerly/northwesterly flow and nearby troughing,
allowing a series of weakening back door cold front to infiltrate
the Mid-Mississippi River Valley. With subtle differences in the
pattern, there is variation in the strength of these fronts and how
much of an impact they will have on the CWA, but they at least have
the potential to nudge temperatures and dewpoints downward
along/east of the Mississippi River.

Pfahler

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1229 PM CDT Wed May 27 2026

The chance of showers impacting the St. Louis metro terminals this
afternoon has lowered, but a PROB30 has been maintained at KCPS, in
closer proximity to greater coverage across southwestern IL.
Otherwise, dry conditions are expected through Thursday evening.
Conditions will be favorable for patchy fog to develop tonight,
primarily in river valleys, if clouds mostly clear. Lower
visibilities may need to be included for river valley terminals once
confidence increases. Winds will remain light through the TAF
period.

Pfahler

&&

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

$$

WFO LSX