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FXUS63 KLSX 111139 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 539 AM CST Mon Dec 11 2023

KEY MESSAGES

- Temperatures vary little through the week with highs and lows near or slightly above seasonal norms.

- Dry conditions are favored with less than 20 percent of the ensemble members supporting measurable rainfall next weekend.



SHORT TERM
(Through Late Tuesday Afternoon)
Issued at 310 AM CST Mon Dec 11 2023

Regional analysis (09z) shows a weak surface low over southeast Nebraska tracking east-southeast into northwest Missouri this morning. IR satellite shows broken mid/high clouds (12k feet)
extending from north-central Missouri southeast along and west of the Mississippi River. Clouds are in response to a wing of weak warm air advection that will have a modest impact on temperatures today.
A nearly stationary boundary extends southeast of the surface low and bisect Missouri. Light/variable winds running ahead of the boundary, eventually turn out of the south over the southwest third of the state.

The features will gradually translate east/southeast today, drawing in slightly milder temperatures this afternoon. Clouds will clear out as the features continue to translate east/southeast through the morning. Mostly sunny skies and strengthening southerly surface flow will allow for a modest boost in temperatures with highs about 5 degrees warmer than Sunday. Conditions remain stable through the short term period with mainly clear skies allowing temperatures to fall back into the mid-20s/low-30s tonight.

A cold front pushes into the region late tonight into Tuesday, which has a greater impact on northeast Missouri and west-central Illinois. The front lacks moisture with mostly clear skies persisting into Tuesday, but does have a sensible impact on temperatures over northeast Missouri and west-central Illinois.
Highs will top out in the low-40s in these locations, while southern sections of the forecast area warm closer to 50 degrees. The front is expected to clear most areas by Tuesday night. However, mid to high clouds are sent into the region from west to east as an upper closed low positions over the southwest CONUS. This should help keep temperatures from falling too much despite the cold front. Lows are similar to Monday night - mid-20s/low-30s.

Maples

LONG TERM
(Tuesday Night through Sunday)
Issued at 310 AM CST Mon Dec 11 2023

NBM ensemble data shows exceptionally tight clustering with 25th- 75th percentile temperature ranges of 3-5 degrees (at most) through Friday. Even heading into next weekend, when spread slightly increases, temperature ranges are only 5-7 degrees. The only precipitation probabilities are introduced late Friday into Saturday with less than 10 percent of the members supporting measurable rainfall (0.01" or greater) across any portion of the CWA. This being said, there is higher than average confidence that current forecasts through the period will play out close to reality with a few minor notes.

A strong surface high (near 1040mb) traverses the Upper Midwest into the Ohio Valley through midweek as a cold front slips south of the forecast area. Cooler air will be fed into the region on the light east and northeast surface flow that works around the eastern and southern periphery of the surface high. Meanwhile, medium and long range guidance shows high RH values (70-90 percent) ejecting eastward from the closed low out of the southwest CONUS, signaling mid/high level cloud cover impeding sunshine through much of the day. With temperatures starting out in the mid-20s to low-30s, lack of sunshine and passage of the front will keep temperatures from moving much with highs more in line with December norms (40s).

Temperatures climb slightly Thursday into Friday on southerly return flow as the high shifts east. The main question has been related to rainfall potential, which continues to look low, but non-zero. NBM remains below 10 percent on precipitation chances through the weekend, along with LREF ensembles showing less than 20 percent supporting measurable rainfall. GFS deterministic guidance shows a northern stream closed low passing overhead from northwest to southeast, while the ECM has the system moving well to the north.
This gives little confidence that any meaningful rainfall will come to fruition with dry and mild conditions through the end of the period.

Maples

AVIATION
(For the 12z TAFs through 12z Tuesday Morning)
Issued at 536 AM CST Mon Dec 11 2023

Mid/high clouds will stream overhead this morning into the early afternoon before skies clear. Light and variable surface flow will turn southerly from mid to late morning and strengthen through the afternoon. Winds subside once again Tuesday evening.
VFR conditions are expected throughout the period with little/no impact otherwise.

Maples

FIRE WEATHER
Issued at 536AM CST Sun Dec 10 2023

The early morning's light and variable surface flow will turn out of the south through mid/late morning. Sustained surface winds increase to around 10 knots this afternoon with an occasional gust possible, especially over northeast Missouri and west-central Illinois. Temperatures are expected to warm into the low-50s over southeast Missouri today, while dewpoints remain in the 20s. This results in minimum RH values around and just below 30 percent around peak heating. Though winds increase and temperatures warm, overlap of conditions result in a short-lived, limited area to support marginal/borderline elevated fire conditions. If elevated fire criteria is met, it would most likely occur between 18z and 22z in a general area south of Jefferson City, along and west of highway 63.

BRC



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

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