Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181709

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1109 AM CST Tue Dec 18 2018

230 AM CST

Through Wednesday...

Mild but otherwise quiet weather conditions expected in the near
term across northern Illinois and northwest Indiana. Ridge of high
pressure stretches from the western Great Lakes to the mid
Mississippi River Valley early this morning and will settle over
the upper Ohio River Valley by this evening. Southwest flow
overspreads the region in its wake and warm air advection and
mostly sunny skies should push temperatures several degrees higher
today than yesterday. Expect afternoon highs to top out in the
mid to upper 40s, and wouldn`t be out of the realm of possibility
to see some locations tag 50F.

Temperature forecast a little more tricky for Wednesday. Cloud
cover is expected to increase through the day. Guidance suggests
a low amplitude shortwave trough will lift across the region
early on Wednesday, coinciding with persistent corridor of WAA.
Forecast soundings show that low level saturation will occur
during this time frame, and couldn`t rule out a few light
sprinkles or possibly some drizzle, particularly over north
central Illinois. Forcing is expected to be strongest north of the
CWA, so will only carry silent 10 percent PoPs at time, but
something to keep an eye on. Cloud cover and precip chances will
affect temps on Wednesday. 850 temps of +5-6C are expected to be
in place over the region, which climo suggests would support
temperatures into the 50s, though this is likely contingent on
cloud cover. Remain a bit conservative with temps Wednesday, but
there is some upward potential from going forecast.



230 AM CST

Wednesday night through Monday...

A deep upper trough is expected to inch east across the Great
Plains midweek. A closed low at the base of the trough is expected
to shift east across the lower Mississippi Valley Wednesday, then
lift to the upper Ohio Valley Friday. A northern wave is expected
to dig across the Great Lakes on Thursday. While the best forcing
stays north and south of the local area with these waves, there
will still likely be a period of precipitation late Wednesday
night into Thursday. Forecast soundings show that most of the
precipitation should fall as rain, but could be a brief change
over to snow before precip ends late Thursday or early Friday.
Upper level flow is highly meridional, and the 0C 850mb isotherm
is expected to spill into the deep south by Thursday, with the
thermal trough shifting east across the local area on Friday.
Expect temps to top out in the upper 30s or lower 40s across the
NW CWA Thursday, but in the mid to upper 40s in the SE CWA. Temps
may peak early in the day, then hold steady or fall into the 30s
with little if any recovery expected on Friday as that thermal
trough moves overhead.

Broad surface ridge will build across the region Friday into early
next week, and should provide generally dry and seasonable
conditions. Temperatures top out in the 30s through the remainder
of the forecast period early next week. May have to keep an eye on
a hybrid clipper/Pacific wave over the weekend, but there is poor
consensus on the track of this system this far out.



For the 18Z TAFs...

VFR conditions are expected through the period, though some MVFR
clouds will be possible late Wednesday morning through the
afternoon. Additionally, LLWS of around 20 knots over the lowest
1.5kft will be possible at all sites tonight.






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