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000
FXUS63 KLOT 191142
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
542 AM CST Sun Jan 19 2020

.SHORT TERM...
347 AM CST

Through Monday...

Today through Monday night...

Arctic blast, with blustery winds and bitter cold wind chills are
the main forecast highlights in the near term, as well as the
potential for some lake effect snow showers later today into
Monday. Some of these lake effect snow showers may affect portions
of Cook county late this afternoon and tonight.

Gusty westerly winds continue to push arctic air into the region
early this morning, within a tight surface pressure gradient
between departing low pressure to the northeast, and 1040+ mb high
pressure building southeast across the Northern Plains. Surface
observations as of 3 AM CST indicate air temperatures have fallen
to around zero across our western cwa, and the lower single digits
across the east. Sustained winds near 20 mph, and gusts of 30-35
mph were producing wind chills in the -20 to -22 range from the
Fox Valley westward, where current wind chill advisory remains
valid through 10 AM this morning. While some slight moderation of
the air mass is expected today, temperatures will struggle to rise
from our morning lows into the low-mid teens at best by this
afternoon. Winds will slowly diminish, but will remain blustery
through the day, keeping wind chills near zero in many areas even
into the afternoon. Areas of cold advection stratus will make for
partly to mostly cloudy skies, with the relatively shallow but
cold cloud layer at ideal temperatures for producing flurries at
times.

With westerly low level winds this morning, lake effect snow
showers have been focused across the lake, east of the forecast
area. However, a subtle surface trough across northern Lake
Michigan will push southeast today in association with mid-level
short wave digging around the trailing periphery of the departing
storm to our east. Winds, particularly just above the boundary
layer, are progged to veer more northerly with time late this
afternoon and tonight, which is expected to allow lake effect snow
showers to edge back into our northwest Indiana counties and even
far eastern and southeastern Cook county. The dry nature of the
arctic air, and relatively shallow inversion heights (maxing out
around 6 kft tonight) should preclude significant measurable snow,
though amounts of perhaps 1-3" will be possible, especially
across Porter County where the longest residence time and
strongest convergence appears likely. Minor accumulations of less
than an inch may occur into the Cook county shore areas,
especially from downtown Chicago southeastward. Backing/decreasing
low level winds should push the weakening band back across Lake
and Porter counties Monday morning which along with a lowering
subsidence inversion should bring lake snow to an end by
afternoon.

Temperature wise, only very slow moderation of the arctic air
mass is expected through Monday. Lows should dip into the single
digits in many areas tonight despite some cloud cover, with
recovery into the teens and lower 20s Monday. Clearing clouds and
diminishing winds Monday night will allow for similar if not
slightly colder mins than tonight.

Ratzer

&&

.LONG TERM...
347 AM CST

Tuesday through Saturday...

Upper ridging builds across the Missouri and Mississippi valleys
and western Great Lakes into mid-week, while surface high pressure
drifts southeast into the Ohio and Tennessee valleys. This will
make for dry and slowly moderating weather Tuesday and Wednesday.

Potentially active weather looks to return Wednesday night
through Saturday however, with medium-range models depicting the
development of a closed upper trough across the central CONUS.
While differences in the details of various guidance remain, the
general consensus is for the development of broad southwesterly
warm/moist advection flow to develop Wednesday night ahead of a
baroclinic frontal zone which lays out from Ontario back into the
Central Plains. While this focus is initially west of the forecast
area, there is evidence of a lead short wave lifting northeast
along the baroclinic zone Wednesday night, which may spread some
very light snow into parts of northern IL overnight. Weak but
persistent warm advection flow will present the potential for
additional light precip Thursday, though the greater potential for
more organized precipitation appears to come late Thursday
through Friday night as the upper trough closes off and a
deepening surface low develops and lifts from the lower
Mississippi Valley toward the Great Lakes. The GFS/CMC/ECMWF and
their ensembles vary with the track of the low at this distance
(GFS warmer and farthest west, the CMC farthest east), so
confidence in details is still low at this distance. It does
appear however that the potential for a storm system to produce
some sort of measurable wintry precipitation exists during the
late week period.

Ratzer

&&

.AVIATION...
For the 12Z TAFs...

542 AM...Forecast concerns include...

Lake effect snow showers late this afternoon into tonight.
Mvfr cigs through tonight.
Gusty west/northwest winds today.

Lake effect snow showers are possible later this afternoon and
especially tonight across northwest Indiana and these are expected
to reach gyy and may move into far northeast IL. Confidence is low
for how far west these snow showers will move and there remains
quite a bit of uncertainty regarding duration at any one point.
For now have opted to include only prob mention this evening for
mdw and for all of tonight at gyy. Scattered flurries will be
possible across the entire area today and tonight.

There have been periodic holes in the mvfr deck early this morning
and this is likely to continue as well as periods with just vfr
cigs but for now maintained mvfr cigs for much of the period.
Lower cigs would be possible with any lake effect snow showers.

West/northwest winds have diminished overnight with gusts now in
the lower/mid 20kt range and these gusts will likely continue this
morning with a slow diminishing trend this afternoon with speeds
under 10kts this evening and wind directions slowly turning more
northerly tonight. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Wind Chill Advisory...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-
     ILZ012-ILZ019-ILZ020-ILZ021-ILZ032 until 10 AM Sunday.

IN...Lakeshore Flood Advisory...INZ002 until noon Sunday.

LM...Heavy Freezing Spray Warning...LMZ740-LMZ741-LMZ742-LMZ743-
     Gary to Michigan City IN until noon Sunday.

     Small Craft Advisory...Winthrop Harbor to Gary IN until 9 AM
     Monday.

     Gale Warning...Gary to Michigan City IN until 6 AM Sunday.

&&

$$

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