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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
542 PM CST Mon Dec 11 2017

340 PM CST

We have contemplated an advisory for the snow band shifting into
Chicago during the evening commute. Snow rates are up to an inch
per hour, but snow of this intensity will not last too long. For
now we will handle with an Special Weather Statement for the northern two tiers of
counties, so this will include northern Cook, northern dupage,
Kendall, and Kane also. Total snow amounts up to 2 inches are
possible, and we have seen over an inch in parts of Rockford


Previous update follows....
229 PM CST

The frontogenetical band has setup across our area instead of
Wisconsin. Fortunately some of this forcing is going into
saturating the atmosphere but also fighting a warmer column. There
are decent mid level lapse rates above this lower level
circulation, and this signal remains strong at least through the
early evening. Once the column saturates after an initial virga
period, accumulations up to an inch are favored across northern
and north central Illinois. This signal holds itself right to
Chicago, but then is forecast to weaken considerably, but do
expect a burst closer to Chicago at least for a brief time.
Pavement temps will initially limit accums closer to Chicago, but
expect a quick drop off in low level temps once the precip
arrives. Latest graphical Nowcast issued around 230pm depicts the
message fairly well.



Short term...
156 PM CST

Through Tuesday...

A compact vort Max across eastern Iowa will continue to dive
southeast into central Illinois late this afternoon, then east
into Indiana tonight. The associated surface low is pulling ahead
of the upper vort, and will quickly allow cooler air to filter
back in to the area. Warm advection ahead of these systems has
allowed temperatures to climb into the mid 30s north to the mid
40s across central Illinois. There has been a persistent band of
mixed precipitation across Iowa ahead of this feature, either as
some sleet or rain, but more predominantly snow. In our neck of
the Woods, the precip/virga has been coming out of a higher cloud
deck and thus some occasional non accumulating sleet has occurred.

As the surface low will move into Indiana late this afternoon and
evening, a cold front will shift across the area. We are starting
to see a band form out ahead of the main forcing which is
beginning as virga but could quickly transition to snow and this
is happening out near Freeport. As the better forcing approaches
north central Illinois there could be a brief burst of snow which
could put down a quick few tenths of an inch of snow, maybe up to
an inch across the northern tier of counties. There could even be
a mix of sleet/snow along the leading edge of the higher
reflectivity values east from Winnebago to Lake County Illinois. The
narre/rap have been fairly consistent in suggesting a narrow
window for several hours from 3-6pm near rfd and between roughly
5-8 PM across the Chicago Metro area where snow will likely be
falling. Better accumulation potential is near rfd as guidance
also suggests a weakening of the band as it gets closer to
Chicago. Surface temperatures have certainly warmed such that when
precip arrives it may struggle initially to accumulate,
especially closer to the Metro, but it is certainly possible for a
coating to a few tenths.

Afterwards, strong northwest winds will bring the core of the
cold air in. The mixed layer eventually gets into a better snow
production region where there could be some flurries overnight.

Tuesday will just be blustery and cold. Northwest winds of 15-25
mph will hold through the day, and highs will remain in the low
to mid 20s. At least there will be sunshine. More clouds will
remain in place across Northwest Indiana and over the lake.
Current wind forecasts would suggest lake effect potential is
higher just to our east.



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

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 County Warning Area. 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 quantitative precipitation forecast 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 least
at this time.



for the 00z tafs...

The narrow, but locally intense, band of snow will continue to
move southeast and should move out of the terminals early this
evening. Under the heavier portions of the band, look for vsby to
bottom out between 1/2sm-3/4sm for a short period of time (less
than an hour). As the snow ends, look for northwest winds to lock
in and begin to quickly increase this evening, likely gusting to
around 30kt at times overnight and likely remaining gusty but
ever so slowly easing Tuesday. Likely to see primarily MVFR cigs
through tonight, though some variability is possible, especially
this evening. Guidance would strongly suggest MVFR cigs scattering
out early Tuesday morning and followed that reasoning in the tafs,
though admittedly confidence is low and wouldn't be shocked to see
MVFR cigs hang on a bit longer than forecast.

- Izzi


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.



Lot watches/warnings/advisories...
In...Lakeshore flood advisory...inz002...midnight Tuesday to 3 PM

Lm...Gale Warning...Illinois nearshore waters until 9 am Tuesday.

Gale nearshore waters until 3 PM Tuesday.

Gale Warning...lmz777-lmz779 until 8 PM Tuesday.



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