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FXUS63 KILX 060500 AFDILX

Area Forecast Discussion...Updated Aviation National Weather Service Lincoln IL 1100 PM CST Tue Dec 5 2023

UPDATE
Issued at 843 PM CST Tue Dec 5 2023

An extensive low cloud shield over the Midwest has developed some breaks over central IL associated with some downward forcing outside a lake-effect cloud/rain band extending southward from Lake Michigan to northeast Vermilion County. As winds shift more westerly overnight, the band and associated cloud breaks should shift eastward as well, returning skies to overcast. Temperatures Range from the mid 30s near I-74 to lower 40s south of I-70 as of 8 pm, and should be able to fall several degrees with some weak cold advection overnight, so lows ranging from upper 20s to lower 30s look on track. Have updated cloud cover and PoPs to account for trends mentioned above.

37

DISCUSSION
Issued at 222 PM CST Tue Dec 5 2023

------------ ---------->>> KEY MESSAGES <<<---------- ------------

* Departing low will leave breezy northwest winds rest of today gusting to around 20 mph into the evening along with a few spotty light showers or sprinkles through around sunset.

* Southwest winds gusting to 25 to 30 mph are expected Thursday which will help result in temps warming well above normal into the 50s and possibly 60s to end the week.

* A multi-faceted storm system will traverse the region this weekend bringing the potential for severe weather, heavy rain followed by a changeover to wet snow, though details remain uncertain at this distance.

------------ -----------<<< DISCUSSION >>>----------- ------------

This afternoon, low pressure has shifted to our east into Indiana while a trailing cold front has pushed across the forecast area.
In its wake, breezy northwest winds are expected the rest of today. Cold air advection into the region is producing steep low level lapse rates with equilibrium levels up to around 4000 feet.
While not particularly deep, low level instability and moisture coinciding with low level cyclonic flow may support a few additional light showers or sprinkles, especially across east central Illinois. Thermal profiles appear warm enough to support mainly liquid precip falling, but wouldn't be completely out of the realm of possibility to see a few snowflakes mixing in especially near/north of I-74. Any precip should diminish around sunset this evening.

Surface ridge will translate across the region Wednesday allowing winds to diminish and generally nice and seasonable conditions Wednesday. Winds will turn SWerly with passage of the ridge axis and a corridor of moderately strong mid level warm air advection will begin to overspread later Wednesday and continue through the day Thursday. Meanwhile, low pressure will track across the northern Great Plains and lift across the Canadian Prairies to Hudson Bay resulting in a strengthening pressure gradient across portions of the mid and upper Mississippi Valley. Gusty southwest winds will develop as a result. NBM indicates a high likelihood (>90%) of gusts in excess of 25 mph across the entire forecast area Thursday and around a 20% chance for wind gusts peaking around 40 mph. Expect the bulk of the gusts Thursday to be in the 25 to 30 mph range with sporadic higher gusts possible.
Temperatures will warm well above normal Thursday and most of the area should warm into the mid to upper 50s. Similar conditions will persist through the day Friday and temperatures may warm a few more degrees with a few spots possibly tagging the 60 mark, mainly west of I-55.

Attention will turn west over the weekend as a deep upper trough is progged to develop over the western CONUS translating across the Great Plains and Mississippi Valley this weekend. Latest runs of the GFS and ECMWF are in reasonable agreement showing a surface low deepening over the southern Great Plains Friday night and lifting from a roughly St Louis to Chicago line through the day Saturday. This multifaceted storm will bring several hazards to portions of central Illinois, including the potential for heavy rain, severe storms, and snow. DESI LREF indicates that around a 30 to 40 percent chance for exceeding an inch of rain is present roughly south of I-72/Danville, and there is a 10 percent chance of exceeding 2 inches roughly along and south of I-70. It also indicates around a 30 percent chance of surface based CAPE exceeding 100 J/kg up to around the I-70 corridor with non-zero chances across the entire forecast area. This dynamic system will be accompanied by strong wind shear, so any instability that is able to develop could result in a high shear, low CAPE severe weather setup. Surface low track from the latest GFS/ECMWF are concerning that parts of the state may see severe weather, but lingering uncertainty in the northern extent of the instability and any potential shifts in the low track lead to murky details at this distance. Behind the low, temperatures will cool enough to support a change over to snow, but this coincides with tapering moisture and weakening forcing. 75th percentile 24-hour snowfall amounts are up to an inch for areas north of I-70 but there are still a number of models that bring minor to moderate accumulations to the region, so this will bear watching.

Deubelbeiss

AVIATION
(For the 06z TAFs through 06z Wednesday Night)
Issued at 1100 PM CST Tue Dec 5 2023

An extensive shield of low cloud cover is over most of the Midwest, with a clearing area extending southwestward from Lake Michigan through parts of central IL. Timing areas of clearing inside this large area of MVFR cigs is the main forecast challenge with the 06Z TAFs. Expect the clearing to gradually shift eastward overnight, with MVFR cigs becoming predominant.
Eventually, the MVFR cigs will shift east of the terminals Wednesday afternoon, but timing remains uncertain. Winds NW 6-12 kts through tonight, shifting to SW and eventually SSW 16Z-23Z.

37

ILX WATCHES/WARNINGS/ADVISORIES
None.

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