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FXUS63 KLSX 061722 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1122 AM CST Wed Dec 6 2023

KEY MESSAGES

- Early morning clouds will give way to sunshine across most of the area today. Wind will turn to the south-southwest which will help temperatures warm up to near or slightly above normal.

- Persistent southwest flow will produce temperatures 15 to 20 degrees above normal on Thursday and Friday.

- A strong storm system will bring rain to the area Friday night and Saturday. There is a 20-30 percent chance for thunderstorms during this timeframe as well. Additionally, there is still around a 20-30 percent chance for the rain to mix with or change to snow late Saturday night into Sunday morning.



SHORT TERM
(Through Late Tonight)
Issued at 343 AM CST Wed Dec 6 2023

A ridge of high pressure will move southeast from western Missouri into eastern Arkansas today. The The wind will turn back to the south-southwest as the ridge axis traverses the forecast area through the morning. The southerly flow on the back side of the ridge will bring warmer temperatures to central and northeast Missouri as well as west central Illinois today. The warmer air will also help break up the low cloud deck from west to east this morning. East central Missouri and southwest/south-central Illinois will see ceilings persist the longest this morning, and miss out on a few hours of insolation. Therefore temperatures won't rebound quite as much. This will result in about a 10 degree temperature gradient from central Missouri east to south central Illinois with highs in the low 50s in central Missouri to low to mid 40s in south central Illinois. Southwest flow will continue tonight which will result in warmer low temperatures in the mid to upper 30s on Thursday morning.

Carney

LONG TERM
(Thursday through Tuesday)
Issued at 343 AM CST Wed Dec 6 2023

Little has changed in the forecast for Thursday through Sunday. The strong upper ridge currently over the Western US will move east ahead of a deep trough. The combination of persistent southwest low level flow and the upper ridge will produce temperatures 15 to 20 degrees above normal in the upper 50s to mid 60s on Thursday and Friday. Meanwhile the aforementioned trough will be digging into the western Plains. The resulting surface low is forecast to move northeast from Oklahoma/Kansas into the Midwest Friday night/early Saturday morning. The positively tilted upper trough will continue to dig and drive the cold front southeast through Missouri and Illinois Saturday. While the operational 06/00Z GFS and ECMWF appear to be in very good agreement on the position of the trough as well as the timing of the cold front Saturday, the EOF patterns in the ensemble cluster analysis indicate uncertainty in both timing and amplitude of the trough. Additionally, there are 3 clusters centered around Saturday and 4 clusters centered around Sunday indicating increasing uncertainty with the trough as it moves east.
This translates into uncertainty with respect to the timing of the cold front, and indeed the 25th percentile high at STL is 48 on Saturday, while the 75th percentile is 63. Also of note, the LREF mean is showing 100-200 J/Kg of CAPE ahead of the front, and the operational GFS and ECMWF are showing 400+ J/Kg. If the front ends up being slower those CAPE values could be woefully underdone. The high shear environment will be supportive of severe storms if there's enough instability, so we will have to keep a close eye on this as the forecast progresses. One other thing to watch is the possibility of snow behind the front Saturday night into Sunday.
Currently the GFS and ECMWF dry out before it gets cold enough, and the LREF shows a maximum of only 25-30% chance for accumulating snow. However there are a few members of the ensembles that produce quite a bit of accumulating snow in the cold sector. Again, this looks like a very low probability at this time. The remainder of the forecast through Tuesday looks relatively quiet behind the weekend system with near normal temperatures.

Carney

AVIATION
(For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1122 AM CST Wed Dec 6 2023

Dry and VFR flight conditions are expected to prevail through the TAF period, leaving LLWS as the only concern overnight into Thursday morning. Following southwesterly winds becoming southerly this evening, a west-southwesterly 50-kt low-level jet will shift eastward over the region late this evening/overnight, leading to LLWS from a combination of speed and directional wind shear within the lowest 2kft of the atmosphere. Once winds veer to the southwest and strengthen with occasional gusts of 17 to 27 kt Thursday morning, impactful LLWS will diminish.

Pfahler

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

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