Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230802

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
202 AM CST Tue Jan 23 2018

202 AM CST

Through tonight...

GOES-16 differential water vapor RGB is simply breathe taking this
morning in its depiction of the large cut off low moving across
the region. Multiple smaller scale vorticity lobes are pivoting
around the overall larger tropospheric circulation. One such vort
lobe shows up nicely over SE WI early this morning and has been
responsible for an area of light to moderate precipitation across
southern WI and moving south into northern IL. As column cools,
rain is mixing with then changing to wet snow. Surface
temperatures are gradually cooling below freezing as well which
has allowed for some accumulation already over McHenry and Lake
Counties as seen on Lake County IL webcam network.

Through early morning, this batch of snow will continue to pivot
south while slowly translating eastward, with guidance indicating
a gradual weakening trend in the precip. As it looks now, Lake and
McHenry County will probably pick up 1 to perhaps locally 3 inches
of snow, while points south likely see an inch of less across the
heart of the Chicago metro area, including the city. Snow should
end by around 7 or 8 AM, but could still have some adverse impact
on the morning rush hour.

Otherwise, look for mostly cloudy, breezy, and much colder
conditions today. Temperatures will likely flat line through the
day with no recovery expected from early morning readings, in
fact, temps could even fall a degree or two this afternoon as cold
air advection continues.

The lower clouds are expected to gradually clear out later
tonight, however, by that time mid-high level cloudiness will
likely have arrived, resulting in mostly cloudy character of the
night tonight. The cloud cover should help limit the temperature
fall a bit and have trended lows toward the higher side of the
guidance spectrum for tonight.

- Izzi


202 AM CST

Wednesday through Monday...

Surface ridge axis moves across the region Wednesday afternoon
with backing winds and the start of warm air advection Wednesday
night. After a couple days (today and tomorrow) with near average
temperatures, we will begin to warm again Thursday into the 40s
and likely into the 50s by Friday. The persistent southerly flow
will do little to advect low level moisture north thanks to
surface ridge axis sprawled out along the length of the Gulf
Coast. The lack of low level moisture should prevent stratus from
becoming an potential issue in holding temps back late in the

A trough and associated surface cold front will move across the
region Saturday. Limited moisture return and strong forcing
passing well north of the area suggest that what rain does fall,
will probably not be very significant Friday night into Saturday.
The amplitude of the trough and magnitude of the cold air arriving
for the weekend has varied a bit in the operational medium range
runs, however, generally not looking like much more than a couple
day return to near average temperatures late Saturday through
Monday. Medium range models continue suggest the pattern will
remain progressive next week with another significant warm up
being advertised mid-week, though forecast confidence at that
distance tends to get pretty low.

- Izzi


For the 06Z TAFs...

Latest radar imagery showing current precip staying confined to
the RFD terminal, with periodic light rain moving elsewhere. This
will likely continue to be the trend, but as colder air arrives,
will see this rain transition over to all snow. This will likely
occur over the next 1-2 hours, and impact RFD first. Still think
the other terminals will see this snow, it will just be delayed
until later tonight into the early morning hours. Have maintained
a period of light snow in the TAFs, with a several hour window of
vis around 1sm. Still monitoring the possibility for a period of
heavier snow, that could drop vis lower. Will need to continue to
monitor trends and adjust accordingly. VFR ceilings in place but
under this snow, will see ceilings fall to MVFR and IFR. LIFR
ceilings still a possibility, and if it were to occur, RFD has the
highest chances of observing. Current southwest winds will quickly
turn to the northwest this morning and then remain this way for
much of today.



124 AM CST

North to northwest gales of 35 to 40 kt will occur today on the
backside of low pressure moving from lower Michigan to Quebec.
Lighter winds are then in store later tonight through much of
Thursday as high pressure moves across the region. The high
pressure will slide east and strengthen Thursday night into Friday
as low pressure moves from the northern Plains to Ontario. This
will result in strong south-southwest winds of at least up to 30
kt, with gales a good possibility into Friday evening and
mentioned in the current open lake forecast. Winds will veer
southwest and ease some Friday night as the cold frontal trough
approaches. There is some uncertainty on the surface pattern over
the lake on Saturday into Saturday evening due to the possibility
of a weak low pressure area moving over or south of the southern
portion of the lake.

For the nearshore waters, the upcoming forecast does not appear
favorable for significant ice redevelopment, with an up and down
temperature pattern. Hazardous conditions for small craft are in
store this morning through tonight for the Illinois shore and
through Wednesday morning for the Indiana shore. Gale force gusts
appear less likely for the Indiana shore today, so will remove
mention from the nearshore forecast. The next period of concern
for hazardous conditions for small craft is Friday through Friday
night with the strong south-southwest winds. Depending on how deep
mixing on land is, gale force speeds/gusts are possible.



LM...Dense Fog Advisory...LMZ777-LMZ779 until 3 AM Tuesday.

     Gale Warning...LMZ777-LMZ779 until 6 PM Tuesday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742...6 AM Tuesday to 4
     AM Wednesday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...6 AM Tuesday to 10
     AM Wednesday.




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