540
FXUS63 KLSX 250931
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
431 AM CDT Mon May 25 2026

.KEY MESSAGES...

- Memorial Day remains dry with near normal temperatures.

- An active pattern brings rain chances back to the region Tuesday
  with additional chances through the end of the work week.


&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 237 AM CDT Mon May 25 2026

Surface ridging extends from the Great Lakes Region into the mid-
Mississippi Valley this morning with mainly clear skies overhead.
Surface observations show winds have settled or lightened to
calm/variable. The light surface flow and narrow dewpoint
depressions could support patchy, mainly light fog again this
morning. Fog potential will be best adjacent to river valleys and
low-lying areas with higher soil moisture content from recent
rainfall.

Memorial Day closes out the holiday weekend with near normal
temperatures and dry conditions. Light, easterly surface flow will
keep moisture at comfortable levels with some scattered afternoon
clouds primarily south of I-70. HRRR/RAP are rather bullish with
afternoon mixing, allowing dewpoints to drop into the 40s. This
signal isn`t as strong in medium/long range guidance, but even then
low to mid-50s is comfortable with highs near 80 degrees. Scattered
afternoon clouds develop primarily along and south of I-70 and that
may be the extent of impact on sensible elements with trends
migrating precipitation chances to the south of the CWA.

Between tonight and Tuesday, a stagnated upper level low over
eastern Texas begins to lift to the northeast as mid-level height
rises subtly build northwestward. The weak front that just slipped
to our south will lift back to the north Tuesday advecting moisture
back into the region. Diurnal-driven showers and a few thunderstorms
are possible, but will largely be limited to areas of southeast
Missouri and southwest Illinois. A narrow ribbon of MUCAPE reaches
between 2000-25000 J/kg along the advancing boundary, coinciding
with higher HREF probabilities (40-70%) for measurable rainfall.
However, even these probabilities are highly variable between
locations within the broader spread of precipitation chances. This
is typical of convective potential with highly variable rainfall in
short distances. Fortunately, with shear below 20 knots, what forms
is not likely become well-organized, but will have the potential to
provide a brief period of heavy rain at effected locations. Most
activity dissipates after sunset as diurnal support is lost.

After Tuesday morning lows in the mid-50s to low-60s, high
temperatures return to nearly identical levels as today with highs
in the upper 70s to low-80s.


Maples

&&

.LONG TERM...  (Tuesday Night through Sunday)
Issued at 237 AM CDT Mon May 25 2026

The upper low that originates from eastern Texas progressively opens
and becomes indiscernible Tuesday into Thursday. The northward
progress of the weak front slows and eventually stalls as fragments
of vorticity linger overhead. This draws a pool of higher theta-e
northward with higher precipitation chances following diurnal
trends. Disperse QPF plumes do little more than provide confidence
that rain will become more likely from Tuesday onward. 24-hour run-
to-run trends in the ECMWF and GFS ensemble show a delaying trend at
the front end of the potential Tuesday afternoon/evening,
maintaining a mostly dry forecast outside of southeast Missouri and
southwest Illinois.

By Wednesday, the weak lifts to its destination, lining up somewhere
along the I-70 corridor. PoPs become less confined and cover the CWA
through the day Wednesday with isolated to scattered showers
becoming relatively more broadly represented through the afternoon
period as instability builds. Despite surface instability peaking
between 2500-2700 J/kg, weak flow persists with shear values at 20-
25 knots or less. Given the weak flow, thunderstorms will become
more vertically structured, leaving little room for well-organized
convection. The greatest impact from this activity will be lightning
potential and pockets of locally heavy rainfall as PWATs climb to
1.5-1.7 Wednesday afternoon.

Diurnal thunderstorm potential will be the theme through the end of
the work week. Ensemble IQRs show a subtle increase in spread
heading into next weekend, though the characteristic to temperature
behavior is persistent and nearly flat several days out. This
provide confidence that air will remain mild with variability driven
by the extent of cloud cover and cooling caused by afternoon
thunderstorms. One of the main drivers of the spread is likely to be
a surface high that builds southeast from Canada into the Great
Lakes Thursday into Friday. This shifts the weak boundary back
toward the south/southwest as a backdoor cold front stalls in the
Midwest. The southwest extent of the ridge orients the boundary
along the Mississippi River, serving as the dividing line between
dry conditions to the east and precipitation chances to the west.
Its along this divide where precipitation data spread is highest due
to placement differences between solutions.

The divergence between ensemble guidance keeps at least low chances
going into next weekend, though there`s a downward trend in the
potential. If deterministic guidance comes to fruition, especially
the ECMWF, the forecast would lead drier in time with another
backdoor cold front dropping north to south and shunting
precipitation southward along with it.

Temperatures range from near to slightly above normal with highs in
the upper 70s to mid-80s and lows in the 60s.


Maples

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 426 AM CDT Mon May 25 2026

Patchy river and valley fog is leading to MVFR visibilities
around KSUS this morning and likely at KJEF, as well. Fog quickly
erodes this morning after sunrise. East and southeasterly flow
increases by late morning, but high pressure remains in control
with sustained winds peaking at 5-10 knots through this afternoon.

VFR conditions are expected outside this morning`s patchy fog.

Maples

&&

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

$$

WFO LSX