Area Forecast Discussion


673 
FXUS63 KLOT 112143
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
343 PM CST Mon Dec 11 2017
.UPDATE...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 SPS 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
already. 
KMD
Previous update follows....
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.
KMD
&&
.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 IL. 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. 
KMD
&&
.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 18Z TAFs...
Sort of complicated forecast this afternoon and evening as low
pressure tracks through the area. The first issue is wind
directions, followed by precipitation type, timing, and duration,
and then focus shifts back to gusty nw winds. 
The core of low pressure will pass just to the south of the
terminals, and then spread east of the area tonight. This will
cause SE winds to hold in the 8-11 kt period for a few hours in
the better pressure falls ahead of the low, then as winds shift
further to the E, expect some decrease in speeds. Warm advective
precipitation is attempting to occur farther upstream of the upper
level low across NE IA/SE MN. Expect during the afternoon some
attempt to precipitate over the coming hours, but with drier low
levels expect this would be some light snow or sleet, non
accumulating. As the better forcing approaches RFD around 21-22z,
there could be a brief burst of snow which could put down a quick
coating of snow, possibly even a few tenths of an inch in a short
period. Cigs will fall to MVFR and vsby too, possibly briefly 
IFR. As this snow axis shifts closer to the Chicago terminals, 
there appears to be some weakening to the stronger forcing, but 
there is still a small window for ORD/MDW in the 0z-02z time 
frame for a quick coating of snow, medium confidence on timing.
Later this evening there could be a few lingering snow showers,
more so wind driven flurries. CIGS will eventually head back to
VFR. Expect NW winds to exceeds 20-25 kt from a 330-340 direction
after 3z or so at least for a few hours if not longer. 
KMD
&&
.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...midnight Tuesday to 3 PM 
     Tuesday.
LM...Gale Warning...LMZ740-LMZ741-LMZ742...8 PM Monday to 9 AM 
     Tuesday.
     Gale Warning...LMZ743-LMZ744-LMZ745...8 PM Monday to 3 PM 
     Tuesday.
     Gale Warning...LMZ777-LMZ779...8 PM Monday to 8 PM Tuesday.
&&
$$
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