Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 120844
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
244 AM CST Tue Dec 12 2017

.SHORT TERM...
243 AM CST

Through tonight...

Blustery and colder weather is in store for the area today, in
the wake of Monday`s Clipper. Clouds and a few flurries early will
give way to partly cloudy skies, though with chilly temps/wind
chills.

Surface low pressure associated with Monday`s system continues to
track away to the east early this morning, with it`s trailing
cold front well south of the Ohio River. Gusty north-northwest
winds 25 to 30+ mph were pulling colder, arctic air into the
region early this morning, with lingering cold-air stratocu deck
producing a few flurries and light snow showers across the eastern
2/3 or so of the cwa. Cloud layer is thinning per GOES 11-3.9
micron imagery and model forecast soundings indicate this trend
should continue this morning, thus would anticipate any flurries
to end fairly early. The exception will be in northwest Indiana,
where lake effect snow showers will develop, though the best low
level convergence focus should remain just east of the
Porter/LaPorte county line. Still, snow showers may produce some
minor accumulations through midday mainly across eastern Porter
county. Skies become partly cloudy over the remainder of the area,
with model thermal fields supporting daytime highs in the low-mid
20`s, and coolest temps north. While winds will very slowly ease
today, wind chills are already in the single digits over far
northern IL as of 2 am, and these will likely dip to around zero
to 5 above across the area this morning before rebounding slightly
later today.

Weak surface high pressure ridge drifts across the forecast area
this evening, allowing winds to diminish further. Decreasing
winds, new snow cover especially across far northern IL, and dry
arctic air may allow for a quick drop-off in temps after sunset
this evening. Have leaned toward coldest guidance temps for areas
toward the IL/WI border where 2-3 inches of snow fell Monday
afternoon in a few spots, and the typically colder Fox Valley,
where temps may fall quickly into the single digits early. Another
quick-moving clipper system will be approaching from the
northwest overnight however, with increasing mid-level cloud cover
which may inhibit further temp drop. Surface winds will also back
to the south overnight as the ridge drifts east and surface low
pressure moves into the upper Midwest. This combination of
increasing/thickening cloud cover and developing south wind will
likely allow for steady or slowly rising temps after midnight.
Precip looks to hold off until Wednesday morning.

Ratzer

&&

.LONG TERM...
201 PM CST

Tuesday Night through Monday...

The long range pattern remains active around the eastern North
America long wave trough. There is a surface low passing through
the region about every 24-36 hours. As such, there are several
precipitation chances, mainly snow as the precipitation type.
Temperature fluctuations will occur, similar to what we are seeing
today with the clipper passage, with any warming being fairly
short-lived.

The next clipper behind today`s is expected to ride the same
baroclinic zone south-southeast into the region on Wednesday.
Ahead of this, depending on the speed in which the system`s
clouds spread in, Tuesday night could have some single digit lows
in outlying areas. The clipper surface low path for Wednesday
would make sense to be close to today`s, and could see it end up
inching a bit west too.

Continue the precipitation chances north and east on Wednesday,
and precipitation could have a similar evolution to today, just
with the system remaining stronger a tad further south resulting
in slightly better snow chances in parts of the northern and
eastern CWA. The same gradient, even tighter, and slope in snow
looks to exist. Given the quick system speed and low amplitude,
snow accumulation should again be limited but there is potential
for over an inch in northeast Illinois and far northwest Indiana.
Also, conceptually there could be brief rain/graupel/snow mix at
onset as the column saturates, but again will depend on how far
warm sector spreads into the area. While on the lower confidence
side with details with this given Day 3, would not expect a major
shift or change in speed with this system.

The next system is forecast Thursday night into Friday and is a
weaker, broader, and slower moving feature on global guidance. As
such, the guidance blend has very little QPF and thus little for
chance mention. While at least flurries would seem like a good
likelihood, there could also be some snow showers.

Uncertainty grows into the weekend as the upper air pattern
briefly flattens over the area, yet still is indicated to be
disturbed. Both the GFS and EC for their past two solutions also
have shown some subtropical energy translating up the Ohio River
Valley late in the weekend. Very tricky to time that exactly from
this distance, and the temperature fluctuations with them.
Saturday looks like it has potential to get to the 40 mark plus
for a good part of the area, while Sunday looks cooler...at least
at this time.

MTF

&&

.AVIATION...
For the 06Z TAFs...

Gusty northwest winds 25-30 kt, gradually improving MVFR to VFR
cigs and a few flurries are the primary aviation concerns in the
near term.

Surface low pressure was pulling away from the region through
Ohio late this evening, while high pressure was building southeast
across the Plains. Strong low level cold advection and steep
lapse rates were aiding the gradient in producing gusty winds,
and these winds will only gradually diminish into Tuesday. MVFR
stratocu was already lifting to VFR from the northwest as drier
air spreads in, with cigs improving to 3500-4000 ft over the next
few hours before scattering early Tuesday morning. A few flurries
or light snow showers evident in radar returns over far north
northeast IL, and will move across the terminals but with no real
impact or restriction to vis.

Winds continue to diminish during the day Tuesday, eventually
backing to the southwest Tuesday night ahead of the next low
pressure system which will approach the area Wednesday. VFR mid
clouds gradually lower late Tuesday night.

Ratzer

&&

.MARINE...
305 PM CST

Concerns for the open waters of the lake are north-northwest
gales tonight and through Tuesday as low pressure passes east and
strengthens. The gales should slowly ease into Tuesday evening.
Weak ridging will move across the lake on Wednesday morning and
behind this winds will briefly turn southwest. The next low
pressure will drop southeast just to the west of the lake on
Wednesday, and behind this low the winds will turn north-
northeast. It is possible waves briefly build to Small Craft
Advisory criteria along the Illinois and Indiana shore during
Thursday. Winds will show fluctuations in direction and at times
some gusty speeds Friday into the weekend with the passage of more
clipper systems.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Lakeshore Flood Advisory...INZ002 until 3 PM Tuesday.

LM...Gale Warning...IL nearshore waters until 9 AM Tuesday.

     Gale Warning...IN nearshore waters until 3 PM Tuesday.

     Gale Warning...LMZ777-LMZ779 until 8 PM Tuesday.

&&

$$

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