River Flood Warning 18, NWS St. Louis, MO

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
658 PM CDT Thu Apr 2 2026

.KEY MESSAGES...

- Strong to severe thunderstorms are expected this afternoon and
  evening where all hazards are possible. The greatest threat for
  severe thunderstorms is across northeast Missouri and west-
  central Illinois where a Tornado Watch is in effect until 8pm
  this evening.

- Another round of strong to severe thunderstorms are expected
  Friday evening into the early nighttime with damaging wind and
  tornadoes the greatest hazards.

- A cold front on Saturday will bring cooler temperatures and
  drier weather until a warming trend starts midweek.

&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 252 PM CDT Thu Apr 2 2026

Latest surface analysis reveals a strong surface cyclone centered
near southern Iowa as it lifts to the northeast. The result for us
today has been a tightened surface pressure gradient that has aided
in strong southerly winds with gusts near 40mph at times. However,
cloud cover has limited our ability to mix out well, which has
restricted our surface winds as the 40-45kt winds at 925mb have not
been able to mix efficiently down to the surface. The quasi-
stationary front that lingered over the area yesterday has continued
lifting north as a warm front, thanks to the approaching cyclone,
and is now stretched across southern Iowa and northern Illinois.
This has placed our entire area within the warm sector and allowed
for efficient moisture return with surface dewpoints now in the
low/mid 60s. The good surface moisture in place will help set the
stage for the potential of strong to severe storms this afternoon
and evening.

As mentioned on the previous forecast shift, the greatest
uncertainty with the afternoon/early evening severe weather threat
is the destabilization of the environment today before the cold
front. This uncertainty still remains, as cloud cover has been
persistent, which has limited destabilization across the area.
Currently, a partial clearing line is punching northeast into
central Missouri within the cyclone's dry slot with the cold front
across western Missouri. The latest HREF and REFS both resolve this
dry slot and clearing moving northeast, and with it, mean SBCAPE
values approaching 1,500 J/kg where clearing occurs. The mid-level
trough is nearing the area, with the greatest height falls and
forcing occurring between now and early this evening from west to
east, which should help lead to convective initiation this
afternoon. A majority of HREF members still do resolve convective
initiation happening momentarily out ahead of the cold front, across
central Missouri. Something that is more certain is that wind shear
is plenty strong, as the HREF and REFS both have 0-3km SRH of >300
m^2/s^2 collocated with the expected convection. As the surface low
continues to lift northeast and enters eastern Iowa later this
evening, a weak cold front moves into the area from west to east.
Some members of the HREF re-fire convection along this southwest-
northeast oriented cold front early this evening in central and
eastern Missouri with convection moving eastward into Illinois by
the late evening. As a result, all severe weather hazards (large
hail, damaging winds, and tornadoes) are possible with thunderstorms
this afternoon and evening. A Tornado Watch is in effect for
northern Missouri and west-central Illinois until 8pm this evening.

With the surface low quickly lifting northeast into the Great Lakes
region, the southern trailing end of the cold front struggles to
progress southeastward and should pivot clockwise, eventually
stalling near the I-70 corridor overnight. A secondary, more robust,
mid-level trough ejects into the Plains tomorrow leading to lee-side
surface cyclogenesis that will help to lift the quasi-stationary
front back to the north as warm front. While this front lifts back
to the north late tomorrow morning into the afternoon, scattered
showers and thunderstorms are possible (30-40%). The latest REFS has
joint probabilities of MUCAPE >1,000 J/kg and 0-6km shear up to 70%
along and just behind the warm front, which means the potential for
a scattered severe storm can't be ruled out in the afternoon,
generally along and north of the I-70 corridor.

A more widespread threat for severe weather will occur later on
Friday evening into early Friday night as a QLCS is expected to form
along a cold front in western Missouri and push east into the area.
Guidance has good consensus in the timing of the QLCS with arrival
in mid-missouri around 00-03Z and eastern Missouri/western Illinois
a few hours later. Confidence is higher in the severe threat across
northern and central Missouri, as the line is forecasted to move
through those areas when diurnally driven instability is still high.
As the QLCS and cold front push east later into the evening,
instability should be waning, which may limit the overall severe
threat across eastern Missouri and western Illinois. Therefore,
forecast uncertainty still exists with the eastward extent of the
severe threat for late on Friday. Given the linear nature of the
QLCS, the main hazards will be damaging winds and tornadoes, with a
lesser threat for hail.

Peine/Maples

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 252 PM CDT Thu Apr 2 2026

A cold front is currently projected to be moving through the
southeast quadrant of the CWA Saturday morning. Ongoing showers and
thunderstorms are expected to accompany the front with cooler, drier
air following closely behind. Before that occurs, anomalously high
PWATs (~1.5") precede the boundary, which may carry the potential
for locally heavy rainfall in/around ongoing convection. The upside
is that this will be during the period of diurnal minimums with
collapsing lapse rates and CAPE, signaling weakening trends are
favored. LREF spreads also show a narrow (50-60 miles) corridor of
roughly 5-10 degrees, providing pretty decent confidence in boundary
timing. While some shower potential hangs on to the southeast
fringes of the forecast area through early afternoon, the trend will
be toward gradual clearing and cooler/drier conditions through most
of Saturday. High temperatures are expected to be in the 50s/60s.

GEFS shows a tandem of upper ridges moving across the country
through next week. The first of the two moves over the mid-
Mississippi Valley Sunday into Monday. GFS/ECMWF ensembles show H8
temperatures (0-5C) tightly clustered (<5C degrees of spread)
between the 25th/75th IQR through 18z Monday. This gives higher than
normal confidence in a cooler, drier pattern through the weekend
into early next week. Mid-level temperatures land in the 30-35th
percentile of climatological means, leaning toward a seasonably cool
(slightly below normal) pattern through at least early Monday.

At the surface, ridging becomes less pronounced as a stronger
surface high builds over the northern Plains. Mid-level temperature
spreads increase Monday afternoon in response to the reinforcement
of the cooler airmass from the north. The depth and southward extent
of this reinforcement is responsible for the spread at this point in
the long range period. The main concern is how cold it gets with
deterministic guidance hugging the freezing mark Tuesday morning.
This will be something to keep an eye on for agricultural interests
with near/slightly below freezing temperatures for morning lows.
Potential is greatest north of I-70, where LREF shows probabilities
for 32F or lower around 15% along I-70 climbing to near 40% around
Quincy, IL.

Spreads increases (10-15F degrees) Wednesday onward as high pressure
moves east and return flow draws warmer air northward around the
more persistent synoptic ridging over the southeast CONUS.
Considering this setup, most convective behavior is steered through
the central Plains with the main question stemming from temperature
trends. While there is decent confidence in a warming trend, the
magnitude of warming ranges between near normal (60s) to above
normal (80s) Wednesday for highs. While similar spreads persist into
Thursday, the IQR collectively warms with above normal temperatures
favored. There is less dispersion in overnight lows (50s) with
little concern at all for freezing temperatures.

Maples

&&

.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 655 PM CDT Thu Apr 2 2026

Current coverage of showers and thunderstorms is limited to only a
few small rain showers across eastern Missouri. Scattered pockets of
agitated cumulus continue to filter northeast into eastern Missouri
with CAMs still showing scattered thunderstorm development along the
cold front later this evening. Therefore, left the PROB30 for
thunderstorms at the St. Louis metro terminals. Widespread MVFR
stratus is across northwestern Missouri with some model guidance
moving it easterly and clipping KUIN later this evening. Expectation
is that this MVFR status field stays to the north of KUIN, so no
mention was included in the TAF, but this will be monitored closely.

Southerly winds will veer westerly as the cold front passes from
west to east this evening with the front stalling near the I-70
corridor overnight, leading to calm and variable winds. The front
lifts back to the north around sunrise, bringing a brief period of
MVFR ceilings from the south before VFR conditions return along with
strengthening southerly winds.

Peine/Maples

&&

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

WFO LSX

Hazardous Weather Outlook
National Weather Service St Louis MO
323 PM CDT Thu Apr 2 2026

ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027-
034>036-041-042-047>052-059>065-072>075-084-085-099-032030-
Greene IL-Macoupin IL-Montgomery IL-Bond IL-Fayette IL-Clinton IL-
Marion IL-Washington IL-Randolph IL-Adams IL-Brown IL-Pike IL-
Calhoun IL-Jersey IL-Madison IL-Saint Clair IL-Monroe IL-Knox MO-
Lewis MO-Shelby MO-Marion MO-Monroe MO-Ralls MO-Pike MO-Boone MO-
Audrain MO-Moniteau MO-Cole MO-Osage MO-Callaway MO-Montgomery MO-
Lincoln MO-Gasconade MO-Warren MO-Saint Charles MO-Franklin MO-
Saint Louis MO-Saint Louis City MO-Jefferson MO-Crawford MO-
Washington MO-Saint Francois MO-Sainte Genevieve MO-Iron MO-
Madison MO-Reynolds MO-
323 PM CDT Thu Apr 2 2026

This Hazardous Weather Outlook is for portions of eastern and
central Missouri as well as west central and southwest Illinois.

.DAY ONE...Tonight.

There is the potential for severe weather in northeast Missouri and
west central Illinois this afternoon through early evening. All
hazards are possible.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

There is a chance of severe thunderstorms Friday evening. Damaging
winds and tornadoes will be possible.

There is a chance of showers and thunderstorms early on Saturday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

Peine