782
FXUS63 KLSX 200352
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1052 PM CDT Sat Apr 19 2025

.KEY MESSAGES...

- Widespread rain with a few thunderstorms are expected to
  continue through Sunday morning, with the highest chances of
  rain gradually shifting northward with time.

- Widespread thunderstorms are expected to move from west to east
  from late Sunday afternoon through late evening. Some of these
  storms will be severe with large hail, damaging winds, and
  tornadoes.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 223 PM CDT Sat Apr 19 2025

Widespread light to moderate rain along with some embedded
thunderstorms has been the rule today mostly along/southeast of I-44
in Missouri and I-70 in Illinois. This is expected to continue
tonight with persistent weak to moderate low-level moisture
convergence centered around the 850 hPa level beneath weakly
diffluent flow. The axis of rain should gradually shift northward
late tonight into Sunday morning as the surface quasi-stationary
front moves poleward in response to cyclogenesis across the southern
Plains. Thunderstorms are expected to be almost exclusively elevated
and weak in nature, though there may be a brief temporal/spatial
window for some wind gusts to around 60 mph in southeast Missouri
between roughly 0500 and 0800 UTC. This is when the surface front
may wobble far enough north to support a small line segment or two
to produce some minor wind damage. The signal is weaker and less
certain compared to last night however.

Rainfall rates tonight still look to stay mostly on the light to
moderate side, given a lack of a stronger, more organized low-level
jet. Only isolated to scattered convection is expected as well and
precipitable water values closer to the 90th percentile also suggest
more modest rainfall rates (similar to what has been observed over
the past 12-24 hours). Therefore, the flash flood threat still looks
low and isolated at best. Some ponding/nuisance flooding is
certainly possible however given the saturated soils along with some
minor river flooding in the days ahead (most likely along the
Meramec basin and its tributaries).

There remains some uncertainty on how fast/far north the warm front
will get by Sunday afternoon. Most guidance is able to get it just
north of the CWA border, which makes some sense given continued
deepening of the surface low tracking into northwest Missouri. The
NAM is slowest/furthest south and only makes it into parts of
northeast Missouri and west-central Illinois. This seems a bit too
slow/far south given the strength of the incoming wave and its
track. However, a slightly slower, further south placement Sunday
afternoon of the warm front may have some merit due to persistent
rain along/north of the retreating boundary. This can retard the
poleward progress of the boundary. The most likely location of the
warm front is very near the CWA border by 2100 UTC, putting
basically all of the bi-state area within the warm sector. There
remains a pretty strong signal that low clouds will hang around much
of the area into the early-mid afternoon, but eventually lift and
scatter out from south to north. This will allow temperatures to
climb at least into the low 70s, with mid to upper 70s possible in
southeast Missouri and southwest Illinois. Steep midlevel lapse
rates will be in place, so exactly how warm the boundary layer gets
will be key in determining how much instability will be available
ahead of the approaching dryline. Probabilities of at least 500 J/kg
remain very high on the latest HREF (70-100%) over the entire area
with exception of parts of northeast Missouri (Knox county and
vicinity). This area is far enough north and west where the dryline
may approach before the warm front is able to move north of that
area. Odds for at least 1000 J/kg of SBCAPE are in the 40-80% range,
and have ticked up about 10% compared to the last cycle. The amount
of instability will likely help modulate how widespread and high end
the threat will be, but even a lower instability scenario is sure to
produce severe weather in the area tomorrow.

The expectation is for supercells to develop across southeast
Kansas/southwest Missouri along the dryline by early afternoon given
a favorable orientation of the deep-layer shear vector to the
boundary (~45 degrees). A couple of CAMs try to initiate a few
isolated (low topped/mini) supercells further east in our area by
mid afternoon ahead of the dryline, but that does not appear to be a
likely scenario given very weak/negligible surface convergence
within the open warm sector and the stronger mid/upper level forcing
for ascent downstream of the negatively tilted shortwave still well
to the west. The dryline should be making its way into central
Missouri around 2100 UTC. CAMs suggest a transition to a more linear
mode, though some embedded supercells are still possible. The
orientation of the shear vector is still favored for discrete
convection well into the afternoon, though the strength of the
mid/upper level forcing for ascent does argue for a transition to a
more linear mode in time/eastward extent. Capping is pretty weak
tomorrow, and given the strength of the mid/upper level forcing,
convection should be very widespread. This could mean a lot of
competition for individual updrafts, and if mergers tend to be
destructive, may help at least slightly limit how widespread and
significant the severe weather will be tomorrow. This is just a
possible limiting factor however along with lingering low clouds
which would help limit instability at least somewhat. Neither of
these negating factors will eliminate the possibility of severe
weather however given the strong dynamics and near-certainty in at
least modest instability available tomorrow.

All severe threats are expected on Sunday, with large hail more
favored earlier on Sunday and further west where supercells are more
favored. The damaging wind threat likely will increase across
eastern Missouri and western Illinois early Sunday evening. The
tornado threat exists across the entire area, though is most likely
along the path of the triple point and just to the east/southeast of
its track. This is where surface winds will be most backed and low-
level shear/helicity maximized. The tornado threat should continue
at least into the mid/late evening hours. The low-level SRH
maintains itself across the river into Illinois as veering of
surface winds is offset by increasing 1 km flow associated with the
nocturnal low-level jet.

Storms should gradually weaken and lose some organization after 0200-
0300 UTC due to weakening instability after sunset and abating
surface convergence along the dryline and weakening mid/upper level
forcing for ascent. By about 0400-0500 UTC, the dryline itself
should exit off to the east of our area ending any remaining threat.


Gosselin

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 223 PM CDT Sat Apr 19 2025

(Monday - Monday Night)

A welcome, but short lived period of quiet weather is in store to
start off the next work week as a weak surface ridge of high
pressure moves across the mid-Mississippi Valley. Dry weather with
temperatures near normal (highs mid 60s to low 70s) are expected.


(Tuesday - Next Saturday)

A return to a more active weather pattern with several rounds of
showers and thunderstorms remains possible for mid/late week as
several perturbations move across the area within zonal flow aloft.
Trying to time any one of these features is fruitless at this time,
which is why a prolonged period of mostly chance (30-50%) PoPs
exists Tuesday night through Thursday. There is a more coherent,
stronger signal centered on Friday as both the GEFS/EPS bring a
stronger mid/upper level trough across the Great Lakes with a cold
front moving through our area. The timing of this frontal passage
could shift though given it is still 6 days out, but does look like
the period right now with the best chance of showers and
thunderstorms (60-70%).

The threat for any organized convection and associated severe
weather/flash flooding appears pretty low given the westerlies are
closer to the US-Canadian border. Multiple rounds of showers and
storms could however lead to renewed river flooding given still
moist soils and elevated base flows continuing in some streams.

Temperatures are expected to be above normal next week, especially
at night. There remains some more uncertainty with daily high
temperatures as widespread precipitation/clouds could yield highs at
least several degrees cooler than forecast.


Gosselin

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1042 PM CDT Sat Apr 19 2025

Low ceilings, fog, showers, and thunderstorms can all be expected
at various times during the 06Z TAF period. At the start of the
period, light rain will steadily spread across the region,
impacting JEF/COU/UIN initially and eventually STL/SUS/CPS as
well. Lightning is not expected initially, but a few strikes and
increasing rain rates will be possible during the early morning
hours, particularly at JEF/COU. Meanwhile, ceilings will steadily
lower overnight through tomorrow morning, with fog likely at times
as well. Ceiling and visibility reductions to IFR are likely at
times in most areas, with a few dips to LIFR possible as well.

Showers will likely diminish late in the morning and early
afternoon, and ceilings/fog should lift for a few hours. However,
a line of thunderstorms, and potentially strong/severe
thunderstorms, is increasingly likely to impact all terminals for
a brief period late in the afternoon and early evening. Strong
winds, hail, and even tornadoes will be possible with these
storms, in addition to lowering ceilings and reduced visibility.

This line will likely only last a couple of hours, and VFR
conditions are expected for at least a few hours thereafter, and
likely until the end of the period. Gusty south winds during the
day will veer to the southwest behind the storms, and remain
breezy. Low ceilings may return to terminals near the end of the
period, but this was not included in this iteration as confidence
in the timing remains low.

BRC

&&

.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