511
FXUS63 KLOT 222046
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
246 PM CST Wed Jan 22 2025

.KEY MESSAGES...

- Accumulating snow (locally up to an inch) to persist
  through tonight resulting in slippery travel at times.

- Lake effect snow to linger in eastern Porter County, IN
  through Thursday afternoon.

- It will be chilly to start Friday but then expect a reprieve
  from Arctic cold through midweek, with 40s possible Mon-Wed.

&&

.DISCUSSION...
Issued at 246 PM CST Wed Jan 22 2025

Through Thursday Night:

Light snow and flurries continue to fester across portions of
northern IL (mainly along and north of I-88) as a shortwave
disturbance pivots over the Midwest and Great Lakes. While the
snow in our area is primarily in the form of flurries, more
robust snow is being observed in eastern IA and southern WI
which will drift into far northern IL over the next hour or
two (as of this writing). The snow will then continue to spread
south and eastward across the rest of the area this evening and
overnight as a cold front swings through the area. The small
snowflake size will limit the accumulations to an extent, but
will make the snow very efficient at lowering visibilities in
the 1-2 mile range with sub-mile values possible under the
heavier bands. Though, some snow accumulations (generally less
than 0.5 inches but locally up to an inch) are still expected
with surface temperatures well below freezing which will result
in some slippery travel through tonight.

The snow is expected to taper from west to east after 3 AM CST
as drier air filters in behind the aforementioned cold front.
However, some lingering light snow and flurries will remain
possible through daybreak Thursday morning when the front fully
clears the area. In the wake of the front, dry and partly to
mostly sunny conditions are expected for Thursday with the
exception of eastern Porter County, IN where some lake effect
snow is forecast to persist through the afternoon. Given the
modest lake effect parameters (ELs around 5000- 6000 ft) and
winds becoming more west-northwest through the day, the lake
effect should focus more into northern IN and southwest Lower MI
and not lead to many impacts in Porter County. Though some
minor accumulations may still occur (amounts around 0.5 inches)
and result in some slippery travel.

Otherwise, expect highs on Thursday in the upper teens and
lower 20s with overnight lows Thursday night in the single
digits to a bit below zero in spots. While winds on Thursday
will become blustery behind the front, speeds should reside more
in the 20-25 mph range which in turn will keep wind chills in
the single digits.

Yack


Friday through Wednesday:

After a chilly start to Friday, with single digits below and
above zero until an hour or so past sunrise, modest southwesterly
warm air advection (WAA) will help temperatures recover to the
lower 20s in the afternoon. There may be a few localized mid 20s
readings. Mid and upper level cloud cover will increase from
the west in the afternoon and early evening, so low temps will
likely occur Friday evening, followed by rising temps overnight.

On Friday night, a short-wave trough will move through the
northern Lakes and south central Canada, with it`s associated
surface low near or just north of Lake Superior. As the surface
low tracks east into Saturday morning, a warm front will lift
across the area, followed closely on its heels by a cold front. A
sheared positively tilted impulse and corresponding mid and upper
level speed max on the southern periphery of the trough will pivot
across the IL/WI state line area in the late evening and
overnight. If enough moisture can materialize, there may be some
flurries near and north of I-88, or even a brief period of light
snow, such as shown on recent NAM runs, 20-40% of SREF members,
and hinted at by the Canadian guidance. The ECMWF/EPS and GEFS/GFS
are much less enthused, so for now will maintain the non-zero
but non-mentionable NBM PoPs.

Saturday will be partly to mostly cloudy and breezy with winds
veering to west-southwest as the cold front passes the area.
There`s a slim chance of a f-gen driven band of snow near the
frontal trough overhead Saturday morning into the afternoon if
better saturation can develop, though the most likely outcome is
dry conditions prevailing. The brunt of the cold air advection
(CAA) behind the front will occur Saturday night, which entails
Saturday likely being the first instance of daytime highs above
freezing since the brief warm-up last Friday. Near to perhaps
slightly below normal temps are then in store Saturday night and
Sunday.

Next work week, cut-off low in the southwest and deep troughing
over eastern Canada will keep the clipper track well to our north
result in a continued dry but notably milder west-northwest flow
pattern. A blustery warm-up on Monday could yield highs near or
above 40F in spots. There may be a weak cold front passage Monday
night, lowering confidence a bit in Tuesday`s temps. Then signs
point toward even milder conditions next Wednesday with highs
in the lower to mid 40s.

Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1135 AM CST Wed Jan 22 2025

Main aviation concerns:

- Periods of -SN expected through early THU AM
- Lingering IFR vsbys early this afternoon at ORD/DPA/RFD
- Another round of IFR vsbys possible this evening between 1-7Z
- NW wind shift behind cold front early Thursday AM

The area of snow has lifted north of MDW/GYY late this morning.
The back edge of the clearing appears to have stalled between
ORD and MDW (at least for now). Accordingly, opted to hang onto
TEMPO groups for IFR vsbys in snow for another hour or two at
ORD/DPA/RFD before the western edge lifts northeast of the
terminals.

Confidence in snow trends for this evening and overnight
remains low as models continue to have a poor handle on things.
Opted to not make too many major changes to the inherited
forecast with a period of IFR vsbys in snow again between the
1-7Z timeframe. Can`t rule out brief dips to LIFR (1/2-3/4 SM)
if a more robust band of snow manages to develop. Additional
accumulations of up to half an inch are possible. Flurries could
linger after daybreak Thursday until low clouds begin to thin
and lift back to VFR or even clear out toward mid-morning.

Winds will remain breezy out of the southwest through sunset
then ease ahead of an approaching cold front. Winds turn
northwest behind the front early Thursday morning with gusts in
the mid-20kt range expected on Thursday.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 6 PM CST this evening for the IL
     and IN nearshore waters.

&&

$$

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