949
FXUS63 KLSX 181931
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
231 PM CDT Fri Apr 18 2025

.KEY MESSAGES...

Key Messages:

- Widespread showers and thunderstorms are forecast tonight. Some
  of these storms may be strong to severe with large hail,
  damaging winds, and a tornado or two all possible.

- A more widespread and higher potential severe event is possible
  on Sunday. There remains some uncertainty in how much
  instability will be available, but all severe hazards are
  possible across the entire area between the afternoon and late
  evening hours.

- A stalling boundary will produce multiple waves of rainfall to
  the region through Sunday night. A Flood Watch remains in effect
  along and south of I-70, where locally up to 5 inches of rain
  is possible.


&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 215 PM CDT Fri Apr 18 2025

A cold front is slowly pushing southeastward and is just northwest
of a KMBY>>KMQB line as of 1900 UTC. RAP soundings and ACARS
observations still show a pretty good cap still in place, with a
temperature inversion between 700 and 800 hPa. Near-term guidance
suggests this cap will erode over the next 2-4 hours, with
isolated to widely scattered convection developing around 2300 UTC
(+- an hour or two). Coverage of storms may tend to stay on the
low side through the evening, before low-level moisture
convergence increases along with a midlevel disturbance also
approaching from the southwest. Convective mode looks messy, but
could see a couple of supercells and/or line segments. Large hail
looks to be the main threat, but damaging winds and even an
isolated tornado are possible given moderately strong 0-1 km
shear/SRH values. After midnight, convection should become much
more widespread, but also may tend to focus more behind the cold
front where the low-level moisture convergence is focused. This
activity should tend to become more elevated through the night,
with marginally severe hail the main threat. There is still a low
chance of some surface-based convection closer to the front
across southeast Missouri and southwest Illinois into the early
overnight hours however. The risk of damaging winds and an
isolated tornado may continue longer in these areas into the night
as a result.

The cold front is expected to ooze southward into southeast Missouri
and southwest Illinois by Saturday morning. Continued convection
north of the boundary should help push it a bit more to the south
during the day. The main concern for much of the day on Saturday
will be heavy rainfall as elevated convection may tend to train over
the same areas in east-central Missouri and western Illinois.
Exactly where this axis sets up is still a source of uncertainty,
and will heavily depend on the placement and evolution of the quasi-
stationary boundary. Rainfall rates however may not be too high
during much of the day on Saturday for several reasons, including:
1) weak elevated instability/lack of deep convection, 2) lack of
stronger low-level moisture convergence, and 3) high, but not very
high (~90th percentile) precipitable water values. All this being
said, it appears likely that there will be a southwest-northeast
axis of heavier rainfall totals (likely 3-5" by Sunday night), but
the threat should tend to be more river flooding vs. flash flooding.

Any severe threat on Saturday north of the boundary looks very low.
Any convection would be elevated with not much MUCAPE to work with.
Small hail should be the only threat. The one possible exception is
far southeast Missouri and southwest Illinois. IF the boundary is
far enough north, there would be a threat for some strong to severe
thunderstorms. Damaging winds, marginally severe hail, and a tornado
or two would also be possible in this scenario. Given the tendency
for boundaries to want to sink a bit further to the south with
persistent convection along/north of it, I have doubts the front
will be far enough north but it is something to keep an eye on.

The front should begin at least to lift back north Saturday night as
surface cyclogenesis commences downstream of a negatively-tilted
midlevel shortwave trough. Additional showers and thunderstorms are
expected across much of the area, with the focus perhaps just
shifting a bit further to the north with time. Similar to earlier in
the weekend, it does not look like a phenomenal setup for flash
flooding given modest moisture convergence, precipitable water
values that are high but not extremely high, along with a shift to
the north of where the rain is falling compared to late
tonight/Saturday. Guidance has very little MUCAPE in place, so other
than some small hail, the chances for any strong to severe
convection appears very low.


Gosselin

&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 215 PM CDT Fri Apr 18 2025

(Sunday - Sunday Night)

The Sunday afternoon/evening period continues to be the period with
the most concern and the highest potential over the next few days.

Deterministic guidance is in much better agreement with the
evolution of the mid/upper level wave and associated surface low.
All guidance has a very dynamic system characterized by a negatively-
tilted midlevel shortwave trough and rapid deepening of a surface
low tracking across northwest Missouri into eastern Iowa late
Sunday. A warm front is forecast to move through our CWA during the
day on Sunday, though how far north and how fast its progress is
still slightly uncertain due in part at least due to elevated
convection north of the boundary Sunday morning. Regardless, much of
if not all of the CWA is very likely to get into the warm sector
ahead of an approaching dryline from the west. This dryline should
serve as the focus for the development of deep convection. Exactly
when and where this occurs varies slightly amongst model guidance,
but should be to the west of the CWA across eastern Kansas or
western Missouri. The convective mode should be supercells at least
initially given about a 45 degree difference between the deep-layer
shear vector and the orientation of the dryline itself. The mode
should shift to more of a QLCS with time given very strong synoptic
scale forcing, but how fast this occurs (and where) is not certain.
Previous experience would suggest this process tends to be on the
quicker side of the envelope in these types of scenarios with very
strong mid/upper level ascent. All hazards are on the table for
Sunday, but backed surface winds (especially near the triple point
closer to central/northeast Missouri) and elongated, curved
hodographs suggest tornadoes would be our primary concern, both
with supercells but also with embedded QLCS mesovortices. Damaging
winds are more likely later on in the event after the transition
to a more linear mode, with large hail a threat with any
supercells. The amount of instability may tend to stay on the low
side however, which suggests very large hail (2+") is unlikely
except for the higher-end instability scenarios.


Speaking of instability, that will be the primary focus for Sunday
afternoon/evening in our area. Very strong (50-60+ knots) deep-layer
shear is all but a given just east/southeast of the track of the
negatively-tilted midlevel shortwave trough. Several factors could
help limit the instability however, including morning
showers/thunderstorms and residual low clouds. LREF probabilities
for SBCAPE of at least 500 J/kg are in the 30-60% range for the
southern half of the area, with a 10-30% chance of at least 1000
J/kg of SBCAPE. Higher-end CAPE values of 1500-2000 J/kg are in the
90-95th percentile range. This type of instability available very
well could lead to a significant outbreak of severe weather with a
much better chance of very large hail and tornadoes (including the
potential for strong ones). A lower end scenario would still bring
severe weather to the area, but would be less widespread (maybe more
confined to east central and southeast Missouri/southwest Illinois)
with a lower ceiling in terms of significance as well.

The threat for any severe weather should end by about 0600 UTC
Monday as the dryline sweeps eastward. Given a bit more delayed
timing of thunderstorms Sunday, did extend the flood watch until
0600 UTC Monday.


(Monday - Next Friday)

A quiet and cooler day is forecast on Monday as the trailing cold
front moves through the area by Monday morning. Dry weather with
high temperatures near to slightly below normal (mid 60s to low 70s)
is expected.

A return to a more active pattern appears likely for mid/late next
work week as subtle midlevel perturbations move across the mid-
Mississippi Valley. The good news is the westerlies are well to the
north, indicative of weak deep-layer shear. Multiple rounds of
showers and thunderstorms are possible, but the chances of any
organized severe weather or heavy rainfall at this time appears very
low.

Temperatures should be anomalously warm overnight given plenty of
cloud cover. Highs each day though will be heavily dependent on the
timing of any rain/storms. That is why the spread between the
25th/75th percentiles of the NBM are quite large for this time
period, generally close to 10 degrees. Most days should be above
normal however given the warm starts each day. The 25th percentile
for instance is almost exclusively still slightly above
climatological normals.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1235 PM CDT Fri Apr 18 2025

A cold front will progress through the area tonight, increasing
the chances of showers and thunderstorms along/behind it and
switching winds to the northwest with lowering ceilings. The best
chance of thunderstorms this evening is along the front at the metro
St. Louis terminals. Added a TEMPO for this activity, with MVFR
visibilities and ceilings expected. Heavier downpours may bring
visibilities lower. Some stronger storms are also possible, with
hail and strong winds both possible. Was not confident enough at
this stage however to add any wind gusts. Showers and embedded
thunderstorms will continue through Saturday morning at the
central Missouri and St. Louis metro terminals. Ceilings will
lower to around or just below 1000 feet AGL, with prevailing
visibilities in the 3-5SM range. Brief downpours further reducing
visibility are possible.


Gosselin

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch from 7 PM CDT this evening 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 from 7 PM CDT this evening 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