137
FXUS63 KLSX 162229
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
529 PM CDT Wed Apr 16 2025

.KEY MESSAGES...

- An active weather pattern will commence tonight and continue
  through Sunday featuring multiple rounds of showers and
  thunderstorms.

- A very low threat of severe storms will exist across far
  northeast Missouri and west-central Illinois on Thursday night,
  with a greater albeit still low threat of severe storms across
  the entire area late Friday afternoon and evening.

- There is an increasing threat of heavy rain from Friday night
  into early Sunday with total amounts of 3-5 inches coming into
  focus centered near the I-44 corridor. This rainfall would
  bring at least a low flash flood threat but a greater threat of
  river flooding.

- A conditional threat of severe storms exists on Sunday, highly
  dependent on the track of a low pressure storm system moving
  across the area.



&&

.SHORT TERM...  (Through Late Thursday Night)
Issued at 227 PM CDT Wed Apr 16 2025

An uptick in the coverage of showers and perhaps a thunderstorm will
occur this evening within a broad north-south zone centered through
eastern MO. This will be in response to a weak mid-level
perturbation tracking across the area, and more importantly,
intensifying low- level WAA. This region of precipitation will
persist into the overnight hours and shift E/NE into IL with
time. Overnight and primarily after 09-10z a separate area of more
organized showers and thunderstorms will spread/develop into
central MO attendant with a more well-defined mid-level impulse
along with a veering and increasing southwesterly LLJ and
associated deeper moisture transport/lift and instability.

Showers and thunderstorms are expected to be ongoing on Thursday
morning, mainly centered west of the MS River. This activity should
shift east of the region by early Thursday afternoon in association
with the eastward migration of the mid-level disturbance, leaving
much of the afternoon void of precipitation. Temperatures should
warm nicely in the wake of the morning precipitation as skies
clear and WAA increases.

Present indications are there will be another threat of showers and
thunderstorms on Thursday night. Warming aloft, mid-level height
rises, and advection of the low-level moisture pool to the northeast
would suggest this threat is limited in both coverage and area and
largely confined to NE MO/WC IL. Better forcing and instability is
forecast further north into IA, however weak but sufficient
MUCAPE and steep mid-level lapse rates would support a small hail
threat with any elevated storms which may form, a low chance of
possibly marginally severe hail with the strongest of storms.

Glass

&&

.LONG TERM...  (Friday through Next Wednesday)
Issued at 227 PM CDT Wed Apr 16 2025

The weather/pattern will become increasingly active Friday
through the weekend. Confidence is low on the severe weather
threat with numerous caveats, however the threat of heavy
rainfall continues to increase.

The jury is still out on the convective evolution and trends on
Friday and Friday Night, but there is an increasing amount of data
suggesting that Friday itself will be largely void of surface-
based convection due to the presence of a stout EML/strong capping
inversion through the daylight hours throughout the warm sector
ahead of the advancing cold front. Like discussed yesterday the
more plausible solution appears to be that height falls/cooling
aloft on Friday evening (and perhaps some cooling from mid-level
showers) will help create a more favorable environment for
thunderstorms along and in the wake of the advancing cold front as
forcing with the upper system and LLJ increases. In this
scenario, thunderstorms are largely rooted above the boundary
layer and a primary severe threat would be large hail and to a
lesser extent strong winds. Even with any storms that might
manage to from along the front itself, the flow is quite veered
leading to unfavorable hodographs for a tornado threat.

An increasing threat of heavy rain will unfold later Friday night
into Sunday as the cold front moves southward and eventually stalls
across SE MO/SRN IL with a slow-moving deep upper trof upstream. The
evolving pattern closely resembles the hybrid synoptic-frontal
pattern for heavy rain recently highlighted in Gerhardt (2024)
supporting multiple waves of showers and thunderstorms and training.
The location and axis of the heaviest rainfall and overall extent of
the rainfall footprint has been nudging northward and there is
normal uncertainty still in the exact location. However the AXIS
of greatest rainfall appears to be setting up near or just north
of the I-44 corridor. The latest probabilities of rainfall in
excess of 3 inches have increased to 50-70 percent. The 100-member
LREF shows the total QPF distribution from the 90th percentile in
the 4-5+ inch range, which largely supports our new messaging of
3-5 inches of rainfall. If these rainfall amounts materialize,
then the most likely impacts would be AT LEAST minor river flooding
on the Meramec, Big, and Bourbeuse, and Black Rivers/Basins and
other small river systems in central and east-central MO. The
flash flood threat will be dependent on high rainfall rates, with
at least a higher threat of nuisance flooding.

We will also need to keep a close eye on the potential for severe
storms. Perhaps immediately along the stalled front across SE MO/SRN
IL Saturday afternoon/eve. Any threat on Sunday will be dependent
on the eventual track of the surface low and position of the warm
front associated with the ejecting upper trof. The models
continue to diverge on specifics with the timing/location of the
upper trof and surface systems. The more northern track shown in
the operational ECMWF showing a negatively tilted trof, bigger
warm sector and more favorable instability would support greater
severe thunderstorm potential, both in coverage and the magnitude
of threats.

Glass

&&

.AVIATION...  (For the 00z TAFs through 00z Thursday Evening)
Issued at 524 PM CDT Wed Apr 16 2025

VFR conditions are expected for the remainder of the evening.
Overnight through tomorrow morning, waves of showers are expected
to affect all terminals, with the most prolonged period of showers
likely at St. Louis area terminals (roughly 05 to 16Z). A few
thunderstorms will also be possible at all terminals, but
confidence is low in terminal impacts due to sparse coverage.
Ceilings are also likely to lower overnight through tomorrow
morning, perhaps to MVFR. Category reductions due to ceilings are
most likely at UIN. Meanwhile, breezy south winds are expected
from late this evening through the end of the period. Low clouds
are expected to gradually diminish through the day tomorrow, and
most sites should end the period with VFR conditions.

BRC

&&

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

$$

WFO LSX