Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 020824

Area Forecast Discussion
National Weather Service Lincoln IL
224 AM CST Thu Feb 2 2023

Issued at 219 AM CST Thu Feb 2 2023

Seasonable today, but a cold front will usher in much cooler air
by tonight, with wind chill values below zero Friday morning. A
warmup is on the way this weekend though, with temperatures in the
mid 40s to low 50s by Sunday. Temperatures remain above normal
into next week. It will be breezy on Saturday, with southerly
winds gusting over 30 mph. No precipitation is expected through


.SHORT TERM...(Today and tonight)
ISSUED AT 219 AM CST Thu Feb 2 2023

The main forecast concern for the short term period is a glancing
blow of Arctic air that arrives tonight, resulting in sub-zero
wind chills early Friday morning, with wind chill values
approaching wind chill advisory criteria (-15F) in our far north

Broad, weak sfc high pressure remains in place just south of the
area, but the main features of interest for the short term period
are located to the north across Canada. An upper level pattern
consisting of ridging over western North America and a deep trough
over the Hudson Bay result in an occluded low pressure system
over the Hudson Bay and a strong (1040+ mb) high pressure system
over the Canadian Prairie. These systems will eventually bring
much cooler air into the region tonight, but ahead of the cold
front, southwesterly flow will push temps into the mid 30s to low
40s by early afternoon. While the front is expected to move
through during the afternoon, the cold air will lag behind by a
few hours. Winds/gusts increase behind the front, thanks to a
tightening pressure gradient and efficient PBL mixing in the CAA
regime. An investigation of HREF members reveals a 6-hour window
where NW gusts of 25-30 mph winds can be expected. For most of the
CWA, such gusts will occur between 6 PM and midnight, but that
timing window may be an hour or two earlier NW of the IL River,
and a few hours later along/south of I-70.

By Fri AM, the cooler air mass will be in place, with 850mb temps
below -10 degC. As noted above though, this is just a glancing
blow from the Arctic airmass, and the 850mb temps across the upper
Midwest, northern Ohio Valley, and New England will be much
cooler, falling below -20 degC. As the center of the sfc high
quickly approaches the area overnight, winds will trend downward,
but remain sustained above 10 mph for most of the night. With
temps falling into the single digits north of I-72, sub-zero wind
chills can be expected. Guidance has been consistent in showing
that the most likely outcome is wind chills remaining just above
advisory criteria (-15F) north of I-74. Still, it`s worth noting
that wind chills as cold as -20F are within the range of realistic
outcomes (25% chance of such values occuring north of a Macomb-
Bloomington line). The coldest wind chill values are expected to
occur between 3 AM and 9 AM on Fri.


.LONG TERM...(Friday through Wednesday)
ISSUED AT 219 AM CST Thu Feb 2 2023

While temps will be well below normal on Fri, a warming trend is
on the way, and confidence is high that Fri will be the coldest
day through at least the next work week. It will be dry through
the weekend, then the pattern becomes a little more unsettled into
next week, with rain chances returning to the forecast. Other
forecast highlights include breezy southerly winds on Sat and Mon.

Sfc high pressure will move over IL by Fri afternoon, resulting in
weakening winds. With a cold airmass situated over the region,
high temps will range from the teens north of I-72 to the low 20s
south of I-72. The sfc high shifts east overnight, resulting in
winds turning to southerly. Lows still fall into the teens, but
then the warmup begins on Sat. Breezy southerly flow is expected
to develop on Sat, but as we`ve often see in WAA regimes, the
presence of a low- level inversion leaves questions as to what
extent mixing will be able to transport stronger winds to the
surface. The expectation is that gusts of 30-35 mph will be
prevalent Sat/Sat night. Temps return to the low 40s on Sat, then
highs climb to the mid-40s to mid- 50s for Sun-Tues.

Into early next week, attention turns to a new trough progressing
out of the western US. This trough will have disturbances in both
the southern and northern branches of the jet stream, and
deterministic models remain split in the evolution. The GFS
continues to show the greatest disconnect between these two waves,
with the southern stream low cutting off and progressing later in
the week. The ECMWF has trended towards the GFS solution, but
still progresses the southern stream wave faster than the GFS,
albeit not quite in phase with the northern wave like it was
showing yesterday. WPC`s predictability discussion noted that
ensembles have been underdispersive and clustering near their
parent model, which decreases confidence in the probabilistic
guidance generated by the ensembles.

Despite some of these uncertainties, the key takeaways for early
next week remain on track: breezy SSE winds on Mon, with gusts to
35 mph, and a low chance (<20%) for rain courtesy of a cold front
late Mon night into Tues. With minimal instability and PWAT values
less than 1", any QPF is expected to be light. Forecast soundings
support this notion, showing considerable dry air in the mid-
levels. A more robust chance of rain could come later in the week,
courtesy of that slower-moving southern stream wave, but forecast
confidence is low as there are substantial differences in the
model handling of that wave by the middle of next week.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1123 PM CST Wed Feb 1 2023

VFR conditions will prevail through the period across central
Illinois with only high clouds expected at times. A weak cold
front will pass through the region without any precip Thursday
afternoon. Ahead of the front, south winds overnight will veer to
the southwest Thursday morning, then veer to the NW behind the
front Thursday afternoon. Winds will increase late afternoon into
the evening with NNW gusts into the 20 to 25 kt range.




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