Area Forecast Discussion
Issued by NWS Chicago, IL

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853
FXUS63 KLOT 122138
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
338 PM CST Tue Dec 12 2017

.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...
320 PM CST

Wednesday night through Tuesday...

Main forecast concerns/challenges are with continued snow chances
Wednesday evening through early Thursday morning, tied to the
backside of the next system to affect the area as well as lake
effect snow development.

It`s now appearing that additional chances for snow are likely
Wednesday evening through early Thursday morning, at least for
areas near the lake in far northeast Illinois and northwest
Indiana. As surface trough axis swings down the lake on the
backside of the anticipated system late Wednesday, system snow
should be departing. However, during the evening hours, will see
flow on the lake focus into northeast Illinois. With thermodynamic
conditions over the lake more than sufficient for lake effect
snow production and with low level convergence increasing, will
see lake effect snow develop over the lake and push inland
initially into northeast Illinois. With some model variability,
confidence is low with regard to timing. However, at this point,
mid to late Wednesday evening is appearing to be favorable time
frame for a resurgence of snow. This snow should remain transient
with residence time in anyone location remaining on the low side,
with residence times of a couple to few hours. Do think its
possible to get some heavier bursts of snow to occur with this
lake effect, especially as it moves into northwest Indiana during
the overnight hours and into early Thursday morning. In this
setup, would not be out of the question to see additional amounts
of around one inch, or even up to two inches in some spots.
Slightly higher amounts are possible in northwest Indiana, and
will need to continue to monitor early Thursday morning for this
area as even higher amounts than currently forecast are possible.
Lower confidence on this possibility, but some guidance is showing
this lake effect development holding in this spot and continuing
longer into the morning on Thursday. If this were to occur, which
is definitely a possibility, then will need to monitor amounts and
likely resultant impacts.

High pressure is expected to build across the region throughout
the day, with any lingering development shifting to the east by
later in the day. After a brief lull in precip, monitoring
additional vorticity maxima to likely swing south through the
region late Thursday into Friday and bring the next chance of
snow. Confidence with coverage and placement are low, and so have
kept pops lower at this time. However, it is appearing possible
that some locations will receive minor snow accumulations. A push
of warmer air looks to occur by the start of the weekend, but
phasing issues with two separate systems into the weekend is
showing itself with model variability on the next system to affect
the region. Pops limited during this time, but will need to
continue to monitor later trends and possible inclusion of pops.

Rodriguez

&&

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

Northwest winds over the area this afternoon will continue to ease
through the afternoon, but the main focus during the period is
with another clipper system set to shift over the area on
Wednesday. The current track of this low would put if over
northern IL by midday Wednesday. Given this track, the wind
forecast will be tricky at the terminals as any minor fluctuation
in the expected track of this low could result in changes in the
favored wind directions and speeds. Currently we expect the winds
to become south-southeasterly by early Wednesday morning, then
shift west-southwesterly later in the morning as the low
approaches, and finally back to the northwest again during the
afternoon.

In addition to the winds, it appears that they may be a period or
two of snow over the eastern terminals on Wednesday. While the
better chances for snow is likely to remain north of the area in
Wisconsin, it does appear that there could be a short 1 to 3 hour
period of snow early Wednesday morning over northeastern IL in
association with increasing warm air advection. There is still
uncertainties in the extent of this band of snow, but if it
materializes could produce a short period of low visibilities in
moderate to heavier snow before it quickly shifts out of the area
by mid to late morning. While the snow is likely to end during the
morning, it appears that another quick moving band of snow could
shift over the terminals during the afternoon, with another period
of reduced visibilities possible. Given uncertainties in timing
and the extent of the snow, I have only mentioned the lowest
visibilities in tempos at this time.

KJB

&&

.MARINE...
337 PM CST

Low pressure along the eastern U.S. will continue to push further
to the east tonight, as a ridge of high pressure swings through
the Great Lakes region. This will allow further diminishing of the
winds, which already has been occurring this afternoon. Have left
the Gale warning headlines over the open waters into this
evening, but with speeds diminishing, could see gales ending
slightly sooner. However, several reports of gales noted this past
hour, and once again, have left the gale warnings in place. Gales
are no longer being observed over the nearshore waters, but
expect hazardous conditions for small craft to continue in both
the Illinois and Indiana nearshores tonight. Low pressure expected
to quickly drop southeast through the region late tonight into
Wednesday and as this occurs, will see the diminishing trend
quickly transition back to an increasing trend. This is especially
the case by early Wednesday morning when hazardous conditions for
small craft likely return. Winds to 30 kt now expected over the
lake Wednesday morning through Wednesday night. Will also need to
monitor brief periods of possible occasional gales during both the
Wednesday morning and night time frames.

Rodriguez

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Warning...LMZ777-LMZ779 until 8 PM Tuesday.

     Small Craft Advisory...IL nearshore waters until 10 PM Tuesday.

     Small Craft Advisory...IN nearshore waters until 4 AM Wednesday.

&&

$$

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