970
FXUS63 KLSX 301058
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
558 AM CDT Tue Apr 30 2024

.KEY MESSAGES...

- Widespread dense fog across southwestern IL, southeastern MO,
  and portions of the Ozarks will persist this morning, before
  dissipating an hour or two after sunrise.

- A few strong to severe thunderstorms are possible across
  central, northeastern MO and west-central IL late this evening
  (10 pm to 1 am) with damaging winds and marginally severe hail
  the main hazards.

- Opportunities for showers and thunderstorms will persist at
  times through the rest of the work week and through the upcoming
  weekend, but more of that time will be dry than wet. The
  greatest chance is late Thursday into Friday at 60 to 80
  percent.


&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 402 AM CDT Tue Apr 30 2024

Dense fog with visibilities of 1/4 mi or less has become
increasingly widespread across southwestern IL, southeastern MO, and
portions of the Ozarks as residual BL moisture continues to
radiationally cool in the presence of a nearly stationary front with
light/calm winds. A Dense Fog Advisory remains in effect for these
areas through mid-morning. To the west, any dense fog is expected to
more patchy/localized and confined to river valleys. Fog will
dissipate within an hour or two of sunrise as diurnal BL mixing and
heating commences. Plentiful insolation and low-level southerly WAA
will support warmer, above average high temperatures today than
Monday and in the mid-70s to mid-80s F along with dry conditions
beneath slight rising heights within quasi-zonal upper-level flow.

An upper-level shortwave trough ejecting over the Central Plains
into Upper Midwest and associated cold front is progged to promote
thunderstorm development across western IA, northwestern MO, and
eastern KS this afternoon and evening. These thunderstorms will
organize into one or more east-southeastward-moving broken lines,
which CAMs indicate will move into central, northeastern MO and west-
central IL during the 03 to 05z timeframe. Conditions will be more
supportive of these thunderstorms being severe earlier and to the
west and northwest of the CWA. Exactly how quickly thunderstorm
intensity wanes as they enter the CWA is uncertain but thunderstorms
will be shifting southeast of stronger deep-layer wind shear,
instability/ moisture, and large-scale ascent. Therefore, the window
appears brief for any strong to severe thunderstorms with hazards of
damaging winds and marginally severe hail in central, northeastern
MO and west-central IL. Subsequent thunderstorm dissipation could
also be gradual overnight as they continue east-southward, in
conjunction with a slow weakening of a southwesterly LLJ ahead of
the approaching cold front.

CWA-wide dry conditions are anticipated to return by sunrise
Wednesday. The cold front will stall across the CWA somewhere in the
vicinity of the I-70 corridor as low-level WAA and moisture
transport continues to the south and over the front. With daytime
insolation, short-term model guidance advertise 1500 to 2000 J/kg of
MLCAPE being generated by the afternoon along and south of the
front. Mid-level heights will again be rising and frontal
convergence is not anticipated to be particularly strong, but little
convective inhibition may be sufficient for isolated to scattered
showers and thunderstorms during the afternoon and evening. Deep-
layer wind shear will be modest, decreasing confidence in any
organized strong to severe thunderstorms at this point. Along and
south of the front, high temperatures in the low to upper 80s F are
forecast and, north of the front, high temperatures in the mid to
upper 70s F are anticipated within a relatively cooler airmass.

Pfahler

&&

.LONG TERM...  (Wednesday Night through Monday)
Issued at 402 AM CDT Tue Apr 30 2024

An increase in showers and thunderstorms is forecast Wednesday night
as a southerly LLJ develops and increases low-level WAA over and
north of a northward retreat of the front as a warm front,
especially across central and northeastern MO in closer proximity to
the core of the LLJ and further away from the most vigorous mid-
level height rises associated with upper-level ridging across
the eastern CONUS.

Global model guidance is in agreement that another upper-level
shortwave trough will eject across the Central Plains into Upper
Midwest on Thursday with another trailing cold front reaching the
CWA. Along and ahead of this cold front, there is a signal in model
guidance for one or more multicell clusters of thunderstorms to
develop between late morning and afternoon and traverse the CWA
during sometime during the Thursday evening into early Friday
timeframe, with timing differences among model guidance. Stronger
mid-level flow and deep- layer shear will remain to the west and
northwest of the CWA, but the threat of isolated strong to severe
thunderstorms will still need to be monitored depending on whether
or not thunderstorms are in the CWA within or shortly after peak
diurnal instability.

Friday through the upcoming weekend, the large-scale wave pattern
over the central CONUS will remain generally similar with ejecting
upper-level shortwave troughs over the Central Plains into Upper
Midwest, permitting additional opportunities for showers and
thunderstorms at times as associated fronts also oscillate across
the Mid-Mississippi River Valley. Following late Thursday/early
Friday`s frontal passage, temperatures will cool to near or
slightly above average, persisting through the weekend. Global
model guidance diverge early next week with respect to whether a
more noteworthy upper-level pattern change takes place or not, but
a relatively active pattern of showers and thunderstorms and
seasonably warm temperatures appears on track in either case.

Pfahler

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 558 AM CDT Tue Apr 30 2024

Fog will continue to impact KSUS, KCPS, and, for a period, KJEF.
However, the most significant impacts with IFR flight conditions
will remain at KCPS. Fog will dissipate an hour or two after sunrise
with flight conditions improving to VFR area wide. Winds are
anticipated to become increasingly southerly with even some gusts
around 20 kt occasionally at KCOU and KUIN. Scattered showers and
thunderstorms will move into the area late this evening into
overnight. Currently, the greatest chance of thunderstorms is in the
vicinity of KUIN, with confidence lowering with time and
southeastward extend overnight at the remainder of terminals.

Pfahler

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until 9 AM CDT this morning for Crawford MO-
     Iron MO-Jefferson MO-Madison MO-Reynolds MO-Saint Francois
     MO-Sainte Genevieve MO-Washington MO.

IL...Dense Fog Advisory until 9 AM CDT this morning for Bond IL-
     Clinton IL-Fayette IL-Marion IL-Monroe IL-Randolph IL-Saint
     Clair IL-Washington IL.

&&

$$

WFO LSX