Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 171748

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1248 PM CDT Tue Apr 17 2018

334 AM CDT

Through tonight...

Large upper level system continues to depart this morning, with
solid stratus slowly drifting southeast. With the departure of
these clouds, locations will see partly cloudy to partly sunny
skies early this morning. However, with cyclonic flow still in
place, will likely see stratocu develop close to midday and with
this cloud cover remaining into the afternoon. Surface pattern and
winds will weaken this afternoon, with onshore winds developing
later today.

Dry conditions today into this evening will likely remain,
however, there will be a slight chance of precip late tonight into
early Wednesday morning for locations west of I-39 ahead of the
system which will impact the area Wednesday.



334 AM CDT

Wednesday through Monday...

Main forecast concerns are with a wintry mix of freezing rain,
sleet, and snow possible Wednesday morning into Wednesday evening
along with a chance of thunderstorms Wednesday morning. A Winter
Weather Advisory has not been issued at this time given lower
confidence with duration of the possible freezing, and lower
confidence with any impact from the sleet during the day Wednesday
and snow during Wednesday evening.

The period will begin active as a fairly progressive upper level
low and associated surface low track east into the area on
Wednesday. Anticipate strong WAA to ramp up out ahead of late
tonight into early Wednesday morning. Drier air initially in place
should help to limit any associated precip development late
tonight into early Wednesday morning. However, some locations
across the far western CWA, west of I-39 could possibly see some
light precip development during this time. Given warming thermal
profiles, anticipate precip type to be mainly liquid initially.
This will be concerning as temps tonight into Wed morning will
likely start off below freezing, and support freezing rain for
areas along/north of I-88. With the continued strong WAA, should
see temps steadily warm into Wednesday morning and limit the
duration of possible freezing rain to only a few hours.

The arrival of stronger forcing associated with this system on
Wednesday morning should allow for a quickly blossoming precip
shield for much of northern Illinois. After the window of possible
freezing rain, anticipate mainly rain. During this time though,
guidance indicating the possibility for deeper moisture/crystals
arriving and with some slight cooling of the column. In this
setup, a mix of rain and sleet would be possible for areas north
of I-88 Wednesday morning into Wednesday afternoon. Another hazard
to monitor Wednesday morning into midday Wednesday will be
thunderstorm development. Most guidance indicating the high
likelihood for steep mid level lapse rates to be in place and with
quickly increasing forcing, this would support a higher chance of
thunderstorms. With this possibility, did introduce chance
thunder for much of the area Wednesday. Not overly confident with
how intense any storm could get, but will definitely need to
monitor as this setup could favor a few more robust storms.

As this system begins to shift to the east Wednesday afternoon,
anticipate colder air to push across the area. This will support a
transition to all snow by early Wednesday evening across northern
Illinois, and then for areas to the south later in the evening.
With the earlier transition and with decent forcing to remain in
place, areas north of I-88 will have a higher chance for some
minor snow accumulations. Did slightly increase totals and think 1
to maybe two inches of snow is possible in this location, with
totals lowering further to the south.

Once again, have decided to hold off on a Winter Weather Advisory
for Wednesday/Wednesday evening given some lower confidence.
However, will need to continue to monitor closely as it`s still
possible that later forecasts may need to introduce a headline.
Air temps tonight into Wednesday morning will need to be monitored
as well as the more convective nature of precip Wednesday that
could help to increase the freezing rain potential despite the
small window of possibility.



For the 18Z TAFs...

Gusty NNW winds will gradually weaken while becoming NNE through
early this evening at all sites. The wind shift should be gradual
except for at GYY, where the shift will likely occur over just an
hour late this afternoon. A surface ridge crossing the area this
evening into the overnight will allow for east winds under 10 knots.
A few patches of fog may develop if cloud cover significantly
diminishes, but confidence was too low to introduce that into this

MVFR ceilings will gradually develop overnight ahead of an
approaching low pressure. Low-end MVFR to high-end IFR ceilings are
expected by Wednesday morning, with precip spreading over the area
by late morning. Precip type will range from mostly sleet at RFD to
mostly rain at GYY at onset, then transition to snow from NW to SE
through the afternoon. Precip rates at onset may also be high enough
to allow for some minor sleet accumulations.



217 AM CDT

High pressure will build over the region through this evening, and
northwest winds will gradually diminish.  Winds and waves look to no
longer be a threat to small craft in the IL nearshore waters so I
will cancel the small craft advisory with the nearshore forecast
update. Hazardous waves will continue across the IN waters into this
afternoon so I left the IN small craft advisory as is.

Winds become northeast to east by Wednesday morning ahead
of the next low.  The low will pass just south of the lake Wednesday
afternoon and reach New England Thursday afternoon. Some models
suggest north gales are possible Wednesday night, but I did not have
enough confidence to put gales in the forecast. As the low continues
east, high pressure will build over the Plains.  The high will reach
the Western Great Lakes Friday and dominate the pattern through the



LM...Small Craft Advisory...IN nearshore waters until 3 PM Tuesday.




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