Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 130946
AFDLOT

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

.UPDATE...
940 PM CST

Low confidence forecast for tonight into tomorrow regarding
precipitation/snow trends as a vigorous clipper type low pressure
system affects the area. Current surface analysis places the 998
mb low near the northeast North Dakota/Manitoba border. This low
will dive south-southeast tonight, reaching northern Illinois by
midday. Strong warm advection aloft in advance of the system has
resulted in virga extending down to Illinois, working on a stout
dry layer just below 850 mb noted on the 00z DVN sounding.

The big question is will sufficient saturation occur for the warm
advection portion of the system during the earlier morning hours.
While much (but not all) of the guidance verbatim casts doubt on
much if any snow occurring in parts of northern Illinois into
extreme northwest Indiana, observational trends are mixed. On the
one hand, the stout dry layer noted, but on the other hand there
are already observations of light snow/flurries ongoing in
Wisconsin as far south as Madison, which is earlier than
anticipated.

Should saturation occur, a look at the model forecast soundings
then becomes more concerning for the brief period of snow in the
morning. Due to the strengthening warm advection/isentropic lift
and low-mid level frontogenesis, soundings indicate strong
lift/omega also aided by steep mid- level lapse rates. For this
reason, did not make a huge change to the going forecast, which
leans toward more supportive guidance such as 00z RGEM, 12z
ECMWF). This favors a corridor from far northern Illinois down to
parts of Chicago for the brief period of snow from roughly 4 or 5
am to 8 or 9am, likely lasting only 1 to 2 hours at most in any
given location. The spectrum of impacts to the commute in the
Chicago area (and possibly Rockford) ranges from non-accumulating
light snow/flurries to a period of moderate snow dropping a quick
half inch to an inch of accumulation. Unfortunately, there remains
too much uncertainty to discount either outcome, so the best
suggestion is to check for latest forecast/radar update in the
morning before heading to work.

Guidance is also at odds with the next potential round of
precipitation in the early to mid afternoon hours, so confidence
is also quite low in that period regarding trends. Finally, the
track of the clipper, with parts of the area getting into the warm
sector, suggests that milder temperatures are possible, especially
southwest, where highs were conservatively bumped to around 40
degrees (with potential for mid 40s). Also increased winds with
the sub 1000 mb low driving stronger south-southwest winds ahead
of it and stronger northwest winds behind it.

Castro

&&

.SHORT TERM...
234 PM CST

Through Wednesday...

The main forecast concern through the period will focus on the
next clipper system expected to move across the area on
Wednesday. This system may result in two short periods of snow
over portions of northern Illinois/northwestern Indiana on
Wednesday.

After a cold evening, warm air advection will begin to kick up
late tonight in association with our approaching clipper system.
As a result, temperatures will likely bottom out this evening and
then begin to raise a few degrees overnight. The fast speed of
this approaching clipper will also induce a rapid increase in
west-southwest winds a couple thousand feet off the surface by
early Wednesday morning. As these winds interact with the
baroclinic zone over the region, expect a decent band of
frontogenesis to set up by early Wednesday morning, especially
over eastern sections of Wisconsin and possibly as far south as
northeastern IL. This will likely drive a progressive band of
moderate to perhaps briefly heavy snow.

The main question that remains at this time is how extensive will
this band of snow be over northeastern IL? Model guidance does
not typically handle these events the best, and I think they may
be under doing the band of snow over northeastern Illinois early
Wednesday morning. As a result, I have continued to mention a
period of snow Wednesday morning, especially over northeastern IL
and portions of northwestern IN. Given the progressive nature of
this band of snow, I am not expecting large amounts of snow,
especially considering it may only last an hour or two in any
given area. However, given that a quick inch of snow could occur
with this band during the morning rush, it could result in rather
high impacts over the Chicago area.

Once this band of snow moves over the area early in the morning,
expect things to quite down for a period during the late morning
as the surface low begins to shift into northern IL. Temperatures
are likely to warm near, or even a couple of degrees above
freezing prior to the arrival of the low, but as it begins to
shift east of the area, expect the colder air to spill back
southward over the area during the afternoon as the winds become
gusty (35 to 40 MPH) out of the northwest. It also appears that
another decent band of snow will move quickly across the area with
the cold during the afternoon as the parent mid- level
disturbance shifts over the area. Similar to the morning activity,
it appears that this band of snow would also be short lived,
likely moving out of the area by mid to late afternoon.

Total snow amounts from both bands could amount to an inch or two,
especially over far northeastern IL. However, additional lake
effect snow showers may add to these totals some Wednesday night.

KJB

&&

.LONG TERM...
344 AM CST

Thursday through Tuesday...

Overview...Lake effect snow continues over northwest Indiana
Thursday morning likely impacting the Thursday morning commute. An
upper level wave may force light snow or flurries Thursday night
and Friday. Temperatures rise to around 40 Saturday and the next
chance of precip arrives Sunday.

While the low pressure system continues to the east coast, the
lake effect snow engine will continue to run. Delta T`s are
forecast to be in the low 20s, and lake induced equilibrium levels
will be around 800 mb. Forecast boundary layer winds direct the
plume over northwest Indiana and slowly shift the band east
through the morning. I currently only have 1-2 inches of snow
forecast in NW IN, due to low confidence in exact band placement
and how quickly it will shift east. However, if the band remains
stationary, snow totals could be higher. Local guidance suggests
totals could be up to 6-8 inches especially if the band sits over
an area for awhile.

An upper level wave moves over the region Thursday night and
Friday. Guidance differs on how much snow the system will produce
so I kept precip chances at chance or lower. Forecast soundings
show saturation in the snow growth zone, so I also included
flurries outside of where the best chance for light snow
showers/accumulating snow may occur. Breezy conditions are also
possible as a deepening low passes over the northern Great Lakes.

Temperatures warm into the 40s over the weekend. The next chance
for precip is Sunday. The GFS keeps the system mainly to our
southeast while the ECMWF brings it over eastern Iowa and southern
WI. The two models have very similar upper level patterns with a
positively tilted trough extending from the upper Midwest through
the desert southwest. If precip does occur, rain looks like the
most likely precip type.

Normal to slightly above normal temps are then expected through
early next week.

JEE

&&

.AVIATION...
For the 06Z TAFs...

Several aviation forecast concerns through Wednesday night,
including a few distinct periods of light snow potential, and then
gusty north-northwest winds developing behind a cold front by this
evening.

Fairly vigorous clipper system will dig southeast across the area
through this afternoon. A short 1-2 hour period of light snow is
possible around sunrise as warm advection ascent strengthens in
advance of this system, although very dry air in low levels is
limiting atmosphere`s ability to saturate and produce organized
snow at ground level. Confidence is pretty high in light snow
development, though coverage and exact timing is lower. A lull in
any precipitation is then expected from mid-morning to mid-day,
until the surface low moves through and shifts winds from south
and southwest to west-northwest this afternoon. A second period of
light snow is possible with the cold front during the early to mid
afternoon. IFR or possibly brief LIFR conditions are possible for
a couple of hours with this band of snow. Winds increase with the
frontal passage as well, and are expected to gust 25-30 kts during
the late afternoon and evening hours, while veering more
northerly. Winds may turn briefly northeast during the evening,
which may produce a period of lake effect snow showers which could
affect Chicago area terminals a couple of hours. This band of lake
effect is expected to shift southeast into northwest IN overnight
as winds turn more north-northwesterly. Any lingering flurries or
light snow showers should diminish and end before sunrise
Thursday, with cigs improving to VFR.

Ratzer

&&

.MARINE...
344 AM CST

Ended the current Small Craft Advisory for NW IN, but I did not make
any changes to the next Small Craft Advisory that begins later this
morning.

Low pressure over Minnesota will pass over southern Lake Michigan
this afternoon. South winds will increase to 30 kt over the southern
half of the lake ahead of the low this morning. Winds diminish and
display a counterclockwise circulation as the low moves over the
lake, and east winds increase to 30 kt over the northern half of the
lake. It looks like there may be a very brief period of gales over
the far southern end of the lake this evening. I only included
occasional gales in the GLF due to the short time frame and medium
to low confidence I have in gales.

The low continues to the east coast while winds gradually back to
the west.  Low pressure forms over Lake Superior Thursday night and
deepens as it reaches Lake Ontario Friday. West winds to 30 kt are
expected behind the low.

Winds become southeast as high pressure passes over Ontario
Saturday. While guidance differs on when the next low will arrive, a
weak low will pass over the lake early next week. Winds are either
west or northwest to 30 kt through the remainder of the forecast as
a low pressure system passes over central Canada.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...7 AM Wednesday to noon Thursday.

&&

$$

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