Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 072105
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
305 PM CST Wed Dec 7 2016

.SHORT TERM...
218 PM CST

Through Thursday...

Temps warmed more than expected today as cloud cover briefly gave
way to sunshine.  Cirrus has spread over the region again bringing
temps back down by a few degrees. Cloud cover will remain overnight,
but colder air moves in with the upper level trough.  Low
temperatures will be in the mid to upper teens. Combine that with
wind gusts up to 25 MPH and min wind chills Thursday morning will be
+1 to +5 degrees near the lake, and +4 to -1 away from the lake.

Gusty winds continue Thursday with gusts to 30 MPH, possibly higher
at times. High temps are forecast to be in the low to mid 20s, but
it will feel like +5 to +10 degrees.  Flurries are possible
throughout the day as the snow growth zone will be just above the
surface. Soundings lack significant forcing, but with the growth
zone so close to the surface and adequate saturation, flurries are
possible. Accumulating snow is NOT forecast through Thursday.

JEE

&&

.LONG TERM...
305 PM CST

Thursday night through Wednesday...

A rather active and cold weather period is likely across the
region through the period. The main concerns are: the potential
for several inches of accumulating snowfall this weekend, along
with the potential of a period of dangerous cold by the middle of
next week.

Weather conditions will be mainly quiet across much of the area
through Friday night...albeit cold. However, Lake effect snow
showers are expected over Lake Michigan and the eastern shores.
While some of these showers could produce some accumulations over
portions of northern Porter County in Indiana during this period,
by far the main axis of heavier lake effect snows are expected
over the eastern shores of Lake Michigan.

The main period of active weather looks to be sometime Saturday
afternoon and into at least early Sunday. A quasi-zonal upper-
level pattern is forecast to set up over the CONUS by late Friday.
As this occurs, upper level energy, currently over the eastern
Pacific will likely shift eastward over the Plains and into the
central CONUS over the weekend. While confidence is fairly high
that this weather pattern will result in a period of accumulating
snow over the area late Saturday and Saturday night, there remains
a large amount of uncertainty in snow amounts, especially into
Sunday and Sunday night.

The 12 UTC run of the ECMWF and its ensembles suggest that the
the main upper wave could be slower to shift onshore over the
Pacific northwest, which could allow for a stronger storm system
over the lower Great Lakes and Ohio valley regions Sunday into
Monday. If this were to verify, it could mean more snow, with
possible P-type issues in my southeast Sunday night. While this
type of solution cant be ruled out at this time, model
disagreements and an inconstant signal from the ECMWF puts its
latest solution into question. As a result, I have only continued
slight chance POPS for snow later Sunday and Sunday night.

In spite of these uncertainties, it is very probable that we will
experience some accumulating snowfall this weekend as isentropic
upglide (warm air advection) sets up over the Mid-Mississippi
Valley. Even the GFS and GEM solutions, which favor late Saturday
through early Sunday for the snow potential, still support over 3
inches of snow for a majority of the area. Given the cold thermal
profiles over the area Saturday and Saturday night, snow ratios
could end up much higher than this past event, helping support
amounts in excess of 3 inches.

The other concern following this weekends accumulating snow will
the potential for a period of dangerous cold by the mid to end of
next week. Forecast guidance suggests that an arctic push of cold
air will shift southward out of northern Canada and into the
northern CONUS next week. There are questions as to how far the
deepest cold will sink into the area, but either way it looks cold
next week. The potential is there, however, to be several degrees
colder on Wednesday than current forecast. Wind chills could also
be nearing -20 degrees, which could result in the need for a wind
advisory at some point by Wednesday. This period will need to be
monitored. Additional snow accumulations, albeit lighter, are
also possible during the week, though lake effect snow will likely
continue to be favorable, mainly over the eastern shores of the
lake.

KJB

&&

.AVIATION...
For the 18Z TAFs...

Broken high level cirrus is over the region with west winds
occasionally gusting to around 20 kt. Cloud cover increases this
evening as the upper level system shifts overhead. Ceilings lower
to low end VFR/high end MVFR by tomorrow morning. Ceilings could
be lower than forecast, around 1500 ft, but do not have enough
confidence to go that low. West winds increase with gusts arnd 25
KT possibly up to 30 kt Thursday. Scattered flurries are also
possible throughout the day, but no accumulation nor impact are
expected from the flurries.

JEE

&&

.MARINE...
218 PM CST

Headlines: No changes were made to the ongoing gale warning.
Extended the small craft advisory due to lingering hazardous waves.

The low over James Bay continues east through the end of the week
while high pressure builds over the plains. West winds will remain
in place into Saturday as the high moves over the lake.  The next
low forms over the plains this weekend. Guidance still show
significant differences in the low`s path and strength; however,
the low will impact the lake over the weekend.  Winds become south
to southeast ahead of the low Saturday evening.  The low itself
passes over or near the lake early next week, and winds become east
behind it.  Have low confidence in the wind and wave forecast
Saturday evening onward due to low confidence in the weather pattern.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 6 PM Friday.

     Small Craft Advisory...ILLINOIS NEARSHORE WATERS UNTIL 9 AM Friday.

&&

$$

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