Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 050135

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
735 PM CST Sun Dec 4 2016

735 PM CST

Allowed the winter storm and winter wx advisory to expire at 6 pm
as scheduled. Still had some accumulating snow falling at 6 pm,
but knew it was winding down quickly and didn`t see a need to
extend advisory an hour. Radar would suggest accumulating snow has
ended and will fire out one more ZFP shortly to pull all snow.

Have made some adjustments to going forecast beyond this evening.

1) Have hit dense fog/dense freezing fog potential harder in the
grids. Really a pretty atypical set-up, but many signs are
pointing to the potential for a potential widespread dense fog
event late tonight and particularly Monday morning. Typically,
following a big snow, we see dry/cold air advection in its wake,
however all of the forcing with today`s big snow came from aloft.
This evening, however, a narrow surface ridge is over the area now
and will scoot off to the east and allow for weak low level
warm/moist advection. So fresh snow cover will work to hold temps
in check and perhaps try and fall a degree or two, while
developing southwest winds attempt to advect in mid 30s dewpoints,
setting stage for what appears to be a good dense fog set up. I`ve
seen many seemingly perfect dense fog set-ups bust with no fog, so
plan to take a wait and see approach regarding any headlines, but
did hit the fog harder in the grids/text products. Assuming fog
does indeed develop as expected, then it would likely peak after
sunrise Monday and could even linger into the afternoon.

2) Most guidance high temps for tomorrow were not based on the
deep snow pack that now blankets the region. The combination of
the deep snow pack and the potential for dense fog and likelihood
of a low overcast would all suggest that our temps tomorrow aren`t
going much of anywhere. Have run with the NAM 2m temps for highs
tomorrow, which basically suggests only a rise of a couple degrees
diurnally, more reason why if dense fog forms as expected, that it
could linger much of the day.

3) Finally, haven`t added it to the forecast, but also will need
to watch for some drizzle/freezing drizzle late tonight and
Monday. Confidence is low, since model guidance varies on the
depth of the stratus/low level moisture. NAM/HRRR look more
supporting with cloud depth nearing 2000ft agl with 20-25kt winds
at the top of the cloud layer. This would result in 15-20kt of
shear within the stratus promoting collision & coalescence and
drizzle production. Temps could inch above freezing, lessening the
freezing potential Monday if drizzle does fall, but it is
something to keep an eye on.



209 PM CST

Through Tonight...

Moderate to heavy snow continues mid afternoon across northern
Illinois and northwest Indiana. This morning, a slow moving band
of snowfall set up across western portions of the CWA where over
6 inches of snow has fallen and that area had to be upgraded to a
Winter Storm Warning. The band eventually shifted east across
portions of the Chicago metro area where a few inches of snow
quickly accumulated and the band is now in the process of
reorienting along the I-80 corridor. Latest RAP shows the best
F-Gen focused along this corridor and shifting east out of the CWA
by late this afternoon or early evening. Already seeing some signs
of improvement west of the I-39 corridor and expect improvement
into the Chicago metro by around 23-00Z. There is some possibility
eastern counties may need a brief extension in headlines for an
hour or two but going to let the timing ride for now. Storm total
snow of 3 to 6 inches appears likely within the advisory area
including the Chicago metro with 1 to 4 inches south of the
advisory. Meanwhile, the warned counties of Ogle, Lee and Dekalb
are expected to finish out in the 5 to 9 inch range.

Expect overcast to mostly cloudy conditions to persist overnight
with temperatures only dropping off a little...into the mid to
upper 20s west of the Fox River Valley and low 30s farther east.
Areas of fog appear possible, especially in the west with weak
surface ridging building into that area. BMD



Monday through Sunday...

231 pm...Forecast concerns included potential for mixed precip
Monday night/Tuesday morning...chance of snow Wednesday night/
Thursday morning and much colder for the middle/end of the week.

High pressure over the Ohio Valley will be moving east Monday
afternoon into Monday night. With the new snow pack and low sun
angle...expect high temps Monday to be below guidance levels even
if some sunshine develops though low level moisture fields would
suggest that some low cloud cover could remain with increasing
mid/high clouds later Monday.

Models remain fairly similar with one low moving northeast
through the Ohio Valley/Mid Atlantic Tuesday and a second low
moving across the upper midwest Monday night and into Ontario
Tuesday...with a trailing cold front moving across the area on
Tuesday. There has been some consistency with the low moving
across the Ohio Valley remaining far enough to the southeast that
precip would only reach the far southeast cwa late Monday night
into Tuesday morning. Only the NAM remains further northwest
across the cwa and confidence with this solution is low. Mainly
chance pops with this system but confidence regarding where the
precip actually occurs is low. Thermal profiles continue to
suggest that precip would be a mix and possibly rain or freezing
rain depending on surface temps. With the new snow pack...its
likely that temps will drop to then hover near freezing Monday
night into Tuesday morning...before slowly warming. Too close to
call from this distance but some mention of light freezing rain
seems prudent at this time. If temps end up a few degrees warmer
or the precip shield continues to shift to the east...then changes
to precip type/timing can be expected.

Cold air begins to spread into the area Tuesday and continues
through the end of the week. Models continue to try to develop
light snow across the area Wednesday afternoon into Wednesday
night as the coldest air arrives. The Ecmwf is most developed...
though still a weak surface low through the Ohio Valley.
Confidence regarding system development and associated snow
remains low with just chance pops for this time period.

Another wave rotates across the area Thursday/Thursday evening
that could bring some snow showers or light snow to the area.
Wind direction with the colder air should keep any significant
lake effect snow east of Porter County but still chance mention
for far northeast Porter County later in the week. A clipper like
system is possible next weekend but too early for any specifics
with this system. cms


For the 00Z TAFs...

The heaviest of the snow is pushing off to the east, with just
some lingering light snow expected for a few hours. Vis has
improves to 1sm or higher at all TAF sites except RFD which
remains at 3/4sm at issuance time. Latest satellite imagery shows
the layered high cloud over far nern IL/nwrn IN with a widespread
area of st/fg to the west. Expect that over the next few hours,
the higher cloud will push to the east, with the lower st/fg
spreading over the entire area. Have made some adjustments to the
cig/vis for the overnight and morning hours. With a blanket of 5
inches or more snow on the ground and temps expected to trend down
to just below freezing, and no drier air likely to move in
overnight, have trended cigs/vis back down to ifr, with the
potential for lifr conditions for the early morning hours. The
highest probability of 1/4sm will be for the more rural areas,
but with the snow cover in place, do not expect much of an urban
heat island effect overnight, so even the more urbanized area will
have the potential for fg/fzfg to develop. Given some uncertainty
in the situation for the the urban areas, have only gone with a
tempo 1/2sm for ORD/MDW with prevailing 1/4sm for RFD/DPA/GYY.
However, there is some potential for ORD/MDW to see the prevailing
1/4sm, but confidence is too low at this point to include this in
the TAFs at this time. Expect that ifr cigs should remain at least
through the morning and into early afternoon. Have gone with
improving conditions to mvfr by early afternoon, but there is a
high probability that the timing of improvement may be too fast.



342 pm...A trough of low pressure will move across the lake this
evening as high pressure moves across the Ohio Valley overnight
into Monday. The gradient will tighten this evening with southerly
winds turning westerly and increasing into the 15-25kt range and
there could be some gusts to 30kts. Low pressure will move across
the upper midwest Monday afternoon and into Ontario Tuesday. Winds
will turn southerly ahead of this low and may increase to 30 kts
as the gradient tightens Monday night. High pressure will spread
across the northern and central plains Tuesday into Wednesday and
then strengthen as it moves across the central part of the country
Thursday into Friday. A tight gradient along with much colder air
spreading across the lakes region will allow strong west/northwest
flow to develop mid week and continue through late week with a
period of gales possible Thursday into Friday. cms





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