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FXUS63 KILX 080511 AFDILX

Area Forecast Discussion...Updated Aviation National Weather Service Lincoln IL 1111 PM CST Thu Dec 7 2023

UPDATE
Issued at 750 PM CST Thu Dec 7 2023

00z surface map indicated sprawling low pressure centered at 988 mb over ND, with high pressure off the southeast US coast. The resulting pressure gradient was producing brisk south winds across central IL, gusting as high as 35 mph. The gradient winds are being augmented by a 45-55 kt 925 mb jet. The low level jet will continue through the overnight hours, so we expect the gusty winds to persist, highest over eastern IL under the jet core. Strong low level warm advection will keep lows unseasonably mild and not far from current readings in the mid 40s (near typical highs for early Dec). Only high cirrus to affect the region tonight, with lower clouds currently over eastern OK and northwest AR spreading northeast into western IL after sunrise.

25

DISCUSSION
Issued at 203 PM CST Thu Dec 7 2023

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

* Unseasonably warm and breezy conditions will continue in the near term ahead of an approaching front this weekend. Temps will run well above normal with highs well into the 50s to around 60 degrees again Friday. Similarly warm conditions are expected ahead of a cold front east of I-57 Saturday.

* Impacts from a weekend system continue to trend down with the threat for severe weather and heavy rain now appearing much lower compared to previous forecasts.

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

This afternoon, a broad 990mb low is located over the Dakotas while a broad high is over the southeastern US. A tight southwesterly gradient is located over central Illinois in between these two features, while aloft, a very strong upper ridge spans much of the Great Plains and Mississippi Valley. The combination of SW flow with the upper ridge is contributing to unseasonably warm conditions across central Illinois today, and these conditions will continue into the day Friday as synoptic features slowly translate east. Temps in the upper 50s to around 60 degrees will be in place areawide Friday with gusts into the 20 to 30 mph range.

Meanwhile, a strong shortwave will begin to dig out a deep trough over the Great Plains Friday through the weekend. This wave will begin to lift across IA/MO Friday evening to the Great Lakes Friday night. Weak surface reflection will develop in response tracking from near Quincy late Friday evening to near Milwaukee overnight. A trailing cold front will sweep across central Illinois Friday night into Saturday morning. Low level saturation will increase tonight into Friday morning ahead of these features, and could begin to see some drizzly conditions Friday evening followed by light rain overnight associated with the upper wave. Given the continued faster trends with frontal passage, instability now looks unsubstantial and the threat for thunderstorms and heavy rain has diminished. NBM probabilities of exceeding an inch of rain is only around 20 percent now in the far southeastern forecast area south of I-70 and less than 5 percent further north. The probability of exceeding 100 J/kg SBCAPE is less than 30 percent south of I-70 with no expectation for surface based instability further north. This makes sense given the timing of the frontal passage and overcast conditions that develop through the day Friday. With this in mind, the threat for severe weather should remain at least south of I-70 with most of central Illinois out of this threat.

Saturday evening, frontal boundary will stall over Indiana and the lower Ohio Valley while another upper level disturbance propagating along the frontal zone causes another round of showers that lift back north slightly towards or into the area. Areas south of I-70 have the best chance for seeing this next round of rain. These chances will diminish again around midnight as the wave exits the area. Most of central Illinois will remain dry Saturday evening.

A large area of high pressure will build across the Great Plains this weekend, spreading into the Mississippi Valley Sunday and continuing to spread across the eastern CONUS through the first half of next week. This will result in several days of dry weather across central Illinois with temperatures back closer to their seasonal norms.

Deubelbeiss

AVIATION
(For the 06z TAFs through 06z Friday Night)
Issued at 1111 PM CST Thu Dec 7 2023

A strong low level jet remains overhead tonight. Sustained south winds have been around 10-15 kt, with sporadic gusts near 25 kt.
Therefore will keep marginal LLWS in the TAFs trough 11-13z. MSLP gradient remains tight through Friday evening, so similar speeds and direction can be expected. Stratocu is already advancing northeast through MO, and will overspread the terminals near/after sunrise. Currently expecting this leading batch to be just above MVFR levels, but lowering to MVFR during the afternoon. During the evening saturation increases ahead of a cold front, so widespread IFR ceilings are forecast, along with scattered showers.

25

ILX WATCHES/WARNINGS/ADVISORIES
None.

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