Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 090911
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
311 AM CST Fri Dec 9 2016

.UPDATE...
925 PM CST

Evening update...

Minor tweaks to going forecast this evening primarily in respect
to lowering cloud cover over far northeast IL. Otherwise, mostly
cloudy, cold and somewhat blustery conditions expected to persist
overnight with a few flurries at times. Further out, the new 00Z
WRF appears to come in line with other guidance and increase
confidence in a significant measurable snow beginning late
Saturday across the the northern half of the LOT cwa.

In the near term, the old, slow-moving low pressure system
continues to drift east across the Canadian Maritime provinces
this evening, with weak surface troughing lingering across the
Great Lakes region. While surface pressure gradient was slowly
weakening across the Midwest, allowing winds to gradually
diminish, west- northwest cyclonic flow continues to produce
extensive low cloud cover and scattered flurries. Early evening
satellite imagery had depicted an area of clearing across eastern
WI, which has worked steadily southward into far northeastern IL
as of 9 pm. Guidance still suggests potential for this to fill in
somewhat given persistent steep low-level lapse rates beneath a
sharp inversion noted in the 900-925 mb vicinity, though lower
clouds have eroded fairly impressively across eastern WI. Some
patchy higher clouds noted across the area, though have adjusted
forecast sky cover to partly cloudy overnight and removed flurries
generally northeast of a Rockford-Gary line. Suspect enough west-
northwest gradient wind to keep things mixed and prevent a sharp
temp drop where clouds thin out, so have maintained overnight lows
in the mid- upper teens.

Updated grids/text forecasts available through tonight.

Ratzer

&&

.SHORT TERM...
210 PM CST

Through Friday...

WSR-88D depicts one west-east snow band in the counties of Grundy,
Will and Kankakee this afternoon. A similar band extends SE of
Davenport toward our western CWA. Snow within these bands may
produce visibilities down to 4 miles. All other radar returns of
light snow showers or flurries have produced virtually no
restrictions. With an upper low in SE Canada, cyclonic/NW flow
continues across northern IL. This supports the low level moisture
in place which will be under an inversion overnight. The moisture is
quite shallow and has little to no upper forcing. The model PVA
depictions and satellite do not indicate any more waves upstream, so
no real uptick in snow is expected other than the established band
south of Morris. There are a couple subtle kinks in the pressure
pattern that would indicate a trough sweeping through northern IL in
the very short term. This could spark a brief uptick in the already
gusty winds. Expecting gusts near 30 mph through the remainder of
the afternoon with a slight decrease to 20 mph during the evening.

Friday brings another cold and overcast day. Winds will not be
nearly as gusty, but temperatures trend near 15 degrees below normal
for this time of year. H85 temps are expected to be near -14C which
supports max temps near 20F. The pressure pattern starts to regulate
across the CWA as we remain in-between weather systems.

MM

&&

.LONG TERM...
306 AM CST

Monday through Friday...

Dangerously cold temperatures will be the main forecast story
next week with sub-zero conditions possible as we head through
midweek. Area of low pressure that will bring our snowy conditions
over the weekend is progged to lift across the eastern Great Lakes
to New England on Monday. GFS spits outs some light QPF early in
the day despite forecast soundings indicating quickly drying mid
levels with subsidence associated with building upper ridge
overspreading the area in the afternoon. Could be some spotty
light flurries or possibly even drizzle given the shallow depth of
the moisture which eventually tops out in the -8 to -10C range for
ice nucleation, but in general think the models may be overdoing
QPF for Monday given the shallow moisture and weak forcing.

Meanwhile on Monday, a strong jet topping an anomalously strong
upper ridge over northern Alaska into the Arctic Ocean will help
dislodge an extremely cold Arctic airmass and push near -40 850MB
temps across the Canadian Prairies on Tuesday with -20C to -30C
temps overspreading portions of the upper Midwest Wednesday and
Thursday. Highs around 30F on Monday will give way to single
digit or teens for lows Wednesday morning behind the Arctic front
with little recovery during the day Wednesday. Models continue to
show some spread in exactly how cold we get, with the latest
forecast remaining conservatively on the warmer side of solution
envelope mainly due to the influence of warmer MOS guidance. Raw
output from the latest runs of the GFS and ECMWF indicate the
possibility of sub zero highs on Thursday over some if not much of
the CWA with widespread sub-zero conditions both Wednesday night
and Thursday night. In addition, breezy conditions in advance of a
strong 1040mb high building into the northern Plains Wednesday and
Thursday will result in (again conservative) wind chill values of
10 to 20 below.

Deubelbeiss


&&

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

High mvfr/low vfr cigs scattered earlier this evening and this
clearing line continues to slowly move south of the terminals.
Appears the only clouds for the rest of the night will be
sct/bkn 4-6kft clouds drifting south from WI. Guidance still
showing the potential for mvfr cigs to redevelop Friday morning
but confidence is low so went with scattered mvfr mention with
this forecast. With the loss of cloud cover...winds have
diminished as well but will likely stay in the 10-12kt range
overnight and into Friday afternoon. High pressure will build
across the region Friday afternoon and Friday night which will
allow winds to diminish under 10kts Friday evening. cms

&&

.MARINE...
306 AM CST

Moderate northwest winds gusting to near 30 kts are in place early
this morning across Lake Michigan but should very gradually ease
into the 15 to 25 kt range through the day as high pressure builds
towards the region. A small craft advisory remains in effect
through late this morning for the Illinois nearshore waters, and
through the afternoon into the early evening for the Indiana side
when waves should finally diminish below criteria. Low pressure is
expected to develop over the Central Plains Saturday night into
Sunday turning winds southerly across Lake Michigan, then the low
is expected to lift across southern Lake Michigan Sunday night
with north to northwest flow overspreading the lake behind the low
Monday. Another cold front associated with a low over Canada will
push across the lake on Tuesday with a bitterly cold airmass
moving in behind the front. West to northwest gales are possible
especially Wednesday into Wednesday night and there is also a
concern for freezing spray.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 6 PM Friday.

     Small Craft Advisory...ILLINOIS NEARSHORE WATERS UNTIL 9 AM Friday.

&&

$$

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