Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 130340

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

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

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.



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

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.



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.



For the 00Z TAFs...

531 pm...Multiple forecast concerns this period with three
potential periods of generally light snow...duration of each
likely only 1-2 hours.

A clipper low pressure system will move across the area early
Wednesday afternoon. A period of light snow is possible around
sunrise as this low approaches. Confidence on the timing of this
potential light snow is high...but confidence of where the snow
develops is only low/medium. Its possible the snow may develop
just over the terminals or just east of the terminals but its
close enough to maintain mention at all the taf sites. Duration
would likely only be 1-2 hours with ifr vis possible. Westerly
winds this evening will shift southerly tonight and then become
south/southeast by sunrise...with speeds possibly into the 10-15
kt range. A lull in the precip is expected mid/late Wednesday
morning as winds turn southwesterly. As the low pressure passes
east of the area...a cold front will move across the terminals
with winds shifting northwest by mid afternoon with speeds/gusts
likely increasing. The second period of snow is possible with this
cold front during the late morning at rfd and into the early/mid
afternoon at the Chicago area terminals. Ifr vis...possibly
briefly lower will be possible...again for 1-2 hours as this band
shifts east.

Winds will shift northerly by early Wednesday evening...possibly
remaining north/northwest but a band of lake effect snow is
expected to develop over the west side of Lake Michigan and this
may affect northeast IL including ord/mdw during the early/mid
Wednesday evening. Confidence is again only low/medium but if this
were to materialize...a period of moderate snow is possible and
trends will need to be monitored with later forecasts for how
significant this band may become. Duration should again only be
1-2 hours as it quickly shifts southeast and into northwest
Indiana by late Wednesday evening. Winds may become northeasterly
with this band of lake effect snow Wednesday evening and opted to
include that potential in the tempo for now.

Outside of any possible light snow...prevailing cigs are likely
to remain vfr through mid/late Wednesday morning and then
prevailing mvfr cigs are expected Wednesday afternoon into
Wednesday evening. cms


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.



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

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




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