Severe Thunderstorm Warning 134, NWS St. Louis, MO

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1230 PM CDT Sat May 13 2023

.UPDATE...
Issued at 1225 PM CDT Sat May 13 2023

Thunderstorms have begun forming along a convergent line nearing
the Mississippi River just north of St Louis. Instability has
maximized in our area today, with dewpoints in the low 70s and
temperatures warming into the 80s, yielding surface based CAPE in
excess of 3000. Wind shear is not overly strong, but sufficient
for briefly organized storms. Low level easterly flow developing
across Illinois will act to enhance the overall shear as storms
move east. Development further south is expected as the storms
encounter greater moisture and low level southeasterly inflow.
Large hail has been the primary threat so far, but damaging winds
are also becoming a possibility especially as storms form into a
cluster and progress to the southeast through Illinois.

Kimble

&&

.SHORT TERM... (Through Sunday Night)
Issued at 324 AM CDT Sat May 13 2023

(Today - Tonight)

A warm front as of 0700 UTC was located near a KSTJ>>KSTL>>KMVN line
this morning, growing more diffuse with southeastward extent. This
boundary likely will serve as the focus for thunderstorm development
this afternoon into the early evening as the warm front slowly moves
poleward. Further southwest, generally weak convection in
southeastern Kansas and northeastern Oklahoma (nearly in the same
locations as 24 hours ago) is moving eastward. The storms are
outflow dominant. Given this fact and a weakening low-level jet
after sunrise, the expectation is for just some scattered showers
and a few rumbles of thunder to reach parts of central and
southeast Missouri this morning. By late morning, we likely will see
a lull or complete dissipation in activity given the weakening low-
level jet and associated moisture convergence. Around midday or
shortly thereafter, additional storms should form near the warm
front. There is some uncertainty on how widespread storms will be
given that the front becomes more diffuse with east/southeast
extent, and therefore, the convergence along it weakens. There
should be enough instability (HREF probabilities for >2000 J/kg
40-50%) and adequate deep-layer shear (20-30 knots) for a few
strong to severe thunderstorms. Convective mode is likely to be a
combination of pulse storms and multicellular clusters, with a
transient supercell or two possible in parts of northeast Missouri
and west-central Illinois this afternoon where the deep-layer
shear is strongest. Steep midlevel lapse rates and high amounts of
CAPE within the hail-growth zone suggest hail is the main threat,
though damaging winds are also possible along with a brief
tornado potentially near the warm frontal boundary.

A continued chance of showers and thunderstorms exists tonight,
mainly in west central and south-central Illinois closer to the
position of the warm front. Convective coverage is once again a
question mark, as deterministic guidance suggests the best low-
level moisture convergence to be well to our northeast overnight
tonight in eastern Iowa and northern Illinois. In terms of severe
chances, the expectation is for these storms to be sub-severe
overnight due to lessening instability.

(Sunday - Sunday Night)

On Sunday, a midlevel shortwave trough is forecast to drop across
the western Great Lakes and into the Ohio Valley. This feature is
expected to help push the surface boundary southward as a cold
front. This cold front is likely to serve as the focus for showers
and thunderstorms through Sunday night. There remains some
uncertainty on the exact timing of this front, but the most likely
scenario is for thunderstorms to blossom in the afternoon hours
along/behind the front and continue through the evening. The best
coverage of storms will most likely be in parts of south central
and southwest Illinois where surface convergence is expected to be
strongest. A few of the storms on Sunday may be strong to severe,
with large hail the primary threat given continued presence of
steep midlevel lapse rates and tendency for storms to become more
elevated with time on Sunday.

Gosselin

.LONG TERM... (Monday through Friday)
Issued at 324 AM CDT Sat May 13 2023

(Monday - Wednesday Night)

Behind Sunday's cold front, there is high confidence in a cooler and
drier pattern for early/mid next week. While some shower activity
may be possible with weak/subtle northwest flow shortwave troughs
and associated secondary cold frontal passages, dry low-level air,
nebulous/weak forcing for ascent, and instability remaining well
south and west of the CWA all suggest most if not all of the area
will stay dry. Temperatures on Monday will be the coolest of the
period as 850-hPa temperatures drop back to near +10C and the
anomalously strong surface high to our northwest has its most
influence over the region. Highs in the low to mid 70s are forecast
Monday afternoon, with a gradual warming trend through Wednesday.
The coolest night of the period is expected to be Monday night into
Tuesday morning, with a clear sky and light/variable winds in close
proximity to the surface anticyclone. Forecast lows range from the
low to mid 50s, but given dewpoints in the mid 40s, would not be
surprised to see some 40s for lows in northeast Missouri and west-
central Illinois.

(Thursday - Friday)

Ensemble guidance is in high agreement that mid/upper level
northwest flow will continue to end the work week. Another shortwave
trough is likely to transverse the Upper Midwest into the Great
Lakes during the period, with a cold front moving across the
Mississippi Valley. The specifics with these features is uncertain
at this time, including but not limited to: the strength/timing of
the midlevel shortwave trough and the timing of the cold front. This
front should bring a chance of showers and thunderstorms. Ahead of
the front, above normal temperatures are likely on Thursday, with
cooler temperatures behind it on Friday. Ensemble guidance show that
the air mass behind this front is likely to be cooler than the front
moving through late Sunday. Anomalies of 850-hPa temperatures on
both the GEFS/EPS are in the 4 to 6C range below normal for late
May.

Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1225 PM CDT Sat May 13 2023

Primary impact for TAFs is the timing of thunderstorms this
afternoon. Storms have begun to form and so confidence has
increased in the timing of storms moving through the metro in the
next couple of hours. This looks like the primary threat window
for today, with conditions improving after the storms move
through. A cold front moves in from the north on Sunday and may
bring some MVFR ceilings with it when it arrives at Quincy. It
could also serve as a focus for more thunderstorm development on
Sunday as it pushes southward.

Kimble

&&

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

WFO LSX

Hazardous Weather Outlook...CORRECTED
National Weather Service St Louis MO
453 AM CDT Sat May 13 2023

ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027-
034>036-041-042-047>052-059>065-072>075-084-085-099-140845-
Greene IL-Macoupin IL-Montgomery IL-Bond IL-Fayette IL-Clinton IL-
Marion IL-Washington IL-Randolph IL-Adams IL-Brown IL-Pike IL-
Calhoun IL-Jersey IL-Madison IL-Saint Clair IL-Monroe IL-Knox MO-
Lewis MO-Shelby MO-Marion MO-Monroe MO-Ralls MO-Pike MO-Boone MO-
Audrain MO-Moniteau MO-Cole MO-Osage MO-Callaway MO-Montgomery MO-
Lincoln MO-Gasconade MO-Warren MO-Saint Charles MO-Franklin MO-
Saint Louis MO-Saint Louis City MO-Jefferson MO-Crawford MO-
Washington MO-Saint Francois MO-Sainte Genevieve MO-Iron MO-
Madison MO-Reynolds MO-
453 AM CDT Sat May 13 2023

This Hazardous Weather Outlook is for portions of eastern and
central Missouri as well as west central and southwest Illinois.

.DAY ONE...Today and tonight.

Scattered thunderstorms are forecast today. A few of these storms may
be strong to severe between 1 and 7 PM with large hail the main
threat. Damaging winds and a tornado are also possible.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

There is a chance for thunderstorms Sunday through Sunday night.
A few of these storms may be strong to severe Sunday afternoon and
evening with large hail the main threat once again.

Another chance of thunderstorms exists across the area Thursday
through Friday.

.SPOTTER INFORMATION STATEMENT...

Spotters are encouraged to report any hail or damaging winds to the
National Weather Service in St. Louis.

Gosselin

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