Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 151816

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1216 PM CST Mon Jan 15 2018

300 AM CST

Through Tuesday Night...

Early this morning, low pressure is centered near Dubuque, IA with
a cold front stretching south along the Mississippi River
Valley. Widespread snowfall is occurring out ahead of the front
within a broad area of low/mid level warm air advection that will
continue through mid to late this morning across the forecast
area. Steady light to moderate snow will continue to accumulate
and the area still looks mostly good to end up with storm totals
in the 3 to 5 inch range, or an additional 1 to 3 inches through
midday in addition to what has already fallen. The surface low is
progged to track across southern Lake Michigan around midday and
the trailing cold front should overspread the entire CWA by early
to mid afternoon with breezy west winds in its wake. Deep moisture
is lost with frontal passage which will allow snow to taper in
intensity, but still could see some occasional flurries or very
light snow showers as steep low level lapse rates develop with
lingering low level moisture falling well withing the dendritic
growth zone. Given the breezy conditions this afternoon, any
additional snowfall and existing snow on the ground will be
subject to blowing and drifting, especially in open and rural
areas. Wind chills this evening and tonight will fall between -5F
to -20F (coldest south of I-80).

Late this afternoon into the evening, 500mb upper low and a
secondary vort max will pivot across southern Wisconsin and far
northern Illinois which will allow additional light accumulations
up to an inch for the northern tier of counties and possibly
into portions of the second tier of counties. In addition, there
will be the increase in lake effect snow showers as cold air moves
into the region behind the departing low. Scattered lake effect
snow showers will move into far northeast Illinois this evening
and gradually shift southeast across the Chicago metro as
northeast flow backs to the northwest. A lot of the high-res
guidance keeps the better showers very near the shore with little
inland progress. Given the shifting winds, residence time may not
be long in any given area so will call for localized additional
accumulations of up to an inch near the Lake Michigan shores in
Illinois as well as Lake County in Indiana.

As we get into the day Tuesday, LES parameters further improve and
should see a single band develop as sfc-850mb delta Ts rise to 17-
18C and inversion height rise to around 10kft after 12Z. In
addition, some models do indicate the presence of a meso-low
developing. Long northerly fetch is expected to take aim on Porter
County and areas east and should produce heavy snowfall rates
along with the potential for additional heavy snow accumulations.
Any mesolow developing does add additional uncertainty as to where
and exactly how much snow but at this time there is sufficient
concern for and confidence in heavy snow accumulations to hoist a
Winter Storm Watch for Porter County through the day Tuesday.



331 AM CST

Tuesday through Sunday...

By Wednesday, high pressure will be in place along much of the
Mississippi Valley which will provide fair albeit still cold
conditions. Afternoon highs will be in the high teens to low 20s.
Ridge axis shifts to our east late Wednesday into Thursday setting
up southwest flow through the remainder of the week into the
weekend. This will allow a warming trend through the latter half of
the week with temperatures back into the 40s by this weekend and 50s
possible Sunday. Past couple days, models have been latching onto a
deep low lifting from the southern Great Plains into the Upper
Midwest over the weekend. This looks to be a potentially significant
system, though at this range there remains considerable
uncertainty in the low track and resulting forecast details. The
local area has been favored to be well into the warm sector thus
far, though, with even some (very meager) surface based
instability noted in the latest GFS. For now, this system will
bear close watching with showers and thunderstorms favored south
of the low track and significant snow to the north.



For the 18Z TAFs...

The ongoing snow event has been a bit slower to taper off than
had been expected, though the overnight period did indeed see the
heavier amounts as anticipated. Have had to extend lower cigs and
vsbys a couple of times this morning for moderate snowfall and
this will continue into the early afternoon. Regional radar
imagery and sfc observations suggest another couple of hours of
lower conditions in the 2-3 SM range or perhaps briefly worse with
around an inch of additional accumulation, then very gradual
improvement with flurries through the remainder of the afternoon
and evening.

Winds have turned westerly behind the passage of a cold front, and
gusts to around 20kt should start to pick up as per upstream
observations. Pockets of blowing snow could become an issue later
in the day given the fairly dry nature of the existing snow cover.

Overnight another shot of lake enhanced snow is possible, most
likely between midnight and dawn. This appears most likely to be
a transient band producing a quick shot of lower cigs/vsbys and
light accumulations of an inch or so.




119 am...Low pressure over northeast IA early this morning will
move to south central Lake Michigan by late this afternoon and
then begin to weaken as it moves to the eastern lakes region on
Tuesday. Southeast winds to 30 kts with a few higher gusts early
this morning will slowly diminish this morning and turn easterly
this afternoon and then northeasterly tonight. However...based on
the current expected track of this low...winds will likely remain
westerly on the far southern portion of the lake tonight. Strong
high pressure will build across the Plains tonight and Tuesday as
low pressure passes well north of the lakes region midweek. This
will tighten the pressure gradient over Lake Michigan and a period
of gale force winds are possible Wednesday and Wednesday night.
Temperatures will still be below freezing during this time so
freezing spray is possible. cms


IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     ILZ022-ILZ023-ILZ032-ILZ033-ILZ039 until 3 PM Monday.

IN...Winter Weather Advisory...INZ001-INZ002-INZ010-INZ011-INZ019
     until 3 PM Monday.

     Winter Storm Watch...INZ002...6 AM Tuesday to 6 AM Wednesday.




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