819
FXUS63 KLSX 031925
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
225 PM CDT Thu Apr 3 2025

.KEY MESSAGES...

- A Flood Watch remains in effect through Saturday night due to multiple
  rounds of rain and thunderstorms. The heaviest rainfall is
  expected Friday afternoon into Friday night.

- Severe thunderstorms are possible Friday afternoon and evening,
  with large hail the primary threat.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 225 PM CDT Thu Apr 3 2025

Widespread rain along with a few embedded thunderstorms has
impacted much of the region this afternoon, with exceptions of
parts of northeast Missouri and west-central Illinois. This round
of rainfall is mainly due to a midlevel disturbance moving through
along with enhanced divergence aloft beneath the right-entrance
region of a 130+ knot jet streak.

The rain should become less widespread later on this evening as the
stronger mid/upper level forcing for ascent departs to the
northeast. However, did keep some low PoPs (20-40%) going most of
the night in parts of southeast Missouri and southwest Illinois
closer to the stalled surface frontal boundary.

By late Friday morning, another midlevel impulse is expected to move
quickly northeast across the mid-Mississippi Valley. More widespread
showers and a few rumbles of thunder are forecast. Low-level
moisture convergence intensifies by early afternoon, so we are
expecting even greater coverage of rain showers along with a better
chance of thunderstorms. This activity should be all elevated, but
there is enough effective shear for elevated supercells which would
be capable of producing large hail. The surface quasi-stationary
front is expected to buckle at least slightly northward into the
early evening hours. How far north it gets remains a question mark,
but may at least approach our far southeast Missouri counties
(Reynolds/Iron/Madison). Some of the extended HRRR/RAP and even the
new 1200 UTC NAM backed off earlier runs which had the boundary
quite a bit further to the northwest. With convection along/north of
the boundary and a lack of strong surface cyclogenesis, think that
the boundary should stay to our south and east. This is something we
will continue to keep a close eye on however as any shift to the
northwest would introduce the threat for surface-based convection in
parts of southeast Missouri. Surface-based convection would have the
threat for damaging winds and tornadoes, especially given very
strong low-level shear and helicity in close proximity to the front
itself. In terms of the large hail threat, that should be confined
to east central and southeast Missouri as well as southwest Illinois.

In addition to the severe threat, the best flash threat looks to be
Friday evening into the early overnight hours. This is when the low-
level moisture convergence maximizes and showers and elevated
convection may tend to train over the same areas. The atmosphere
will also be primed for efficient, warm rainfall processes due to
deep warm cloud depths, tall/thin elevated instability, and
anomalous precipitable water values (>1.50" - near 99th percentile
of climatology). The axis of this period of heaviest rainfall rates
is most likely along and just southeast of I-44 in Missouri and I-55
in Illinois. Rainfall amounts of 2-3+" are likely in this time
period, and given fairly saturated soils, this is certainly the
period of highest concern regarding flash flooding.


Gosselin

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 225 PM CDT Thu Apr 3 2025

(Saturday - Sunday)

Rain should continue into early Saturday, mainly in southeast
Missouri and southwest Illinois. The good news is that rainfall
rates should really be on the downturn as the precipitation looks to
be more stratiform vs. convective. Another (weaker) wave moves along
the surface front, bringing more widespread stratiform rain a bit
further to the north during the day. Saturday`s round of rain looks
an awful lot like this afternoon`s. The bad news with that is the
same areas that don`t need any more rain will get more rain. The
good news is that rainfall rates should be low, with little to no
convection expected. One difference is that it will be cooler, with
some areas struggling to hit the 50 degree mark.

Steady light rain should exit most of the area late Saturday night,
and then we watch the parent mid/upper level trough incoming from
the west. There is still some uncertainty with the track/strength of
this feature, but the operational GFS remains a strong outlier.
Given the synoptic setup depicting strong mid/upper level confluence
downstream across the Great Lakes, this system really should get
sheared apart and weaken. Therefore, the expectation is just for
some lingering very light rain to continue across southern portions
of the forecast area into the day on Sunday. In the unlikely event
the stronger GFS is closer to being correct, a bit more rain would
occur with even a chance for some of the rain to mix with wet snow
Sunday morning.

Sunday will be a pretty raw, chilly day regardless with low clouds
and light rain chances. Highs in the upper 40s to low 50s are
forecast, with more leeway to be even chillier if the stronger GFS
type solution comes to fruition.


(Sunday Night - Next Thursday)

A very welcome dry and quiet period still looks to be in the offing
early next week as mid/upper level flow become northwesterly. There
is still a strong signal for a very deep mid/upper level trough to
carve out across the Great Lakes early next week, sending our area a
shot of some cold air. Probabilities on the LREF for subfreezing low
temperatures early next week are very high, on the order 80-95+%. A
hard freeze (<=28F) is also likely (50-70%), mainly across northeast
Missouri and west-central Illinois. Given the rapid onset of the
growing season northward over the past few weeks, these types of low
low temperatures could certainly cause damage to sensitive
vegetation. Freeze watches/warnings may be needed for early next
week (with Tuesday morning likely the coldest) in the days ahead.

The next chance of rain comes midweek as a northwest flow
disturbance crosses the Mississippi Valley. This system will not
have a ton of moisture to work with, and any rain that does fall
looks light. In other words, this system should not impact rivers
and should allow them to continue to fall through the remainder of
the work week.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1216 PM CDT Thu Apr 3 2025

Widespread rain, mostly light, will impact the central Missouri
and metro St. Louis terminals this afternoon/early evening. Some
heavier downpours and even some light in southwest Missouri have
been observed. This activity should move through metro St. Louis
over the next few hours, but should weaken. Brief periods of MVFR
visibilities (and possibly ceilings) are expected. A thunderstorm
may impact the metro terminals this afternoon, but confidence was
too low to add mention at this time. Mostly dry weather is
forecast late this evening into the overnight hours with the next
round of rain holding off until later Friday morning. Ceilings are
expected to drop gradually overnight, with low MVFR conditions
expected by early Friday morning at each of the terminals other
than KUIN.

Gosselin

&&

.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
     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-Madison IL-Marion IL-Monroe IL-Randolph IL-Saint
     Clair IL-Washington IL.

&&

$$

WFO LSX