Area Forecast Discussion
Issued by NWS Chicago, IL

Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS63 KLOT 212347

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
547 PM CST Mon Jan 21 2019

154 PM CST

Tonight through Tuesday Night...

A rather complex forecast ahead as low pressure lifts from the
central Great Plains into the Upper Midwest Tuesday. This
afternoon, high pressure stretches from the eastern Great Lakes
region across the upper Ohio Valley. Meanwhile, low pressure is
analyzed over front range of the Colorado Rockies. For the rest of
this evening and tonight, expect increasing and thickening cloud
cover in advance of this approaching low, as well as
strengthening southeast winds. Anticipate non-diurnal temperature
trends overnight with temps very gradually rising into the upper
teens or lower 20s by Tuesday morning.

Precipitation associated with the low is expected to overspread
the forecast area from the west from mid morning into the early
afternoon. There are a number of factors on Tuesday that result in
lower than average confidence in details of the forecast,
particularly with respect to precip type. For one, there are a
number of hours on both the front end and back end of this
precipitation event where saturation is not deep enough and cloud
top temperatures not cold enough for heterogeneous ice nucleation
and resulting in drizzle (freezing or not depending on surface
temps). Next, temperatures both aloft and at the surface indicate
that we may see the full range of precip types at various points
in the day. A modest 850mb warm nose with temps around +1-2C will
spread across northern Illinois through the afternoon atop the
bitterly cold air mass still in place. Sleet and freezing rain
both appear possible as temperatures slowly warm aloft.
Eventually, as the boundary layer warms through the day and into
the evening, precipitation should change over to rain from the
south. Anticipate the rain/snow line to set up near the IL/WI
state line Tuesday evening. Pavement temperatures are an
additional wild card with respect to the impacts of any freezing
rain/drizzle. Given the expected cloud cover, think that pavement
temperatures may be slow to respond to any advection driven
warming. Alternatively, if skies do have partial clearing,
insolation should help pavement temperatures quickly respond and
will mitigate impacts.

Surface low is progged to lift across north central Illinois around
midnight Tuesday night, with a trailing cold front sweeping across
the CWA overnight. Temperatures do cool back down for a transition
to snow, but a narrow dry slot punching across the area may end up
resulting in some additional freezing drizzle before precip ends.
That said, forcing drops off pretty quickly as well, so precip may
just end before additional impacts are realized.

Will be hoisting a winter weather advisory to cover the freezing
rain and possible sleet threat for Livingston, Kankakee counties and
counties to the north in north central and northeast Illinois. This
area represents the highest confidence in hazardous conditions from
freezing drizzle/rain and sleet. The advisory will cover the initial
freezing drizzle/rain threat from mid morning through the afternoon.
While the forecast will mention some potential an icy mix in the
remaining counties, confidence remains too low at this time to issue
headlines for the remaining counties.



200 PM CST

Wednesday through Monday...

The message for the mid to latter part of the week into the
weekend is a much colder than normal pattern, with the cold air
mass reinforcing over the central and eastern U.S. during this
time. The pattern will also remain active around the periphery of
the eastern North America long wave trough and polar lobe centered
over Hudson Bay and into Ontario. In the northwest flow through
Saturday, any snow makers look to be low amplitude clippers or
driven by strong cold advection and not particularly robust in
snow magnitude, at least at this distance.

Wrapparound light snow will be possible Wednesday morning with the
departing system. Temperatures will struggle to climb much from
where they start the day in the mid 20s with northwest winds
likely to gust at times over 20 mph. A potent short wave will drop
over the Great Lakes Thursday into Thursday night, swinging a
strong cold front (slightly modified Arctic front) through the
area Thursday afternoon. Forcing along and behind this front may
be enough to compensate for very limited moisture to produce
scattered snow showers. The 850mb temperatures are forecast to
drop quickly to -20C by Thursday evening. Blustery conditions and
falling temperatures will occur in the afternoon and night, with
below zero temperatures forecast prior to midnight. Wind chills
presently look to drop to -20 to -25 later Thursday night into
early Friday morning, which could warrant the first Wind Chill
Advisory of the season.

As numerous mid-level jet streaks and associated waves ride from
western/central Canada into the middle of the U.S. Friday onward,
it is challenging at this distance to time snow chances Friday-
Sunday. Again these generally are clipper systems and do not look
to have much snow per say, but some snow with blustery and cold
conditions can still result in minor impacts. Highs look to be in
the teens with lows dependent on cloud cover and whether in the
cold advection envelope of any of these systems, but at or below
zero in outlying areas is favored. At the end of the weekend into
early next week, most global guidance does indicate a stronger
clipper/northern Pacific hybrid system cutting into the Upper
Midwest/Great Lakes region, which would steer a tighter
baroclinic zone and stronger forcing back over the area for
possibly a more robust snow maker somewhere in the region.
Obviously at nearly a week out in this active pattern, confidence
is low at this point.



For the 00Z TAFs...

Several aviation forecast concerns in this TAF period, including:

* LLWS potential later tonight.

* Period of snow/sleet/freezing rain during the day Tuesday
  (lingering into Tuesday evening for RFD).

* Gusty south winds.

Surface low pressure was developing in the lee of the Colorado
Rockies early this evening, with strong surface pressure falls
already developing into the Mississippi Valley. This will result
in increasing southeast winds across the forecast area tonight,
while winds above the boundary layer turn southerly and ramp up
into the 40-50 kt range after midnight. This is expected to
produce a period of LLWS into Tuesday morning, with 45 kt winds
at 2000 ft AGL. Surface winds will gust around 25 kts from the
south during the day, gradually diminishing Tuesday evening.

VFR mid and high clouds will thicken and lower Tuesday morning
as the low moves towards the Midwest, with precipitation expected
to develop from west to east during the morning. With strong warm
advection occurring aloft, and retreating arctic air at the
surface, precipitation will likely begin as a brief snow/sleet
mix and then transition to freezing rain. Freezing rain will then
change to rain across the Chicago terminals by mid-late afternoon
as surface temps warm above freezing. Rockford, farther northwest
into the colder air, will likely see the snow/sleet/freezing rain
mix persist through the afternoon however, as surface temps remain
near freezing. The surface low is forecast to track through
northern IL after midnight, with a change-over to light snow
expected beyond the end of the current TAF period. Ceiling/vis
conditions are expected to deteriorate to MVFR once precipitation
develops, lowering to IFR by afternoon. Ceilings may dip into LIFR
range later Tuesday night as winds diminish near the surface low



IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ008-ILZ010-ILZ011-
     ILZ019-ILZ032...9 AM Tuesday to 6 PM Tuesday.

     Winter Weather Advisory...ILZ005-ILZ006-ILZ012-ILZ013-ILZ014-
     ILZ020-ILZ021-ILZ022-ILZ023...noon Tuesday to 6 PM Tuesday.

LM...Small Craft Advisory...IL nearshore waters until 6 PM Tuesday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...9 PM Monday to 6
     AM Wednesday.




WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.