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FXUS63 KLSX 021734 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1134 AM CST Sat Dec 2 2023

KEY MESSAGES

- Confidence has increased in the chance for light rain both tonight (up to 90%) and Sunday night (up to 45%).

- Seasonable temperatures are expected today through the middle of next week, with confidence remaining high (90%+) in above normal temperatures by Thursday and continuing into the weekend.



SHORT TERM
(Through Late Sunday Night)
Issued at 321 AM CST Sat Dec 2 2023

Water vapor imagery shows southwesterly flow over the Middle Mississippi Valley, which continues to keep an area of low pressure at the surface moving eastward away from the area near the Great Lakes. Lingering low-level moisture has kept the CWA beneath a blanket of low stratus overnight, with surface observations showing patchy fog across portions of the area. While the fog may dissipate through this morning, guidance consensus is that low-level moisture will continue to pool behind a front south of the CWA, leading to the cloud cover hanging around through the day.

Within the southwesterly upper-level flow, a vorticity maximum currently seen in water vapor imagery over the Southwest will eject into the Southern Plains and head toward the Middle Mississippi Valley tonight. As this occurs, an upper level jet streak will strengthen, pairing with the incoming vort max to increase lift over the region and deepen a low moving into the CWA. As a result, rain chances will increase this evening from west to east, with the greatest probability for rainfall (80-90%) being just ahead of the low across northeastern Missouri and west-central Illinois. Rain will exit the area to the east Sunday morning as the low quickly shifts eastward.

In this low's wake, subtle ridging will build into the CWA, correlating with a bump in temperatures Sunday over what is expected today. Much of the day will be dry, but this calm weather will be short-lived, as yet another vort max digs southeastward into the Ohio Valley. As it does so, its surface reflection will move into central Missouri during the evening and track roughly along I-70 during the overnight hours. This track favors another round of light rain along and north of the I-70 corridor, ending by early Monday morning as the low continues eastward.

Elmore

LONG TERM
(Monday through Friday)
Issued at 321 AM CST Sat Dec 2 2023

Through Monday morning, the vorticity maximum responsible for our light rain Sunday night will be digging into the Southeast as deep northwesterly flow remains in place over the Middle Mississippi Valley. Within this northwesterly flow, yet another vorticity maximum will dive southeastward into the Midwest late in the day and into the night, its surface reflection following a similar trajectory. There will still be enough moisture across the Midwest to lead to a low chance for precipitation with this system Monday night, though ensemble consensus is that the surface low will pass north of the CWA - an unfavorable track for precipitation in the CWA. As a result, a majority of ensemble members (80-90%) keep precipitation out of the area, and I've maintained a dry forecast. If the low tracks further south, light rain could fall across portions of west-central and southern Illinois.

Ensemble and deterministic guidance consensus is that this will be our last vorticity maximum for the period, as the upper-level trough kicks eastward as weak northwesterly flow remains in place over the CWA. As a result, ensembles are clustered around near- normal temperatures through Wednesday.

For Thursday into the weekend, ensembles show upper-level ridging building into the Middle Mississippi Valley, with high pressure at the surface shifting eastward and returning southerly low-level flow to the CWA. This pattern favors warming temperatures, though ensembles show varying degrees of the ridge building over the region. Despite this spread in solutions, even the 10th percentile of ensemble temperatures is above normal Thursday and Friday, leading to high confidence (90%+) that warmer than normal temperatures will be experienced area-wide during this stretch.

Elmore

AVIATION
(For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1133 AM CST Sat Dec 2 2023

Impacts will persist at all terminals through the duration of the TAF period. Low stratus will produce IFR/low MVFR and continue through the overnight hours. This evening and overnight, a surface low pressure system will push across the region southwest to northeast and bring light rain to the area. Light rain will lower visibilities slightly with KUIN expected to have the lowest visibilities and more persistent rain. Confidence remains low that more persistent light rain will directly impact KCOU, KJEF, and the St. Louis Metropolitan terminals after the initial "push" of light rain, therefore decided to keep VSCH through early Sunday morning at the aforementioned terminals.

Winds are expected to be light through the early evening and then increase as the low pressure system moves across the area. Winds will then become more southerly on Sunday.

MMG/Byrd

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

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