175
FXUS63 KLSX 091139
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
639 AM CDT Thu May 9 2024

.KEY MESSAGES...

- Showers and thunderstorms are expected across portions of the
  area this afternoon into tonight, and then again at times
  through the first half of next week. Strong to severe
  thunderstorms are not currently anticipated.

- Temperatures will be cooler than normal Friday, but seasonable
  the rest of the forecast period.


&&

.SHORT TERM... (Through Friday Night)
Issued at 317 AM CDT Thu May 9 2024

The low responsible for yesterday`s severe thunderstorms is spinning
through central Illinois currently, its cold front entering
southeastern Missouri and southwestern Illinois. Moisture pooling
along the front has led to a patchy area of low stratus and fog
currently along the Mississippi River that will continue to drift
slowly eastward this morning before dissipating with the rising sun.

Today, a broad upper-level trough will begin to shear apart as a
shortwave dives through the Midwest, with part of the trough
becoming cut off over the western CONUS. The shortwave will aid in
forcing isolated to scattered showers and weak thunderstorms among
moisture wrapping around the backside of the departing low. This
convection will mainly occur across portions of northeastern
Missouri and west-central Illinois this afternoon before drifting
south-southeast this evening and tonight as the axis of the
shortwave digs into the Ohio Valley.

Tomorrow, the eastern portion of the upper-level trough will deepen
across the eastern CONUS, placing the Middle Mississippi Valley
beneath deep northwesterly flow. Within this flow, cooler air will
move into the CWA, causing temperatures to run below climatological
normals. Weak warm air advection in the low to mid-levels ahead of
another shortwave digging into the Midwest may lead to widely
scattered showers and maybe a thunderstorm on Friday. However, the
signal for convection is currently weak, so a dry forecast has been
maintained for now.

Elmore

&&

.LONG TERM... (Saturday through Wednesday)
Issued at 317 AM CDT Thu May 9 2024

Early Saturday, guidance consensus is that the previously mentioned
shortwave will be diving southward through the Midwest along the
western periphery of the eastern upper-level trough. In turn,
another cold front will pass north to south through the CWA.
However, a majority of the global ensemble members show mid-level
temperatures warming as ridging builds into the Middle Mississippi
Valley. Skies are expected to be mostly clear, allowing for deep
mixing and surface temperatures to be warmer than they are expected
to be on Friday despite the cold front and northwesterly surface
winds. Confidence is high that this will be a dry FROPA, as the
ensemble-based probability for measurable rainfall (0.01") is 10% or
less across eastern portions of the CWA.

On Sunday, ensemble clusters show upper-level ridging building
further into the area ahead of a shortwave over the Great Plains. As
a result, a surface high south of the region will move eastward and
return southerly low-level flow to the CWA. This will advect warmer
air into the CWA, with ensemble means supporting temperatures rising
back to or just above seasonal normals. The shortwave will move into
the Midwest on Monday, deepening and tracking a surface low through
approximately the Mid-South and then into the Ohio Valley. Ensemble
guidance shows a wide spread in the track and timing of this low,
leading to a medium chance (roughly 50%) for showers and
thunderstorms Monday through Tuesday. However, the chance for rain
is much higher and will likely be confined to a 12-24 hour period or
at the very least not a constant threat Monday through Tuesday.

Ensembles and deterministic guidance diverge on the phasing of the
upper-level pattern as we reach the middle of next week. Solutions
vary from zonal to southwesterly flow ahead of a deepening trough
over the Plains to northwesterly flow over the much of the Midwest.
If the former two solutions are realized, the CWA may experience
additional rounds of rain and a warming trend with temperatures. If
the latter solution is realized, calmer and cooler weather may take
place. Confidence in any of the scenarios is not high given guidance
spread, so the NBM initialization as been left as is.

Elmore

&&

.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 628 AM CDT Thu May 9 2024

Moisture pooling behind a departing cold front is producing low
stratus and patchy fog. Relative to the local terminals, this fog
and stratus is dissipating and moving eastward with VFR flight
conditions expected shortly. However, low stratus will return from
the north later this morning, impacting KUIN. This terminal will
be on the edge of this stratus through much of the day, so there
may be rapid and frequent changes in flight conditions through the
late morning into the afternoon. Additionally, isolated to
scattered showers and thunderstorms are expected this afternoon
around KUIN. Confidence in direct impacts is low at this time, but
this convection will be capable of brief reductions in visibility
and frequent lightning. This convection may also shift southward
toward the other local terminals during the evening and overnight
hours, but confidence in this occurring is very low right now. Low
stratus will expand further tonight to impact all local terminals
through the early morning hours.

Elmore

&&

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

$$

WFO LSX