938
FXUS63 KLSX 041301 CCA
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
642 AM CDT Fri Apr 4 2025

.KEY MESSAGES...

- A few thunderstorms could be severe this afternoon and evening.
  Large hail is the primary threat, but there is a conditional
  threat of tornadoes and damaging winds in southeastern MO and
  southwestern IL.

- Confidence is high in flash flooding across portions of the
  Flood Watch area this evening into early overnight from
  thunderstorms with heavy rainfall. Moderate to major river flood
  is also possible in response to heavy rainfall amounts.

- Quieter weather is expected early next week with freezes likely
  Monday and Tuesday mornings.


&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 412 AM CDT Fri Apr 4 2025

Lingering showers across southwestern IL continue to decrease in
coverage as the CWA reaches a lull in shower coverage, owing to
weakly rising mid-level heights and weakening forcing. This lull
will be temporary as numerous showers and scattered thunderstorms
spread quickly back northward into the CWA during mid to late
morning as low to mid-level WAA and moisture transport increase with
the passage of a mid-level perturbation in southwesterly flow. As
additional perturbations arrive and the CWA becomes positioned in
the equatorward entrance region of an upper-level jet streak,
showers and thunderstorms will peak in both coverage and intensity
during the afternoon and evening, as instability is also able to
advect northward. Most of this instability will be elevated, but
MUCAPE of 1000 to 2000 J/kg and strong effective wind shear will be
supportive of at least supercells capable of large hail. Short-term
model guidance are in agreement that a surface front will advance
northward into southern MO during the evening but disagree on if it
will advance far enough northward to reach southeastern MO and
southwestern IL counties in the CWA before it is impeded by
widespread showers and thunderstorms. This front will delineate the
northern extent of strong surface-based instability and, therefore,
supercells and/or QLCS segments capable of tornadoes and damaging
winds with very strong and favorable low-level wind
shear/hodographs. Therefore, even slight northward adjustments in
the front`s forecast position will sharply increase the threat of
tornadoes and damaging winds in southeastern MO and southwestern IL.

In addition to potential severe thunderstorms, the pattern will be
favorable for heavy rainfall and flash flooding this evening into
overnight. Efficient warm rain processes are expected with deep warm
cloud depths, skinny/thin instability profiles, and precipitable
water exceeding 1.5", nearing the maximum climatological values for
this time of year. These processes along with a tendency for
thunderstorms to train due to a large parallel component of deep-
layer flow with linear forcing leading will support periods of
sustained high rainfall rates. The latest HREF LPMM indicates swaths
of 2 to 4" along and southeast of I-44 in MO and I-55 in IL, but
given how favorable the pattern is, localized amounts reaching 6"
(99th percentile) seem attainable. Since soils have already been
moistening and 3-hour FFG is 1.5 to 2.5", flash flooding is likely
in portions of the aforementioned areas. Fortunately, CAMs indicate
that the axis (or axes) of training thunderstorms will shift
southeastward with time, eventually out of the CWA by around the
early overnight hours. In fact, most showers and thunderstorms are
expected to become confined to southeastern MO/southwestern IL by
early Saturday morning.

Yet another mid-level perturbation will drive another wave of
precipitation on Saturday, however, conditions will be much less
favorable for heavy rainfall with lower instability and precipitable
water resulting in rain largely stratiform in nature but widespread
for a period late Saturday morning into afternoon beneath another
equator jet streak entrance region. This rainfall should not
exacerbate any flooding issues but could stall improvements. With
low-level CAA and widespread cloud cover and precipitation,
temperatures will remain in the 40s and 50s F on Saturday.

Pfahler

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 412 AM CDT Fri Apr 4 2025

Late Saturday night into Sunday, the main upper-level trough is
anticipated to eject northeastward from the Southern Plains to the
Mid-Mississippi River Valley but while becoming deformed and
weakening as it encounters increasingly confluent flow. This
evolution should only allow light precipitation to spread back into
the CWA on Sunday, but due to differences in how quickly the trough
dissipates, model guidance varies on how long precipitation persists
on Sunday. Most of this precipitation will be light rain but around
30 to 50 percent of ensemble model membership also have rain mixing
with light snow in central and east-central MO. Since temperatures
will be above 32 F and precipitation rates will be light, no impacts
are expected and it will likely be difficult for any accumulations
to occur.

Much quieter and dry conditions are expected early next week as upper-
level northwesterly flow dominates the Central CONUS in response to
a potent trough crossing the Great Lakes and Northeast. Glancing
bouts of seasonably cold air will lead to below average
temperatures. NBM probabilities of low temperatures less than 32 F
are 50 to 80 percent across much of the CWA Monday morning and
Tuesday morning, increasing confidence in freezes threatening
sensitive plants with the recent onset of the growing season.

There is a signal for an upper-level trough to provide the next
opportunity for rain sometime in the Wednesday-Thursday timeframe.
However, with limited moisture return and progressive nature of the
trough, most ensemble model membership keep rainfall amounts 0.25"
or less.

Pfahler

&&

.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 642 AM CDT Fri Apr 4 2025

Showers and lower ceilings are expected to develop and spread into
the region this morning into early afternoon, leading to MVFR to IFR
flight conditions at times. The heaviest rain, including possibly
thunderstorms, is expected this afternoon and evening primarily at
KCOU, KJEF, and St. Louis metro terminals. A period of dry
conditions will take place overnight into Saturday morning as
showers and thunderstorms shift to the east, but it is uncertain
how long IFR ceilings will persist thereafter. Another round of
lighter rain will move into the area mid/late Saturday morning with
at least MVFR flight conditions. Winds will remain easterly through
this evening before shifting to northerly early Saturday morning.

Pfahler

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch through late Saturday night for Crawford MO-Franklin
     MO-Iron MO-Jefferson MO-Madison MO-Reynolds MO-Saint
     Charles MO-Saint Francois MO-Saint Louis City MO-Saint
     Louis MO-Sainte Genevieve MO-Washington MO.

IL...Flood Watch through late Saturday night for Bond IL-Clinton IL-
     Fayette IL-Jersey IL-Macoupin IL-Madison IL-Marion IL-
     Monroe IL-Montgomery IL-Randolph IL-Saint Clair IL-
     Washington IL.

&&

$$

WFO LSX