Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 141935

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
135 PM CST Thu Dec 14 2017

135 PM CST

Through Friday...

Seasonal colder conditions remain in place this afternoon with
passing higher clouds. Northwest flow aloft will continue tonight,
and will steer another weak disturbance through the region. There
have been some flurries/light snow upstream in Minnesota with this
feature. We see very little if any model guidance painting any
accumulation with this feature, other than some flurries after
midnight into early Friday. Can`t argue against a brief isolated
dusting given the cold temperatures and secondary cold surge with
the wave. This forcing will exit east mid morning. Warm advection
will ensue ahead of another clipper on track for Friday night,
with this one again largely to remain north of the Wisconsin
border. We would just expect some increase in higher clouds
during the afternoon. With the warm advection and least sun breaks
in the middle of the day, we could tack on a few degrees from
today. Temperatures will still be below normal.



307 AM CST

Friday through Wednesday...

Overview...Due to a lack of snow on the ground, I went with slightly
warmer high temperatures through the weekend. Friday will be breezy
with west winds gusting to 30 MPH. Next chance of precip is Sunday
with an additional weak system or two early next week.

An upper level wave passes overhead Friday, and a few flurries are
possible. Soundings feature a deep saturated layer, so while the
upper level wave isn`t overly impressive, it shouldn`t take much to
squeeze out a few flurries.  Low pressure over Lake Superior will
deepen as is shifts east and that should result in gusty west winds
over the forecast area.

I think guidance is displaying a low bias in temperatures likely
caused by the lack of snow on the ground. As such, bumped up high
temperatures Friday through Sunday using local climatology.  I did
not have enough confidence to adjust temperatures further out in the

Warm air advection kicks into high gear Saturday with high temps in
the low to mid 40s. Temps could end up being a big warmer than

Guidance is still struggling with the path of Sunday`s system. The
GFS has the system impacting Appalachia while the ECMWF has a more
westerly path that impacts the forecast area.  I kept the chance of
precip SuperBlend loaded, and if there is precip, precip type will
be a concern Sunday morning. Temperatures may be around freezing,
and forecast soundings would suggest liquid precip above the surface
thanks to a deep melting layer. Surface temperatures quickly warm
and rain should be the primary precip type from that point forward.

An upper level wave may lead to additional light precip Monday
afternoon. High pressure then spreads over the region Wednesday.



For the 18Z TAFs...

Next concern are some flurries overnight into early Friday, and
possibly some MVFR cigs.

No significant weather in the first 12 hours of the TAF cycle.
High pressure across western IL will inch eastward which will
allow NW winds near ORD/MDW to weaken and become
variable/westerly. Another weak northwest flow disturbance will
shift through the terminals overnight into early Friday. Isolated
to maybe scattered flurries remain possible with this wave, mainly
in the 10z-15z window. Cigs remain VFR until MVFR clouds arrive
during this time also. Cannot rule out a brief localized dusting
somewhere, but low confidence that this would occur.



307 AM CST

The gale warning was allowed to expire as winds diminished to below
gale force. Hazardous conditions for small craft will continue
through this morning across the IL waters and through mid afternoon
for the NW IN waters.

The low over PA will continue east reaching the Atlantic Coast this
morning. Another weak low will form over Lake Superior this evening
and deepen as it moves just northeast of Lake Huron Friday evening.
West winds increase to 30 kt Friday as the low strengthens and
pushes east. I have low confidence in gales, but will mention ocnl
gales over the southern end of the lake Friday afternoon. Another
small craft advisory will be needed for hazardous winds and waves

Wind direction will vary across the lake as a frontal boundary will
reside over the southern end of the lake Saturday.  The front will
sink south as high pressure shifts southeast over Ontario. This will
lead to southeast or east winds across the lake Saturday
afternoon/evening at 15-25 kt. Guidance suggests the low will weaken
as it moves over the lake Sunday evening.

A large low pressure system moves over Hudson Bay Monday evening and
continues east through mid week. The low`s cold front will pass over
the lake Tuesday morning and gales are expected behind the front. A
surface high will also form over the Plains. This will result in a
tightened pressure gradient over the lake allowing gales to persist
through Tuesday night. Winds weaken as high pressure approaches



LM...Small Craft Advisory...IN nearshore waters until 3 PM Thursday.




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