Area Forecast Discussion

158
FXUS63 KLSX 121052
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
552 AM CDT Sun Apr 12 2026

.KEY MESSAGES...

- There is a chance for showers and a few thunderstorms today
  into this evening. The best chance for rain will be across
  central and northeast Missouri into west central Illinois this
  afternoon.

- There will be chances for showers and thunderstorms this week.
  The highest chances will be Tuesday night and again Wednesday
  afternoon and Wednesday night. Some of the storms could be
  severe both Tuesday and Wednesday.

- Above normal temperatures are expected at least through Friday.


&&

.SHORT TERM...  (Through Late Tonight)
Issued at 359 AM CDT Sun Apr 12 2026

A long wave trough over the West Coast combined with ridging over
the eastern U.S. is producing deep south-southest flow across the
Plains and Mississippi Valley today. GOES-E water vapor imagery
is showing a short wave moving northeast through Colorado and
Wyoming into the Northern Plains. This wave is forcing low level
cyclogenesis over the Dakotas, which is strengthening the pressure
gradient over the Plains and the low level southwest flow. Short
range deterministic guidance shows areas of generally unfocused
low level moisture convergence over the Eastern Plains in response
to this strengthening flow. This moisture convergence moves east
with corresponding QPF through the day as the short wave continues
east from the Plains into the Midwest. CAMs depict a broad area
of weakening convection moving into central and northeast Missouri
around 18Z today which agrees well with GFS/RAP forecasts.
Instability looks weak with less than 500 J/Kg MUCAPE this
afternoon, and the low level forcing is broad and relatively
diffuse. Have therefore lowered PoPs a little for most areas to
try to depict a convection that is more scattered in nature. That
said, still have 60-80 PoPs for central and northeast Missouri
into west central Illinois this afternoon. Should have some
insolation through mid/high clouds today ahead of the precip,
which will produce warmer temperatures in the mid 70s to low 80s
today, though temperatures will likely cool a little through the
afternoon where rain falls. The short wave moves east into the
Great Lakes Region this evening to be replaced by weak ridging
aloft. However the broad low level jet remains overhead which
continues to produce weak low level moisture convergence. While
convection should decrease in coverage from west to east through
the evening and overnight, some isolated showers will be possible
through most of the night, primarily across south central and
southwest Illinois ahead of the mid-upper level ridging. Southerly
flow and clouds will keep temperatures from cooling beyond the
low to mid 60s.

Carney

&&

.LONG TERM...  (Monday through Saturday)
Issued at 359 AM CDT Sun Apr 12 2026

Next week continues to look warm under deep southwest flow with a
persistent long wave trough over the western U.S. and a ridge over
the east.  Forecast highs are 10-20 degrees above normal each day,
mainly in the low to mid 80s, with parts of central and eastern
Missouri approaching 90 on Tuesday and again on Friday.  The
persistent southwest flow and lack of significant frontal passages
through Friday lends good confidence to these temperatures.
Temperature IQRs from both the NBM and LREF are only 3-5 degrees
through this period...but convection could always produce lower
temperatures.

Precipitation chances have been tough to nail down as the ridge axis
aloft varies from run-to-run, but PoPs are trending lower Monday and
Tuesday.  Monday looks largely dry with subtle mid-upper level short
wave ridging over the Mississippi Valley.  However, moist low level
flow continues and isolated convection remains possible.
Precipitation chances increase Tuesday as the long wave trough
drifts into the Four Corners Region which forces low level
cyclogenesis over the Western Plains.  Deterministic guidance has
been pretty consistently developing an area of strong instability in
excess of 2500 J/Kg of CAPE ahead of the surface boundary across the
Eastern Plains into northern Missouri and southern Iowa.  Deep layer
shear of 40+ kts will coincide with the strong thermodynamics.  This
is certainly enough to produce severe storms, however forecast
soundings are showing a low level inversion over our area through
Tuesday night, and there continues to be weak ridging in the mid and
upper levels.  Think the most likely scenario is that storms which
develop over the eastern Plains will be weakening as they reach
central and northeast Missouri during the evening or overnight
hours, but there will still be a damaging wind and large hail threat
primarily over central and northeast Missouri into west central
Illinois.

The best chance for storms still looks like Wednesday/Wednesday
night as the upper trough moves from the Plains and through the
Midwest.  Severe threat for our area looks conditional though.
Instability isn`t as strong topping out between 1500-2500 J/Kg
according to the GFS and ECMWF, but the deep layer shear will be
stronger with the trough moving through.  However, guidance is
having trouble resolving where these components come together and
persist.  Latest runs keep the strongest instability well west of
our area over northwest Missouri into south central Iowa, and
rapidly diminish the CAPE by 00Z Thursday.  Would expect afternoon
storms to be weakening as they move into our area in the evening in
this case, but a small shift in the guidance could put those higher
instability values into central and northeast Missouri.

The cold front behind Wednesday/Wednesday night`s storms washes out
as its upper level support moves rapidly east.  Another long wave
trough digs into the western U.S. which re-establishes southwest
flow over the Mississippi Valley.  This continues the streak of
above normal temperatures into Friday.  Guidance suggests this
second wave is more progressive than the previous one and it pushes
a cold front through Missouri and Illinois on Saturday.  This is
obviously way out in the medium range and there are large differences
in the timing of the front.  While the 00Z deterministic GFS and
ECMWF are in fairly good agreement with surface features, they
handle the upper features somewhat differently.  These differences
are quite apparent in the LREF cluster analysis which depict a large
variance in both the speed and amplitude of the trough.  This
translates to differences in frontal position on Saturday and large
temperature differences with high temperature IQRs on the order of
20 degrees.

Carney

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 541 AM CDT Sun Apr 12 2026

An area of showers and a few embedded thunderstorms over Oklahoma
and Kansas will continue moving east-northeast today. This
precipitation is expected to overspread central and northeast
Missouri between 17-19Z, and continue moving to the east-northeast
through the afternoon and evening. While much of the area will
see at least a 20-30 percent chance for rain from this afternoon
through this evening, the best chance (60-80 percent) will be
across central and northeast Missouri and west central Illinois
through the afternoon. MVFR ceilings/visibilities, possibly
dropping to IFR are possible with the heaviest precipitation. VFR
conditions are expected outside of areas of precipitation today,
however ceilings are expected to fall to MVFR across most of the
area tonight. Southerly winds will increase this morning with
gusts up to 25-30kts this afternoon. Winds will diminish but
remain southerly this evening. Low level wind shear conditions are
possible tonight due to a strong low level jet, but current
indications are LLWS will not be strong enough to include in the
terminal forecasts at this time.

Carney

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion