213
FXUS63 KLSX 191151
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
651 AM CDT Sat Apr 19 2025

.KEY MESSAGES...

- Widespread showers and thunderstorms continue today with
  additional chances Sunday. Some rain may be heavy at times,
  resulting in ponding of flood prone areas and localized flash
  flooding. Locally up to 5" is possible within the Flood Watch.

- Thunderstorms late Sunday afternoon into Sunday night could be
  strong to severe. There remains some uncertainty where
  thunderstorms will be most intense. All hazards will be
  possible.

- Dry conditions start off next week with near normal temperatures
  (mid-60s/mid-70s) rising back above normal Tuesday. The pattern
  becomes active once again around midweek.


&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 331 AM CDT Sat Apr 19 2025

The Midwest is sandwiched between ridging over the southeastern U.S.
and an amplified, positively tilted trough that extends from the
north-central Plains into the Desert Southwest. This setup sets the
stage for multiple waves of showers and thunderstorms that will
impact much of the weekend.

The leading edge of the showers and thunderstorms has moved into the
mid-Mississippi Valley this morning. Region IR satellite shows a
mass of colder cloud tops from numerous thunderstorms that extend
southwestward from southern Illinois into central Texas. As of 06z,
surface analysis depicts a cold front lined northeast to southwest,
pushing through the middle section of the forecast area. Along and
immediately ahead of the front, thunderstorms are tapping into
MUCAPE values around 1000 J/kg and 0-6 km shear of 60-70 kts per SPC
mesoanalysis. As the front continue southward early this morning, a
couple of thunderstorms may become strong to severe with damaging
wind and potentially a tornado. Hail is a lower threat at this point
considering marginal mid-level lapse rates (5.5-6C) beginning to
trail off. Thunderstorm intensity is expected to wane through the
next couple of hours as the front moves south and instability
subsides with time.

Radar mosaics show additional showers and a few embedded
thunderstorms extending from central Missouri into west-central
Illinois. This activity is post-frontal and elevated in nature.
Additionally, showers and thunderstorms lack the same support that
resides further south ahead of the front. While this activity does
not pose a severe threat, attention shifts to flooding potential as
rainfall amounts continue to add up through the event.

A train of 500 mb vort lobes are ejected out ahead of the amplified
trough and around the northwestern edge of the ridge through at
least early this afternoon. Showers and thunderstorms will continue
to drive over the region with pockets of moderate to heavy rainfall
possible. Saturation extends from the surface through 500 mb with
PWATs generally ranging from 1.3-1.5 inches early this morning.
However, the persistence of the low level jet, duration of parallel
flow in relation to the surface front and varying warm cloud depth
could pose some challenge to overall rainfall performance. While
these indicators initially support training, heavy rainfall, they
either do not persist in perfect harmony throughout the period.
There are indications there will be weakening of the low level jet.
Additionally, as the front continues southward, PWATs that are
nearly 150% of mean values (0.50") drop off slightly through the
course of the day. This suggests at least some downward trend late
tonight, providing some relief from more persistent, heavier
rainfall we are experiencing this morning through early afternoon.
As surfaces become saturated, the threat for flooding will increase
with the highest confidence in river flooding. Flash flooding, while
possible, likely remains localized with widespread rainfall totals
of 2-4" and localized amount around 5" (75th percentile) with the
highest amounts along the I-44 corridor.
.

The upper trough begin to progress eastward out of the southwestern
CONUS tonight into Sunday. Surface low pressure develops over the
southern planes and quickly deeps as it takes a turn to the
northeast. As the surface low tracks northeast from central Oklahoma
into eastern Kansas tonight, it begin to lift the stalled surface
boundary back to the north as a warm front with showers and embedded
thunderstorms. Fortunately, better instability remains further
south, keeping thunderstorms sub-severe. The main question will be
the timing with some minor differences in position and northward
speed of the front. Should the quicker solution come to fruition, we
may get into a lull ahead of the cold front from mid morning through
early afternoon, but confidence low that we have any completely rain-
free period.

A cold front that extends south of the system moves into western
Missouri Sunday afternoon. GFS seems to be the quicker outlier here
with the ECM and short range guidance further west with convective
initiation. Much of the guidance is favoring a low CAPE, high shear
environment with LREF MUCAPE means ranging 500-800 J/kg and 0-6 km
shear of around 60 knots. Latest NAM guidance is somewhat concerning
with a narrow corridor of SBCAPE climbing to around 1500 J/kg
immediately along and ahead of the cold front. There is quite a bit
of uncertainty with instability with the abundance of cloud cover
that will have to clear out to allow instability to build. Even with
lower CAPE values, 0-3 km shear of 30-35 knots, SRH approaching 250
m2/s2, and curved hodographs would suggest the potential for
embedded vorticity along a narrow like of strong to severe
thunderstorms. Mid-level lapse rates initially approach 7C where
thunderstorms initiate, supporting better potential for hail,
leaving all hazards on the table. As thunderstorms become linear
through late evening, damaging winds and tornadoes likely become the
main threat over eastern Missouri into Illinois.


Maples

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 331 AM CDT Sat Apr 19 2025

Pending the speed of Sunday`s cold front, showers and thunderstorms
could linger into late Sunday east of the Mississippi River. Some of
the thunderstorms could be strong to severe, but will likely be in
their weakening phase as they push east of the region Monday
morning. Temperatures cool back to near normal (mid-60s/mid-70s)
Monday as high pressure builds in from west to east.

Quasi-zonal upper level flow starts off the week with ridging
encompassing the southeastern U.S. The ridge migrates slightly
eastward Tuesday with return flow resulting in warm air advection
and temperatures that rise back above normal as 850 mb temperatures
of 10-15C are drawn northward out of the southern Plains.

The upper level pattern remains relatively flat and void of any real
amplified systems through the remainder of the week with a couple of
quick-moving shortwaves traversing the central U.S. Active weather
could return sometime around midweek, but the exact track and the
shortwaves and corresponding surface systems is not certain and will
affect timing and rainfall potential from midweek onward.


Maples

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 642 AM CDT Sat Apr 19 2025

A cold front has shifted south of the terminals with widespread
showers and thunderstorms impacting all but KUIN. KUIN will likely
bounce between showers, while central Missouri and metro terminals
remain in the active beltway for waves of activity to pass
overhead through tonight.

Showers will occasionally be accompanied by thunder at times today
with the number of thunderstorms decreasing as the boundary sinks
further south. It`s difficult to pinpoint exactly when any given
terminal will be affected by a thunderstorm in particular
considering the long duration of the event. Therefore, tsra
mention was placed at points potential is highest with the caveat
that thunderstorms will accompany showers at any time through the
period.

Site will bounce between categories with IFR/MVFR becoming more
common this morning through early Sunday morning. By Sunday
morning, a warm front lifts northward through the region with
additional convection possible.

Maples

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch through late Sunday night for Audrain MO-Boone MO-
     Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Lincoln MO-Madison MO-Moniteau MO-
     Montgomery MO-Osage MO-Pike MO-Reynolds MO-Saint Charles MO-
     Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte
     Genevieve MO-Warren MO-Washington MO.

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

&&

$$

WFO LSX