475
FXUS63 KLSX 090826
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
326 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 06z TAFs through 06z Thursday Night)
Issued at 1035 PM CDT Wed May 8 2024

While an isolated shower/thunderstorm cannot be totally ruled out,
the best threat has shifted well off to the southeast of the
terminals with dry weather likely the remainder of the night into
Thursday morning. Scattered showers may impact KUIN in the
afternoon hours, but they should stay north of the other
terminals.

A cold front is near KCOU/KJEF and should approach KUIN over the
next 1-2 hours. Metro terminals will be closer to 0600-0800 UTC.
Near the front, winds are light and variable. Upstream
observations (including at KCOU/KJEF) have shown some patchy (and
at times dense) fog development in areas of clearing. The recent
rainfall is also helping. Therefore, would not be surprised to see
a brief period (~1 hour or so) of some fog at the metro and KUIN
terminals over the next few hours. However, any fog should be
short lived as winds behind the front pick up enough out of the
west/northwest to improve visibilites back to VFR. The threat
overnight into Thursday morning should be more on low stratus.
Model guidance is all over the place, but I could see some high
IFR/low MVFR ceilings late tonight into early Thursday morning.
Best chances appear to be at the metro and KUIN terminals.
Conditions should improve from west to east late tonight through
Thursday morning.

Gosselin

&&

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

$$

WFO LSX