Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 131945

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
145 PM CST Wed Dec 13 2017

351 AM CST

Through tonight...

Another clipper system will bring the potential for some light
precipitation to the region again today, with the greatest
potential for any light snow accumulation mainly across the far
northern suburbs of Chicago. Winds turn northerly and gusty behind
the departing system tonight, with lake effect snow showers
expected to affect northeast IL with some accumulation potential
this evening, and eventually northwest Indiana later tonight into

Early morning GOES water vapor imagery shows a pair of mid-level
short waves digging southeast from the northern Plains and
Canadian prairies. A fairly stout 995 mb surface low was analyzed
over western Minnesota at 09Z, and guidance is in good agreement
in tracking this low southeast across far northeastern Illinois
this afternoon. Warm advection was noted across the area per RAP
analysis, with westerly 30 kt winds inducing isentropic upglide
across the western Lakes. Regional radar depicts mid-level returns
over parts of WI and Lake Michigan at this hour, though very dry
air in low levels has initially been preventing much of this from
reaching the surface south of the IL/WI border overnight. Slightly
better saturation appears to be occurring with band across
western WI, and high-res guidance generally develops this
southeast and clips far northeastern IL around sunrise. Thus a
short 1-2 hour period of light snow is possible mainly east of
Rockford and north of Chicago early this morning, with perhaps a
dusting possible. Forecast soundings then dry things out by mid-
morning, before the main mid-level vort and surface low approach
the area late morning. Again, forecast soundings indicate a
struggle to really saturate the column south of the IL/WI border,
though it does appear at least some spotty precip is possible
across the area during the midday and afternoon hours as the low
and cold front move through the area. Precip type may be an issue
as warmer, above freezing air is pulled north across the area
ahead of the low, with a mix of rain/snow or just rain along/south
of I-80. Similar to Monday, temps will likely warm into the lower
40`s south of I-80, and the mid-30`s farther north. Quick cooling
of the column then occurs with the cold frontal passage later
this afternoon, which should turn everything over to snow showers.
Again, some light accums of less than an inch are possible this
afternoon, mainly north of Chicago where precip has the best
chance of being primarily snow along/north of the surface low

Winds look to increase substantially behind the low/cold front by
this evening, with gusts of 35 mph as cold air surges back into
the region and steepens up low level lapse rates. Several models
turn winds north-northeast for a time during the evening,
suggesting the potential for lake effect snow showers to affect
northeast IL. Lake to H8 delta-T`s approach 20C overnight, though
a fairly stout subsidence inversion around 800 mb would limit
equilibrium levels to <7000 ft. Thus have fairly high confidence
in snow showers, but accumulations may be rather modest, perhaps
an inch if the band remains transient. Boundary layer flow
gradually backs more north-northwest overnight, shifting focus
into northwest Indiana after midnight, with slightly better
accumulation potential.



144 PM CST

Thursday night through Wednesday...

For the early portion of the long term forecast period,
Thursday Night through Saturday Night, model guidance is in
relatively good agreement on the longwave pattern, with high
amplitude ridging over the wrn CONUS and broad troughing east of the
Rockies.  A series of nrn stream shortwaves dropping through the
fast nwly flow aloft will bring some pcpn chances for much of the
CWA.  Thermal profiles will be cold enough to support snow as the
prevailing p-type, with the main concern being location and amount
of any possible accumulation.  The general shortwave track will be
through srn WI/Lake Michigan/Lower Michigan with the strongest
forcing north and east of the CWA.  However, there should be enough
upper forcing dipping into nrn IL/IN to support some light
accumulating snow, mainly for the nrn and ern portions of the CWA,
flurries more likely for the swrn portions of the CWA.  With weak
forcing and low deep layer moisture in place, any accumulation would
likely be nothing more than a dusting.

For the latter portion of the long term forecast period, from Sunday
through Wednesday, the various longer range models quickly diverge
in how they handle southern stream systems.  The various model begin
to significantly diverge as early as Friday night and Saturday for
the wrn CONUS, but these issues will likely not impact forecast
confidence and concerns until Sunday or Sunday night.  So, given the
model differences, have opted not to make any significant changes to
the going forecast, with a general trend of near to slightly above
normal temperatures.  While confidence in finer scale details is
low, the source of systems impacting the region are of pacific-
origin, thus the trend for generally higher than normal
temperatures. P-type and coverage will be most problematic on
Sunday. While the GFS and ECMWF both indicate srn stream shortwave
energy lifting out of the southwest, which would support mentionable
pcpn, but given the timing differences, will limit PoPs to slight
chance to low chance levels.  There is a chance that much or all of
the pcpn could be rain or a rain/snow mix with short periods of all
snow at more diurnally favored times around time of min temp.


For the 18Z TAFs...

Low pressure will move from southwest WI early this afternoon far
southern Michigan this evening. Gusty SSW winds and VFR conditions
are in place out ahead of the low and attendant cold front this
afternoon. Regional radar shows a narrow band of precipitation
stretching from SW WI into southeast IA this afternoon. This is
associated with the cold front and will continue to move east across
the terminals this afternoon. It does appear that antecedent dry
air is preventing precip from reaching the surface in some areas,
but models still indicate that the terminals will quickly saturate
this afternoon as the cold front moves through allowing snowfall
to reach the surface. Given the dry air concerns and the relative
narrowness of the band, it`s unlikely that the terminals will see
any accumulation this afternoon. Confidence in vsby restrictions
with the snow is low, thus have backed off some in the latest

Winds will shift to the northwest and expect MVFR stratus to develop
behind the front late this afternoon and continue through tonight.
Winds may briefly gust in excess of 30kt but should settle into the
high 20kt range through much of late afternoon and evening hours.
Winds are expected to veer to the north this evening and cold air
filtering into the region behind the departing low will allow
scattered lake effect snow showers to develop over the Chicago area.
Snow will initially be scattered or multi-banded, but eventually
expect a transient single band to evolve and swing from the north
suburbs of Chicago this evening into northwest Indiana overnight
where it will linger into tomorrow morning before diminishing. A
dusting to half an inch or so appears possible at ORD/MDW
depending on how long the band lingers over any one area while
higher amounts of 1-3 inches is possible at GYY.



144 PM CST

The wind pattern over Lake Michigan will be very complicated for
the remainder of this afternoon and evening as strong low pressure
tracks across sern Wisconsin and srn Lake Michigan and then
tracks south of Lake Erie this evening. A small Craft Advisory
remains in effect for the IL/IN nearshore waters with swly winds
to 30 kt early this afternoon then shifting to nwly by late
afternoon. Winds should remain strong and nwly through the night.
While prevailing gales are not expected, there is a chance for a
short period of Gale Force gusts early this evening when cold
advection and pressure rises behind the exiting low are strongest
and would most likely be confined to the far southern end of the
Lake. Winds should diminish through the morning on Thursday as
high pressure builds across the Middle Mississippi Valley and into
the Ohio Valley. Conditions through the weekend should be
relatively quiet. The next chance for impactful weather is not
expected until early to the middle of next week as a strong cold
front pushes across the Great Lakes region. However, the longer
range model guidance is in very poor agreement on how the pattern
evolves next week, so confidence in timing of this next system is


LM...Small Craft Advisory...nearshore waters
     until noon Thursday.




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