158
FXUS63 KLSX 201103
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
603 AM CDT Thu Mar 20 2025

.KEY MESSAGES...

- Elevated fire danger is expected to persist in southeast Missouri
  today, expanding to the whole area for Friday afternoon.

- Widespread beneficial rainfall is forecast Saturday night into
  Sunday, with probabilities of at least a quarter inch of rain
  ranging from 50-80% across the area.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 313 AM CDT Thu Mar 20 2025

The cold front has moved through the remainder of the CWA, with
much cooler air continuing to infiltrate the region. Regional
radar shows some light precipitation continuing across the area
beneath an area of low stratus. Observations show that this
precipitation is very light, and the expectation is chances for
anything measurable is very low. Most of the precipitation should
be flurries given very low wetbulb zero heights, but some
sprinkles are possible in parts of east central and southeast
Missouri as well as southwest Illinois especially where surface
temperatures and associated wetbulb zero heights are higher. In
these locations, the sprinkles/drizzle should end as flurries
early this morning.

Any flurries or sprinkles should quickly exit the area by mid
morning, with decreasing cloudiness for the remainder of the day. A
vast majority of the area should experience plenty of sunshine, but
it still will be pretty chilly for mid/late March. Temperatures
aloft at 850 hPa are in the -5 to -7C range, which are at or below
the 10th percentile of climatology. Highs in the mid 40s to low 50s
are forecast for this afternoon. It will feel colder than that too
given brisk northwest winds. These winds will subside gradually
however throughout the day as a weak ridge of surface high pressure
quickly approaches from the west.

The surface ridge axis is expected to slide into the mid south/Ohio
Valley tonight, with light southerly return flow commencing in its
wake. Lows tonight are forecast to be in the low to mid 30s, with
the warmest conditions in parts of central and northeast Missouri
where southerly wind speeds will be a bit stronger. The exception is
in favored valleys in portions of east central and southeast
Missouri. In these locations, lows in the upper 20s are forecast.
Model guidance has a very consistent warm bias in these retreating
surface high scenarios in/near the eastern Ozarks. Some mid 20s
cannot be ruled out either given dewpoints in the low 20s tonight.

A quick warmup will occur on Friday as southerly winds increase and
veer a bit to the southwest ahead of an approaching cold front.
Highs in the low to mid 60s are forecast, and only look to be
limited by an increase in midlevel cloudiness. Wind gusts of 30-40+
mph are forecast, with the highest speeds in parts of northeast
Missouri and west-central Illinois early-mid afternoon. Model
soundings show speeds of 45-50 knots at the top of the boundary
layer, so cannot rule out at least isolated advisory criteria gusts
(39+ knots). What is more certain is another day of elevated fire
danger across the whole area due to the combination of the
aforementioned gusty southerly winds, low relative humidity values,
and dry fuels.


Gosselin

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 313 AM CDT Thu Mar 20 2025

(Friday Night - Saturday)

A cold front will move through the bi-state area Friday night, with
slightly cooler air moving in behind the boundary. Chances of rain
appear pretty low with this feature, due to weak moisture return and
the stronger mid/upper level forcing for ascent staying to our
northeast. LREF probabilities are now down to 10-20% for any
measurable rainfall, and even that is confined to western Illinois.
These chances have dropped about 15-20% compared to 24 hours ago.
Given these trends, PoPs were removed Friday night.


(Saturday Night - Sunday)

Another midlevel shortwave trough will be quickly on its
predecessor`s heels. This system continues to have a much stronger
signal of widespread, beneficial rainfall across the area.
Deterministic model guidance shows increasing diffluence ahead of
the midlevel shortwave with strong low-level moisture advection just
north of a surface warm front. Widespread rain with embedded
(elevated) convection looks like a good bet Saturday night. At the
moment, the southern half of the area seems favored for the heavier
rainfall totals. LREF odds for at least 0.25" of rain range from 50-
80%, with the highest chance in southeast Missouri and southwest
Illinois. Chances for at least 0.50" drop off, but still are in the
30-45% range. Given the dryness and dry fuels, this rainfall would
be much appreciated and much needed.


(Sunday Night - Wednesday)

Ensemble guidance remains in good agreement with respect to the
mid/upper level flow pattern over the CONUS. A ridge is forecast to
be located near the west coast, with troughing centered across
the Great Lakes/southeast Canada. That leaves northwest flow aloft
across the mid-Mississippi Valley. This should overall be a more
tranquil, quiet weather pattern with dry weather favored.
Temperatures will be the primary concern, with a fair amount of
uncertainty in both highs and lows due to our area being between
the anomalous warmth to our west and anomalous cold centered to
our east. Spread between the 25th/75th percentile on the NBM
ranges from 10-15 degrees for afternoon highs, and 5-10 degrees
for overnight lows.


Gosselin

&&

.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 600 AM CDT Thu Mar 20 2025

MVFR ceilings will advect out of the metro St. Louis terminals
over the next couple of hours, and out of southeast Missouri and
southwest Illinois by mid/late morning. A mostly clear sky is
forecast thereafter, other than FEW-SCT cirrus. Northwest wind
gusts will continue through the morning, but gusts should diminish
from west to east this afternoon. Winds should become
light/variable this evening as a surface ridge of high pressure
moves across the region. Behind this surface ridge, look for
winds to turn out of the south.

Gosselin

&&

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

$$

WFO LSX