177
FXUS63 KLSX 200844
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
344 AM CDT Mon May 20 2024

.KEY MESSAGES...

- Isolated to scattered showers and thunderstorms are expected to develop
  this afternoon, mainly in parts of east central Missouri and
  south-central Illinois. A few of these storms may be strong to
  severe with damaging winds and hail up to the size of quarters
  between 2 and 8 PM.

- The main focus for this forecast package is on convective trends
  Tuesday through Wednesday. The best chance for strong to severe
  thunderstorms continues to be Tuesday evening, mainly in parts
  of central and northeast Missouri and west-central Illinois.
  Another round of severe weather is possible Wednesday afternoon
  in parts of southeast Missouri and southwest Illinois, but is
  more conditional.

- Summerlike temperatures will persist through Tuesday. Highs on
  Tuesday afternoon are expected to be in the upper 80s to low 90s
  and may break the daily record high in St. Louis (93F - 1941).


&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 339 AM CDT Mon May 20 2024

Earlier strong to severe convection in western Missouri has rapidly
weakened over the past several hours, with only some isolated to
widely scattered showers remaining. These showers may move
through parts of central and northeast Missouri as well as west-
central Illinois before completely dissipating this morning. Of
more interest is for the potential for additional thunderstorms
this afternoon. The best chances (30-40%) are in parts of east-
central Missouri and south-central Illinois, roughly between
2-8 PM. This is where the combination of increasing diurnal
instability, a weak surface convergence zone, and a possible
remnant MCV (from ongoing convection in NE OK) overlaps this
afternoon. A few of these storms may be strong to severe, with
damaging winds and hail up to the size of quarters both possible.
Convective mode will likely be a mixed mode of pulse thunderstorms
along with multicellular clusters with the aforementioned MCV
potentially aiding in a bit more wind around the 700 hPa and
increasing effective shear more into the ~25-30 knot range. The
atmosphere should also be at least moderately unstable as there
should be partial sunshine due to breaks in the convective debris
clouds. The 0Z HREF has probabilities for at least 1000 J/kg of
CAPE in the 70-90% range, but does sharply drop to around 10-20%
for at least 2000 J/kg at 2100 UTC. This is quite a bit less
instability compared to yesterday, due to a combination of cooler
surface temperatures (more cloud cover) as well as weaker midlevel
lapse rates. However, the slightly higher deep-layer shear and
subtle, but relevant forcing mechanisms (surface convergence zone,
MCV) may be able to compensate.

Any thunderstorms should gradually exit parts of west central and
south-central Illinois by early this evening, ending any severe
threat. The expectation is for mainly dry weather through the
remainder of the night with stronger forcing for ascent well to our
northwest across the mid-Missouri and Upper Mississippi Valleys. Did
keep some slight chance PoPs (~20%) into the early overnight hours
in central and northeast Missouri as well as west-central Illinois
where there is broad, weak low-level moisture convergence and some
diffluent flow aloft.

Tuesday is expected to be dry across the entire area, at least
through the afternoon hours. Very warm conditions are expected, with
highs reaching into the upper 80s to low 90s. These values were near
the 90th percentile of available model guidance due to strong
low-level warm air advection and plenty of sunshine. The warmest
conditions are likely to be within metro St. Louis where the high
may threaten the daily record for the date (93F - 1941).


Gosselin

&&

.LONG TERM...  (Tuesday Night through Sunday)
Issued at 339 AM CDT Mon May 20 2024

(Tuesday Evening - Wednesday Night)

There has been very little/no change with the expectations for
Tuesday evening. This continues to be the period of highest
confidence with respect to chances for severe weather.

Convective initiation is expected by mid afternoon along either the
synoptic cold front or a prefrontal trough near the Missouri-Kansas
border. These storms likely will initially be discrete/supercellular
given about a 30-45 degree angle of the shear vector off of the
initiating boundary. Given the strong mid/upper level forcing for
ascent associated with a negatively-tilted shortwave, storms should
quickly transition to a line/QLCS before reaching parts of central
and northeast Missouri. The line/QLCS is likely to arrive between 6-
8PM in these locations and move east/southeast in time. The line is
likely to weaken as it does move to the east/southeast as our area
becomes further and further removed from the stronger forcing for
ascent. In addition, instability gradually abates nocturnally and
deep-layer shear slackens a bit with southeastward extent. There is
uncertainty as to exactly when and how fast this downturn in
convective organization occurs, but the best guess is for the best
temporal-spatial window for widespread severe weather to be 2300-
0300 UTC in parts of central and northeast Missouri as well as west-
central Illinois. Damaging winds should be the main threat, though
isolated tornadoes are also possible within any portions of the QLCS
that are able to surge out more to the east/northeast. Otherwise, at
least a large portion of the line may be close to parallel to the 0-
3km shear vector which is forecast to be around 35 knots.

Convective trends/evolution become more ambiguous heading into
Wednesday. At least some thunderstorms should be ongoing across the
area Wednesday morning, but given the factors mentioned in the
paragraph above, the expectation is for much less in the way of
organization by late Tuesday night/Wednesday morning. The surface
cold front is expected to lag a bit back to the northwest, with its
location likely near the I-44/I-70 corridors in Missouri and
Illinois respectively by 1800 UTC. If early morning convection
doesn`t impact atmospheric recovery for the afternoon, this would
put a good portion of southeast Missouri and southwest Illinois in
the threat zone for severe weather. The threat however remains
conditional, owing to some lingering uncertainty with the exact
location of the cold front mid afternoon on Wednesday, along with
the degree of instability that will be available for storms.


(Thursday - Sunday)

A brief break in the more active weather pattern is possible on
Thursday, though the far southern CWA will at least have a lingering
threat for showers and non-severe thunderstorms. This is because the
cold front is expected to stall somewhere south of the CWA, but
exactly where is unknown. Given the expectation for a well-organized
MCS to eventually form though Wednesday night, the effective front
likely will be across the mid south. If this indeed occurs, PoPs on
Thursday may need to be reduced and shifted further to the south
than is currently forecast.

Models are in pretty good agreement that a midlevel shortwave trough
will move through the mid south Thursday night and interact with the
stalled frontal boundary to yield greater coverage of showers and
thunderstorms. Once again, the best chances are in parts of
southeast Missouri and southwest Illinois. Similar concerns as
Thursday exist for Thursday night though. If the front is far enough
south, much of the area may be dry and PoPs (40-60%) may be a bit on
the aggressive side of the envelope.

Ensemble spread has decreased heading into the holiday weekend with
a better consensus that the mid-Mississippi Valley will be beneath
quasi-zonal to southwest flow aloft. None of the 500-hPa height
clusters show mid/upper level ridging like was shown yesterday. What
does this mean in terms of sensible weather? Basically, it looks
like the area will have multiple rounds of showers and thunderstorms
possible through the upcoming holiday weekend. Timing exactly
when/where is impossible this far out, but if you have outdoor
plans, stay tuned for future forecast updates. Daytime temperatures
also won`t be as anomalously warm due to plenty of
clouds/precipitation chances with highs mostly in the low to mid
80s. These values also are a bit skewed toward the higher end of the
spectrum, as widespread convection occurring during the day could
really limit the temperature climb.


Gosselin

&&

.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1043 PM CDT Sun May 19 2024

Primary concern for the 06Z TAF period will be the potential for
scattered showers and thunderstorms, both near the beginning of
the period and again during the later afternoon and evening.
Confidence is growing that at least some showers and thunderstorms
will reach COU/JEF near 06Z, with gusty winds and visibility
reductions possible for a brief period. Some shower activity and
perhaps a thunderstorm or two will also be possible at UIN
overnight, but are not expected to reach St. Louis until during
the day.

After a lull in showers during the mid morning and early
afternoon, redeveloping scattered thunderstorms will be possible
during the late afternoon and evening. Confidence is low regarding
exactly when and where this activity will develop, but there is a
chance for this activity at all of our local terminals.

Ceilings are likely to remain mostly VFR, but dips to MVFR will be
possible at times as showers/thunderstorms move through the area.

BRC

&&

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

$$

WFO LSX