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FXUS63 KILX 300541 AFDILX

Area Forecast Discussion...Updated Aviation National Weather Service Lincoln IL 1141 PM CST Wed Nov 29 2023

SYNOPSIS
Issued at 1141 PM CST Wed Nov 29 2023

Dry and seasonable weather will continue into Thursday morning.
After that, a large storm system will spread rain across central and southern IL, for Thursday afternoon into Friday. Rain amounts of one half to one inch are expected.

UPDATE
Issued at 859 PM CST Wed Nov 29 2023

Light southerly winds continue to blow this evening, helping to advect milder air into the region and enhance mixing to prevent temperatures from dropping as much as the past few nights. Only a couple of degrees more temperature falls should take place. Lows ranging from around 30 on I-74 to the mid 30s in southeast IL look on track. High clouds should keep streaming across the area as well, associated with an upper level shortwave over the central Plains. No significant updates are needed this evening as the forecast has these features incorporated well.

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DISCUSSION
Issued at 223 PM CST Wed Nov 29 2023

KEY MESSAGES

* Widespread moderate rainfall Thursday night, with beneficial rainfall amounts mainly in a 0.6 to 1.1" range.

* Temperatures within a few degrees of normal into early next week.

Quiet weather will continue overnight and Thursday morning. High pressure over the Gulf coast and a low north of the Great Lakes will keep southwest winds blowing, with mild air prevailing and lows around 30-35F. MSLP gradient tightens by midday Thursday ahead of a cold front pressing into the Midwest, leading to a period of breezy southwest winds Thursday afternoon, gusting up to 30 mph in eastern IL.

The focus of the forecast is the evolution of the 500 mb wave currently entering southern CA. As this lifts northeast over the next 24 hours, surface low pressure is forecast to develop over the TX panhandle, then track into central or southern IL Friday afternoon. Several ingredients will be present to produce a swath of widespread moderate rainfall, including a coupled upper level jet (providing increased ascent Thursday night), and an unseasonably moist airmass advecting northward, resulting in PWATs at the 90th percentile of climatology for late November. We expect light rain to increase in coverage from the southwest Thursday afternoon and evening, with maximized ascent for moderate rainfall after midnight. Heaviest QPF axis has continued to edge northwest closer to the I-55 corridor, while higher end amounts have decreased from previous forecasts. Ensemble probabilities show a 95% chance of exceeding 0.50" near/southeast of I-55, with probs trailing below 50% west of the IL River. Probs of exceeding 1" have decreased markedly with the area between I-55 and I-70 peaking at 30-50%. Nonetheless, this will be a widespread beneficial rainfall with much of the region running 1-3" deficits over the past 30-60 days.

We continue to monitor the potential for a brief period of freezing rain at the onset late Thursday evening across the far northwest CWA, primarily north of a Galesburg to Lacon line. Here, surface temperatures will be very near 32F at the onset, before warming overnight. Current deterministic forecast shows temperatures at 33-35F, and ensemble probs show 10-20% chance of freezing rain. So freezing rain is a low probability occurrence, but if it happens impacts would be slick roads for a few hours before warming after midnight.

The bulk of the rainfall will be pushing northeast of the area after sunrise Friday, but we will continue with 20-30% chance of scattered showers into Friday evening, as the low slowly tracks across the state.

Mid level flow remains active from the southwest into early next week, with embedded shortwaves bringing 20-30 PoPs Saturday night and Sunday. Behind this, upper level flow veers northwest resulting in a cooler pattern early next week. Global models indicate a clipper affecting the Midwest or Great Lakes Monday or Tuesday. Large spread on timing and location so slight chance PoPs from blended guidance look reasonable.

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AVIATION
(For the 06z TAFs through 06z Thursday Night)
Issued at 1141 PM CST Wed Nov 29 2023

MVFR cigs and a chance of rain will arrive after 18Z-20Z as a storm system approaches from the southwest. Continued deterioration of cigs/vsbys will take place after 00Z with this system, likely reaching IFR cigs and MVFR vsbys by 06Z. Until then, VFR conditions are expected with increasing high- and eventually mid-level cloud cover. Low level wind shear is expected to develop due to 40 KT WSW winds aloft at KPIA from around 10Z-14Z. Surface winds SSW 5-10 kts increasing to 10-15 kts with gusts near 20 kts after 14Z-18Z. Winds should decrease again to under 10 kts by 01Z.

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ILX WATCHES/WARNINGS/ADVISORIES
None.

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