150
FXUS63 KLSX 260922
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
422 AM CDT Thu Sep 26 2024

.KEY MESSAGES...

- Rain associated with tropical moisture and Tropical Cyclone
  Helene will spread into the region overnight lasting through
  Friday evening, primarily along/south of I-44 in MO and I-70 in
  IL. Total rainfall of 1 to 3" is forecast across southwestern IL
  and southeastern MO with minor flooding possible where locally
  higher amounts fall.

- Breezy conditions are also expected on Friday with occasional
  gusts of 25 to 40 mph, but higher wind gusts are possible in
  southwestern IL Friday evening, depending on the track of
  Helene.

- Mainly afternoon and evening showers are expected Saturday
  through Monday with coverage decreasing each day.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 421 AM CDT Thu Sep 26 2024

Early this morning, an upper-level cutoff low is centered over
southern IL, with much of the CWA under the influence of weak
subsidence in its wake and a weak inverted SLP ridge. With only a
few mid-level clouds present, efficient radiational cooling has
allowed river valley fog to develop, especially along the Missouri
River and in the Ozarks. High temperatures today are anticipated to
be a couple degrees warmer than Wednesday with greater insolation
through only scattered diurnal cumulus and ahead of upper and mid-
level clouds arriving during the evening.

As the upper-level cutoff low drifts southward into the Lower
Mississippi River Valley this evening, tropical moisture will begin
to wrap around the cutoff low and spread into southwestern IL and
southeastern MO. The initial arrival of this moisture could be
accompanied by development isolated to scattered showers and
thunderstorms during the evening while instability is available, but
a more widespread shield of stratiform rain will arrive overnight as
moisture deepens and isentropic ascent increases.

This shield of rain will persist through most of Friday but its
northwestward extent varies across CAMs and other short-term model
guidance due to variability in the exact position of the cutoff low
while it interacts with a weakening, post-landfall TC Helene
tracking northwestward through the Southern Appalachians/Tennessee
Valley and Lower Ohio Valley. HREF 6-hour probabilities of
measurable rainfall are 60 percent or higher along and south of I-44
in MO and I-70 in IL, leading to some confidence that the rain
shield will extend across much of that area through Friday. Under
the shield of rain, high temperatures will only reach the upper 60s
to around 70 F, with 70s F elsewhere. Strong low-level flow (around
40 to 50+ kt at 850 hPa) and a tightening SLP gradient will lead to
breezy conditions throughout the day with occasional gusts of 25 to
40 mph. Late Friday, TC Helene is progged to rotate around the
closed low and reach the vicinity of the Mississippi-Ohio River
confluence as it becomes absorbed into the cutoff low. Not only will
this process potentially augment the rain shield, but it will also
strengthen winds across southwestern IL. The latest HREF has 60 to
80 percent probabilities of wind gusts exceeding 50 mph Friday
evening; however, these gusts are conditional on the track of TC
Helene being nearby the CWA.

There will be a gradient in total rainfall through Friday night
across the CWA, the interquartile range of QPF in ensemble models
are between nothing to 0.25" in central/northeastern MO and west-
central IL, 0.25 to 1.50" near I-44 (MO) and I-70 (IL), and 1.00 to
3.00" in southwestern IL and southeastern MO. With PW above the 99th
climatological percentile, the heaviest rain is expected across
southwestern IL and southeastern MO with rainfall, although little
to no instability will limit rain rates. It is worth noting that the
90th percentile of QPF in those areas is around 4", which cannot be
ruled out locally with minor flooding possible, but flash flooding
is unlikely.

Pfahler

&&

.LONG TERM...  (Saturday through Wednesday)
Issued at 421 AM CDT Thu Sep 26 2024

Saturday through Monday, the upper-level cutoff low is anticipated
to have fully absorbed TC Helene and begin progressing slowly
eastward, away from the Mid-Mississippi River Valley. In response,
large-scale ascent will also depart, but lingering moisture and
modest daytime heating will support diurnal showers, most numerous
on Saturday afternoon. With clouds and coverage of showers lessening
each day, a slight warming of temperatures is expected through the
70s F to near 80 F by Sunday and Monday. Thereafter, global model
guidance signal an upper-level trough traversing the northern CONUS,
sending a cold front through the Mid-Mississippi River Valley around
Tuesday, but there is uncertainty in the strength of the front and
whether temperatures fall from above average to near or below
average following its passage.

Pfahler

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1051 PM CDT Wed Sep 25 2024

Light and variable winds, clear skies, and lingering low-level
moisture will favor the development of fog overnight at KSUS and
KCPS. Fog cannot be ruled out at KJEF but confidence in its
development is too low to include in TAF. Expect northerly winds to
increase Thursday morning with diurnal cumulus. Cloud cover is
forecast to increase in coverage late Thursday evening.

MMG/Glass

&&

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

$$

WFO LSX