Area Forecast Discussion

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FXUS63 KLSX 012358
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
658 PM CDT Tue Apr 1 2025

.KEY MESSAGES...

- Strong to severe thunderstorms are possible after midnight tonight,
  primarily across parts of central and northeast Missouri.
  Severe thunderstorms are more likely on Wednesday afternoon,
  especially in southeast Missouri and southwest Illinois.

- Periods of heavy rain are likely Thursday through Saturday. The threat
  of moderate to major river flooding is becoming more likely
  across parts of southeast Missouri and southern Illinois.
  Significant flash flooding will also be possible in these areas.
  A flood watch has been issued Thursday - Saturday.

- Gusty southerly winds are forecast on Wednesday afternoon, with
  peak gusts of 40-45 mph expected.


&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 230 PM CDT Tue Apr 1 2025

A very busy and complex forecast exists over the next 24-36
hours, and really beyond. A subtle surface warm front will push
north of the area during the early overnight hours. Some scattered
showers and thunderstorms are expected, mainly in northeast
Missouri and west-central Illinois. Some small hail is possible,
but there doesn`t appear to be enough instability for marginally
severe (~1" in diameter) hail. There should then be a several
hour break, with dry weather then for most of the rest of the
night. A line of thunderstorms is forecast then to move
eastward into northeast Missouri just before 1200 UTC. These
storms should be on a weakening trend, due to timing (less
instability) and working into a less favorable environment
overall. Storms also should be elevated in nature, making it more
difficult to mix severe winds down to the surface unless they
remain more organized/robust than currently anticipated.
Marginally severe hail is the main threat with any deeper cores,
followed by damaging wind gusts to 60 mph.

The expectation is for continued slow, but steady weakening of these
storms as they gradually push eastward. However, I do think they
will not completely dissipate due to a continued strong (50+ knot)
low-level jet. The minimum in activity is likely to be around
noon, as a vorticity maxima ejects into the Upper Midwest and its
influences leave our area.

Convective coverage should increase again by early afternoon as
cooling midlevel temperatures and slightly stronger low-level
moisture convergence combine to develop thunderstorms. There is some
capping in place and not exactly the strongest low-level convergence
along the Pacific cold front, nor any pronounced mid/upper level
convergence. All that being said, all CAMs and deterministic
guidance show a nice uptick in coverage of storms. The cooling
temperatures aloft, low-level moisture convergence, and increasing
diurnal instability however should be enough even given the
limitations detailed above. Storms will likely get going near the
I-44/I-55 corridors in Missouri and Illinois respectively by early
afternoon. The exact axis isn`t a certainty however, and will be
dependent on how far east the morning convection gets before
nearly dissipating. If storms get further east, the area
threatened by severe weather Wednesday afternoon will be mostly
confined to parts of southeast Missouri and southwest Illinois.
However, storms weakening quicker and further to the west would
lead to a larger footprint where severe weather may threaten.

Very strong (50-70 knots) deep-layer shear remains a given, and at
least pockets of moderate (750-1000 J/kg) surface-based instability
are likely. This is of course a very favorable thermodynamic environment
for organized strong to severe thunderstorms. While at least some
severe weather in southeast Missouri and southwest Illinois is
all but certain, there remains low confidence in how significant
the severe weather will be in our area. There are a lot of
similarities to Sunday`s event, though with higher magnitudes of
both deep-layer and low-level shear. Forecast hodographs are
mostly straight, with near unidirectional flow in the lowest 3 km.
There is quite a bit of speed shear however in the low levels,
with 0-1 km shear expected to be around 20-25 knots. Straight
hodographs, with even some anticyclonic curvature aloft, suggests
the potential for splitting supercells, with perhaps even a few
dominant left movers. The deep-layer shear vector is also nearly
parallel to the frontal boundary, which means updrafts may tend to
get cut off by the progression of the actual front itself. Right-
movers would have a better probability of deviating off of the
front, and would have the highest probability of being severe, and
potentially with very large (2+") hail and tornadoes. Large hail
looks to be the main threat overall, though the magnitude of the
0-1 km shear is certainly enough for tornadic potential,
especially if there is a remnant outflow boundary around (which
could locally enhance SRH). Tornadic potential is limited by
relatively high LCLs and lack of greater cyclonic curvature
evident on hodographs (low SRH). The parameter space for tornadoes
(and some potentially strong/EF2+) does improve later on in time
Wednesday evening, but storms are expected to be out of our area
around or just after 0000 UTC Thursday.


Gosselin

&&

.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 230 PM CDT Tue Apr 1 2025

(Wednesday Night - Saturday Night)

A relative quiet period for the area looks in store for most of
Wednesday night as storms exit the area near or just after 0000 UTC
Thursday and the Pacific cold front progresses southeast of the
area. This front will get hung up and become quasi-stationary
Thursday morning. Another midlevel shortwave and some increasing low-
level moisture convergence should spread in some widespread rain (and
perhaps a few rumbles of thunder) to much of the region on
Thursday. This round of rain does not look particularly heavy,
with only modest low-level moisture convergence and very limited
instability aloft. However, some moderate rain could fall in parts
of southeast Missouri and southwest Illinois during this time
period. Given what should be more saturated soils after
Wednesday`s rain, will go ahead and start a flood watch Thursday
morning.

Another relative "lull" is expected Thursday night, though continued
showers and elevated convection are expected in/near the stalled
frontal boundary. This means parts of southeast Missouri and
southwest Illinois may continue to see showers and thunderstorms,
further saturating soils and sending creeks/streams upwards. The
time period of biggest concern is Friday night through Saturday
afternoon. This is when deterministic guidance shows the strongest
low-level moisture convergence, with stronger mid/upper level
forcing for ascent downstream of the approaching shortwave. In
addition, the strongest moisture advection is nearly stationary,
suggesting a very prolonged period of potentially heavy rainfall.
Very strong integrated water vapor transport (>95th percentile),
deep warm cloud depths, and anomalous precipitable water values
(1.50"+ - >99th percentile of climatology), and tall/skinny elevated
CAPE all suggest efficient rainfall processes. This is a long way of
saying there is increasing concern for potentially historic rainfall
amounts over a 3-5 day time period in our southeast Missouri
counties. Probabilities for at least 5" of rain by Saturday night
are 70-80% from the LREF, with a 20-30% chance of at least 8". If
there is a lot of training between Friday night and Saturday, I
would not be surprised to see some 12-15+" totals somewhere. The
most favored axis for the heaviest rainfall amounts is just to our
southeast, but it is close and a slight shift northwest could
bring these types of amounts into parts of Reynolds/Iron/Madison
counties in a worst case scenario. Very significant flash
flooding is possible, with widespread moderate to major flooding
along rivers and streams in southeast Missouri including the
Black, St. Francois, and Meramec basins.

Some light stratiform rain is expected Saturday afternoon/night (and
possibly into Sunday morning) with the main shortwave. This feature
gets sheared out however due to strong mid/upper level confluence
across the Upper Midwest/Great Lakes. This type of rain won`t
exactly be helping things, but should be light and devoid of
convection. In other words, this rain won`t make things worse and
should have little/no impact on river crests. Temperatures by
this time will also cool into the 40s for a lot of the area, so a
pretty chilly rain looks in store for early April.


(Sunday - Next Tuesday)

A welcomed quiet period of weather still is in the offing to end the
weekend heading into next work week. Both the GEFS/EPS agree that
anomalous mid/upper level troughing is expected to dig into the
Great Lakes, with 850-hPa temperature anomalies of -8 to -12C. The
ECMWF/EPS is more noteworthy with this push of cold, and would
suggest not only a freeze but a hard freeze (

NWS LSX Office Area Forecast Discussion