Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 141606
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1006 AM CST Thu Dec 14 2017

.UPDATE...
1002 AM CST

Lake effect snow will continue to shift east, with the main precip
shield now into Porter county. Light coatings, a few additional
tenths of snow accumulations are possible before noon before the
convergence signal at the southern tip of Lake Michigan erodes.
Will hold the advisory a bit longer just given the variable
visibility and locally slick roads more so on less traveled roads,
though accumulations should largely remain on the lower end of
the spectrum.

KMD

&&

.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 12Z TAFs...

Only real concern this morning is lake effect snow for GYY. Other
terminals will be VFR with light northwest winds turning more west
this afternoon.

North winds down the length of Lake Michigan has resulted in
development of lake effect snow showers overnight. Light snow
showers had affected the IL shore earlier in the night, though the
main band has become oriented north-south east of the IL/IN state
line across Lake/Porter counties. Main convergence axis appears to
be just east of GYY at this time, and overall intensity of the
snow has been lighter than earlier expected, though IFR vis will
remain possible for 1-2 hours at GYY before focus shifts farther
to the east. Otherwise, dry arctic air has allowed clearing of VFR
clouds across northern IL, and this should remain the case through
the remainder of the day. Winds will shift light southwest this
evening in advance of another weak surface trough which will move
across the area toward Friday morning. VFR cloud cover will
increase tonight, with a few flurries possible late, but not
expected to create an impact in vis.

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...IN nearshore waters until 3 PM Thursday.

&&

$$

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