504
FXUS63 KLSX 041949
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
249 PM CDT Thu Jul 4 2024

.KEY MESSAGES...

- Another round of thunderstorms is expected this evening into
  overnight, primarily along and south of I-44 in MO and I-70 in
  IL. Isolated damaging winds are the primary threat, followed by
  large hail and heavy rainfall that could lead to localized flash
  flooding.

- After slightly below average temperatures on Friday, near
  average temperatures will persist through next week with the
  next opportunity for showers and thunderstorms late Sunday and
  Monday.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 248 PM CDT Thu Jul 4 2024

In the wake of the morning MCS and as clouds have decreased, the
atmosphere has recovered with MLCAPE increasing to between 1500 and
3500 J/kg along and south of I-70. Despite this instability, the
combination of weak convective inhibition and weak forcing has
prevented any additional development of showers and thunderstorms so
far this afternoon in the CWA, but isolated to scattered development
could still occur this afternoon ahead of a weak surface front in
southeastern MO and southwestern IL as heating persists and
inhibition decreases. Greater confidence in development of showers
and thunderstorms exists this evening as broad ascent leading an
upper-level trough arrives and another cold front reaches the
reservoir of instability. CAMs focus showers and thunderstorms
initially across southern and south-central MO before
expanding/progressing northeastward along and south of I-44 (MO) and
I-70 (IL) late this evening into overnight along a strengthening of
the front at 850 hPa. Instability will only slightly decrease and,
with 25 to 40 kt of deep-layer wind shear, strong to severe
thunderstorms are possible with a hazard of damaging winds with an
opportunity for a few microbursts and upscale growth into one or
more clusters. However, large hail is a secondary hazard.

Although the pattern is not quite as favorable for sustained heavy
rainfall as last night with more progressive thunderstorm motions
forecast, ingredients are still present to support a threat of heavy
rainfall with these thunderstorms including anomalous PWs and deep
warm cloud depths. The latest HREF 24-hour QPF LPMM highlights
localized areas of 1 to 4" somewhere along I-44 in MO and I-70 in IL
where brief training of thunderstorms along the 850-hPa front is
possible. Given that some of that area has lowered flash flood
guidance from saturated soils, the Flood Watch remains in effect and
has not been changed.

Showers and thunderstorms will continue to the east of the CWA early
Friday morning, with dry conditions thereafter on Friday. With
scattered to broken diurnal cumulus/stratocumulus and weak low-level
CAA amidst deep broad cyclonic flow, temperatures on Friday will
also be slightly below average accompanied by lower humidity.

Pfahler

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 248 PM CDT Thu Jul 4 2024

Global model guidance are in agreement that the upper/mid-level
cyclonic flow will give way to southwesterly flow within deepening
longwave troughing over the Northern and Central Plains. Multiple
shortwave troughs rounding the longwave trough will provide one or
more opportunities for periods of showers and thunderstorms Sunday
through next week. The largest membership of ensemble models contain
rainfall late Sunday and Monday. NBM interquartile temperature
ranges also straddle daily average temperatures, increasing
confidence in temperatures remaining near average through next week.

Pfahler

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1258 PM CDT Thu Jul 4 2024

MVFR ceilings will continue to scatter and lift generally from north
to south this afternoon with improvement to VFR flight conditions at
all terminals. Another round of showers and thunderstorms is
expected this evening, but its northward extent from
southern/southeastern MO and exact timing is uncertain. St. Louis
metro terminals have the greatest chance of seeing showers and
thunderstorms, with KUIN having the lowest chance (too low to
mention). Thunderstorms will be capable of heavy rainfall, gusty
winds, and MVFR to IFR flight conditions.

Following showers and thunderstorms, a period of MVFR stratus is
possible behind a passing weak cold front at all terminals. West-
northwesterly winds with gusts of 20 to 25 kt are anticipated on
Friday.

Pfahler

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch until 1 AM CDT Friday for Boone MO-Callaway MO-Cole
     MO-Crawford MO-Franklin MO-Gasconade MO-Iron MO-Jefferson
     MO-Madison MO-Moniteau MO-Montgomery MO-Osage MO-Reynolds
     MO-Saint Charles MO-Saint Francois MO-Saint Louis City MO-
     Saint Louis MO-Sainte Genevieve MO-Warren MO-Washington MO.

     Heat Advisory until 9 PM CDT this evening for Iron MO-Madison MO-
     Reynolds MO.

IL...Flood Watch until 1 AM CDT Friday for Monroe IL-Randolph IL-
     Saint Clair IL-Washington IL.

&&

$$

WFO LSX