Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 080521
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1121 PM CST Wed Dec 7 2016

.UPDATE...
910 PM CST

Updates to evening forecast include increasing wind/gust speeds
and adding scattered flurries.

WFO Chicago forecast area remains in blustery westerly cyclonic
low-level flow around deep surface low pressure centered over
James Bay Ontario this evening. Steep low-level lapse rates,
produced by persistent cold advection of arctic air from the
central and northern Plains, continues to enhance mixing of
stronger winds from the upper portions of the boundary layer.
Recent surface observations have indicated gusts in excess of 30
mph in some spots, particularly across northeast IL/northwest
Indiana, and forecast soundings suggest gusts in the 25-30 mph
range will likely continue for much of the night. Blustery
conditions combined with air temps in the teens will yield single
digit wind chills (and a few 0 or -1 values north) through the
night.

In addition to the wind and cold, an extensive area of 2500-3000
ft stratus/stratocu has spread into northern Illinois from the
west/northwest this evening, with a few reports of flurries.
Forecast soundings depict this moist cloud-bearing layer reaching
into the -10C and colder regions between 900-950 mb through the
night suggesting a few scattered flurries will most likely
persist into Thursday.

Updated grid/text products available.

Ratzer

&&

.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
chill 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 06Z TAFs...

Forecast concerns include strong/gusty westerly winds through the
period...flurries through the period and chance of snow showers
Thursday afternoon/evening.

The low levels remain mixed through the period and winds in the
mixed layer remain in the mid/upper 20kt range through Thursday
evening when speeds slowly begin to diminish. With colder air
spreading across the area...expecting these stronger winds to mix
to the surface for much of the period with gusts into the mid/
upper 20kt range with some higher gusts possible especially on
Thursday.

A few flurries have developed this evening and with little change
to the lower levels...a chance of flurries will continue through
the period. A weak upper wave will rotate across the area Thursday
afternoon into Thursday evening and this may be strong enough to
produce scattered snow showers. While duration of these snow
showers will likely be brief...they may reduce visibilities to at
least mvfr and perhaps ifr. So for now have included a prob30 for
Thursday afternoon/evening but trends will need to be monitored.

Mvfr cigs spread back across the area this evening and guidance
suggests these may scatter/lift for a time overnight toward
daybreak but with plenty of mvfr cigs west and northwest of the
terminals...opted to carry mvfr cigs through the period. cms

&&

.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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