Extreme Cold in Illinois, January 1 to 2, 2018

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1032 PM CST Sun Dec 31 2017

.UPDATE...
622 PM CST

Have expanded the Wind Chill Warning one tier of counties
northeast to include the far western Chicago suburbs and the
Kankakee area. The warning time remains the same from 9 pm tonight
through noon Monday.

Radiational cooling has been off to the races since 430 pm, with
several locations below zero in the western and southern CWA.
Accounting for this temperature trend into the forecast, as well
as continuing the idea of winds 7-10 mph tonight, wind chills of
-25 to -35 are forecast for the region that was just added to
the warning.

All in all though the message remains the same...very cold, even
to dangerous levels when considering the wind chill, this New
Year's Eve night and New Year's Day.

MTF

&&

.SHORT TERM...
207 PM CST

Through Monday...

LES band has managed to stay out of Porter County so far and it
looks like it will stay that way, but 30-40 miles offshore to the
north the band extends west far enough to make it difficult to
completely rule out a brief period of moderate snow over the
northeast tip of Porter County. Going to make a last minute
decision about whether or not its safe to cancel early, but
certainly looks safe with snow accumulations over northeast tip of
Porter County likely to be less than an inch or so.

Only other short term item of interest in the weather departments
is with the bitterly cold temperatures and wind chills. No changes
planned to going headlines which look to capture the expected wind
chills well. Still looks like several hours above wind chill
advisory criteria Monday afternoon, particularly in the urban heat
island area of Chicago where snow cover is less and the sun will
help temps moderate some. Despite the likely break in wind chills
Monday afternoon, plan to let advisory ride as is.

As far as ambient temps go, did go a bit lower than guidance over
the deeper snowpack over our southern CWA tonight where isolated
locations could make a run at 20 below by Monday morning.
Elsewhere, mostly looking for 10 to 15 below (except maybe the
typical cold spots like ARR, RPJ, etc). Also, Chicago's urban heat
island should limit lows in the urbanized/highly suburbanized
areas closer to the 5 to 10 below range.

Monday's highs over the deeper snowpack over southern CWA could
struggle to reach zero, while the lower snowpack, urbanized areas
of Chicago could make a run at 10 above, with everyone else
some where in between.

- Izzi

&&

.LONG TERM...
300 PM CST

Monday night through Sunday...

The remarkably long stretch of cold weather will continue through
the first shortened work week of 2018, with finally signs of
pattern relaxation next weekend. It looks to be a continuation of
the drier than normal theme of December, though a few lower
amplitude clipper type waves at least threatening the region with
snow are probable in the Wednesday to Saturday timeframe. Given
the typically low predictability of these systems, stayed close to
blended guidance PoPs, which yields a primarily forecast during
periods in which snow will be possible.

After the record or near record cold high temps on New Year's Day,
the Arctic surface high of 1046-1048 mb, while not perfectly
positioned for ideal radiational cooling, is expected be close
enough for temperatures plummeting to below zero area wide (lowest
confidence for downtown Chicago). The coldest low temperatures
early Tuesday are forecast to be in the mid teens below zero at
the typical cold spots and deepest snow covered areas along and
south of I-88 southward to the southern CWA counties. Boundary
layer decoupling should allow winds to drop off, especially
outside of Chicago, possibly light enough at times for wind chills
to rise back above Wind Chill Advisory criteria in some locations.
With this being said, will not make any changes to the going
advisory, which is in effect through 18z/Noon Tuesday.

The high pressure will settle south of the area ahead of the next
reinforcing cold front. Given warm advection (925 mb temps rising
to around +12 Celsius) and increasing southwest winds, suspect
that much of the guidance is too cold with high temperatures
Tuesday and low temps Tuesday night. Bumped up highs to ranging
from the upper single digits over deeper snow covered areas to
the mid to upper teens in and near Chicago and think these are
probably still too cool, with solidly teens area wide to even
around 20 in spots suggested by local 925 mb temp climatology. The
same can be said for lows, with a bump to low single digits to
around 10 possibly not warm enough. The breezy southwest winds
will still result in a cold feel, but not as cold wind chills as
earlier forecast.

Temperatures should reach the teens on Wednesday with the next
cold fropa, followed by the next shot of very cold to bitterly
cold air for the rest of the work week. As mentioned earlier,
can't rule out some clipper systems Wednesday through Saturday
timeframe with a threat for snow. The long awaited pattern
relaxation to close next weekend and into the following work week
could be accompanied by a storm system somewhere over the region
per recent operational and ensemble guidance, but confidence is
obviously low in any details Sunday-Monday.

Castro

&&

.CLIMATE...
622 PM CST

Records that will be in jeopardy for the first day of 2018:

             January 1 Record Low      January 1 Record Low Max
Chicago          -10 (1969)                   5 (1969)
Rockford         -13 (1924, 1968)             4 (1974)

MTF

&&

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

No significant aviation concerns next 30 hours, with VFR
conditions and west-northwest winds around 10 kts through the
period.

Ratzer

&&

.MARINE...
237 PM CST

Lake induced meso-low now over extreme southwest Lower Michigan
has been responsible for the squirrelly winds over the southern
portions of the lake. This small scale low will dissipate tonight
and a somewhat more uniform northwest wind will lock in over the
lake, though eastern shore areas may still see some land breeze
slackening of the winds. Moderate northwesterly winds will remain
locked in until Tuesday when winds back in advance of an Alberta
Clipper. Period of southwest gales looks likely Tuesday night,
though opted to hold off on a 5th period gale watch given the
expected lower end nature of the gales. Cold front will move
across the lake Wednesday with moderate northwest winds likely the
remainder of the week. Freezing spray will continue to be an
issue much of this week due to the arctic air mass in place,
gradually cooling lake temps and gusty winds.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Wind Chill Warning...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-
     ILZ012-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039
     until noon Monday.

     Wind Chill Advisory...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-
     ILZ012-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-
     ILZ039...noon Monday to noon Tuesday.

     Wind Chill Advisory...ILZ006-ILZ013-ILZ014-ILZ022 until noon
     Tuesday.

IN...Wind Chill Advisory...INZ001-INZ002-INZ010-INZ011-INZ019 until
     noon Tuesday.

LM...Small Craft Advisory...IN nearshore waters until 3 AM Tuesday.

&&

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