Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 050915

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
315 AM CST Mon Dec 5 2016

.UPDATE...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


9z TAF AMDs.

Concerns: Expansion of fog early this morning, how low visibility
will get, and how long it will last. This is coupled with a
potential lowering of cigs to LIFR levels from their current IFR
and the question of whether the cigs even lift very much at all
today. All of these features are only of medium confidence, and
low confidence on ORD/MDW on the extent of vsby reduction. Higher
confidence in lowering of cigs.

An extensive IFR stratus deck is locked in place across the region
this morning. There are no real indications that this deck will go
away, but short term guidance is suggesting a lowering of ceilings
and a corresponding drop in visibility as this occurs. There is a
subtle wind shift to west-southwest across western IL behind a
surface high in an area of warm advection atop the fresh snow
cover. In this area there has been some clearing of the stratus
leading to fog, but in the areas without clearing the stratus is
lowering to LIFR. The trend should continue farther more so after
12z- 13z closer to the Chicagoland termianals. There is some
guidance bringing visibility in the tank after daybreak and
keeping it down for several hours, other sources do show a
lowering cig and vsby trend but not quite as low. At this point
confidence is highest continued IFR stratus with a dip to LIFR
this morning with a downward trend in vsby as well. The low level
wind field turns more southwest through the morning such that
confidence on 1/2sm vsby or lower at ORD at MDW are still very
possible, but not a lock just yet. This is something that will
require mesoscale monitoring here overnight as the warm advective
axis is going to shift right over our area too and ORD/MDW do
have a similar snowpack in place. Forecast soundings from the
RAP/HRRR indicate very little lifting today after cigs retreat
again later today until possibly later tonight as winds shift more

Precipitation chances look fairly low for most of the area. Low
pressure will sneak northeast into the Ohio valley Tuesday
morning, but most guidance keeps the light precip across northwest
Indiana and more so points farther south and east across central
and southern Indiana. There is also a weak trough axis that will
cross northern IL Tuesdsay morning but precip chances with this
also look low. Southeast winds ahead of this boundary will
eventually shift back to westerly.



256 AM CST

Modest west winds to 30 kt will transition to southwesterly and
remain elevated...though possible settling slightly to 15 to 25
kt as high pressure moves east across the Ohio Valley and low
pressure strengthens across the upper Midwest. This low will then
shift east across southern Ontario on Tuesday and then proceed
northeast to James Bay by Wednesday night. South-southeast winds
will strenghten to the south of this low later tonight. A strong
cold front associated with the low will pass across Lake Michigan
later Tuesday. The pressure gradient to the south of this low
coupled with a strengthening ridge of high pressure across the
northern plains and extending into the Ohio valley will create the
potential for low-end gales as early as Tuesday night, particuarly
across the north half of the lake. Stiff west winds shift
northwesterly later in the week in the cold advection behind this
front, occasionally getting near gale levels again though guidance
is not super excited about persistent gales and holds winds are 30
kt or so through an extended period. The gale potential does still
exist though.






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