Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 131136

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
536 AM CST Sat Jan 13 2018

232 AM CST

Through tonight...

Primary forecast concerns are ongoing lake effect snow showers in
northwest Indiana and far northeast Illinois along with cold and
breezy conditions elsewhere.

A broad and deep upper trough is in place across the mid section
of the country this morning. At the surface, strong high
pressure of 1040mb is analyzed over the Dakotas while a sub-
1000mb low is lifting north along the eastern seaboard. A modest
north breeze will continue over the local area between these two
features, but should gradually diminish through the day as the
surface high inches into the Upper Midwest allowing the gradient
to relax.

Conditions will remain favorable for lake effect snow to continue
across portions of mainly northwest Indiana through the morning.
Backing winds with the approaching high should eventually push LES
east out of the CWA by late morning or early afternoon. Until
then, NWI may see additional light accumulations up to an inch,
especially in Porter County which will see the longest residence
time this morning as the focus gradually shifts east.

Overnight, winds will continue to drop off as the ridge axis moves
overhead and may see many areas become light & variable or even
calm. The light winds in combination with mostly clear skies will
allow strong radiational cooling to occur. Overnight lows are
expected to fall below zero for much of the forecast area with low
single digits in the Chicago Metro area.



232 AM CST

Sunday through Friday...

On Sunday, an upper level disturbance is progged to ride down the
lee side of the west coast upper ridge into the northern Great
Plains and will eventually close off at 500mb at the low moves
across the western Great Lakes on Monday. There remain modest
track differences of the low between the various model solutions,
but have increasing confidence in lightly accumulating snowfall
Sunday evening into the day Monday. Moisture and forcing are not
overly impressive with this system. Mid level mixing ratios
average 1.4 g/kg northern CWA to 1.8 g/kg south and suggests in
the ballpark of a 1-3 or perhaps a 2-4 inch snow event. Out ahead
of the wave, low/mid level WAA ramps up Sunday afternoon and
evening but this energy will initially go towards gradually
saturating the dry column. Forecast soundings indicate saturation
will be reached mid to late in the evening allowing precipitation
to make it to the surface. Forcing peaks just after midnight
Sunday night into early Monday morning as the vort max rotates
across the region, but occurs mostly above the dendritic growth
zone which will inhibit snow to liquid ratios some.

Better forcing begins to shift off to our east Monday late morning
or early afternoon allowing accumulating snow to taper off, and
cold air wraps across the region shortly thereafter. 850mb temps
are progged to fall back to around -20C Monday evening and night.
Steepening low level lapse rates and lingering stratus falling
within the DGZ should allow occasional flurries to continue
through the remainder of the day Monday and even into the day
Tuesday before the 500mb low finally opens up and moves off to our
east. Meanwhile, single digit and sub-zero lows will be in place
again Monday night and expect highs only in the teens on Tuesday.

Will have to keep an eye on lake effect potential Tuesday and
Wednesday within the cold airmass, but otherwise expect dry
conditions from midweek on along with a warming trend. West coast
upper ridge will translate east across the Great Plains Thursday
then into the Great Lakes Friday. Temps should moderate and
eventually warm to above normal as this occurs.



For the 12Z TAFs...

Strong area of high pressure will build toward the region today
allowing winds to continue to diminish and shift any lingering
lake effect snow showers farther east away from the terminals.
GYY may see some light snow for another couple hours early this
morning before the best chances move east. Outside the lake
effect snow, VFR conditions will prevail. Winds will back to the
southwest overnight and to the SSW tomorrow as low pressure takes
shape over the Upper Midwest. This next system will bring
accumulating snowfall to the region Sunday evening and overnight,
but beyond the current TAF valid period.



330 AM CST

North winds continue to slowly diminish below gale force early
this morning, as strong arctic high pressure spreads southeast
from the Northern Plains. The gradient will continue to weaken
through tonight, as the center of the high moves across Illinois
and settles into the Ohio Valley on Sunday. As the surface high
passes south of the lake, winds will back to the southwest, and
will begin to increase during the day Sunday in response to an
area of low pressure which will drop southeast from Manitoba. A
period of 30 kt south- southwest winds is expected late Sunday and
Sunday night, until the low moves into Southern Wisconsin and the
gradient eases a bit early Monday. As the low tracks across and
eventually east of the lake, winds will turn north-northeast
initially on northern parts of the lake, while becoming westerly
across the south. A return to north to northwest winds is expected
by Tuesday across the lake, as the low moves away and another
strong high pressure pushes south across the Plains. A similar
progression of this high into the lower Mississippi Valley will
turn winds westerly again by Wednesday night.

Open waters gale warning has been allowed to expire at 3 am as
scheduled. IL/IN nearshore waters have also fallen below gale,
with recent obs showing roughly 30 kts along the south shore.
Small craft advisory headlines will remain in place through
Sunday morning in the far south.



LM...Small Craft Advisory...IN nearshore waters until 6 AM Sunday.

     Small Craft Advisory...IL nearshore waters until 3 PM Saturday.




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