Area Forecast Discussion
Issued by NWS Chicago, IL

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431
FXUS63 KLOT 140908
AFDLOT

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

.SHORT TERM...
246 AM CST

Through tonight...

Only real forecast concern in the near term is the lake effect
snow showers which will continue to impact the IL/IN state line
area and portions of Lake and Porter counties this morning. Winter
weather advisory remains in effect through 11 am, with 1-4" snow
amounts, reduced visibility and some blowing snow this morning.

Low pressure which passed across the area Wednesday continue to
move off well to the east of the region early this morning.
Blustery north winds, still gusting 25-30 mph at 330 am, were
helping to focus a north-south band of lake effect snow showers
along/just east of the IL/IN border. Fairly strong low level
convergence is expected to persist in this location early this
morning, before winds begin to diminish more quickly and begin to
show some directional shear in the boundary layer by mid-late
morning. Lake surface to 850 mb delta T`s approach 20C, though
subsidence in version around 6000 ft agl will limit convective
depth somewhat. With high-res guidance depicting the band hanging
in the same general area over the next few hours, still should see
some accumulation potential, though have decreased amounts just a
tad from earlier forecast. Highest amounts should be in Lake
county, with 2-4" totals. Far western Porter county may pick up
1-3" before the snow band weakens and becomes less organized by
late morning. Will maintain advisory headline, as travel may be
impacted across northwest IN this morning, and gusty winds will
produce some blowing and drifting. Far southeast Cook and eastern
Will/Kankakee counties may see a few light snow showers or
flurries early this morning with little or no accumulation.

Elsewhere, skies were clearing skies will allow for sunshine
today, with temps expected in the mid-upper 20`s. Another clipper
digs into the Great Lakes region tonight, though with a farther
north track than the past few. Clouds will increase, with the
potential for a few flurries mainly after midnight as the weak
surface trough approaches overnight.

Ratzer

&&

.LONG TERM...
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
forecast.

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
forecast.

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.

JEE

&&

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

Lake effect snow showers will affect northeast IL and far
northwest IN terminals overnight/this morning, with MVFR cigs and
MVFR/high end IFR vis at ORD/MDW overnight, and likely more solid
IFR/LIFR at GYY into Thursday morning. Gusty north winds up to 25
kts (a little higher at GYY) will diminish and back northwest this
morning.

0530Z/1130 PM CST regional radar mosaic depicts flurries and light
snow showers along the IL/IN shores of Lake Michigan, with a more
organized plume of lake effect snow north of the terminals from
UGN-ENW-RAC. This plume of heavier snow showers will move south
down the IL shore over the next 1-2 hours, with a period of light
snow expected at ORD/MDW. Based on current surface obs and high
res model forecasts, would expect ORD to remain primarily MVFR
with snow slipping south of the area by 07-08Z. MDW, a little
further southeast, may see a brief period of IFR vis. The lake
effect plume will then likely become oriented more N-S along/just
off the IL shore into GYY, and is expected to linger for a time
past sunrise. Given MDW`s closer proximity to the shore and the
expected location of the snow band, have maintained a VCSH through
sunrise, with SHSN about 10 nm east of the field. Tougher to
determine an end time at GYY, though the band should weaken and
drift east of there by mid-morning or so.

Otherwise, winds continue to diminish and back northwest this
morning, and eventually west-southwest by Thursday evening.

Ratzer

&&

.MARINE...
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
Friday.

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
Wednesday.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Winter Weather Advisory...INZ001-INZ002 until 11 AM Thursday.

LM...Small Craft Advisory...IL nearshore waters until noon Thursday.

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

&&

$$

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