Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 020411
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1011 PM CST Thu Dec 1 2016

.UPDATE...
643 PM CST

Have continued the mention of occasional drizzle or sprinkles
through tonight`s forecast. There is an outside possibility of
some snow grains too. The temperature trend should be a very slow
fall with readings expected to remain above 32F tonight.

Broad cyclonic flow with multiple sheared short waves continue to
rotate east-southeast over the area. This is keeping in place a
cold pocket of 800-900mb layer air (-5C to -6C). The 00Z DVN
sounding indicated the continued steep low-level lapse rates up
through that layer, including roughly a 3,000 ft depth within the
cloud layer. This is helping to support the low-level omega and
light precipitation seen on radars from here all the way up to
Bismarck, ND. So basically just continued mention of the
occasional drizzle/sprinkles through the night, especially
along/north of I-80, and some of this could continue through
Friday morning. The top of the saturated layer may flirt with ice
nucleation so that`s why cannot rule out some snow grains.

Also, some higher mid-level saturation and lift across SD/western
IA at present will translate toward western IL and possibly the
SW/S CWA late tonight into early Friday morning and could support
some seeder feeder processes for flurries, but even if so no
impact expected.

The generally homogeneous air mass and neutral surface advection
will only support temperatures dropping a few degrees from where
they are right now, with lows of 33-36 still forecast across the
area. A steady 10-13 mph wind will keep wind chills in the mid-
upper 20s.

MTF

&&

.SHORT TERM...
141 PM CST

Through Saturday...

Cloudy and generally seasonable conditions will be the rule to
close out the week and kick off the weekend. This afternoon, low
pressure is centered over southern Quebec with troughing
stretching across the Great Lakes into the Upper Midwest. Despite
upper ridging beginning to build over the region, the combination
of low level cyclonic flow, steep low/mid level lapse rates and
sufficiently deep moisture is resulting in overcast skies with
patchy light rain and drizzle across northern Illinois and
northwest Indiana. Mid level lapse rates will weaken through the
latter half of the afternoon as subsidence increases across the
region, and think that any lingering precip should diminish as a
result. Lapse rates steepen up again briefly late tonight into
Friday morning as the surface trough drops across the region and
there is an off chance there could be some spotty
sprinkles/flurries once again, but confidence is pretty low,
especially for anything of impact, so not mentioning in the grids
just yet. Clouds will linger through the day Friday with temps a
degree or two cooler north of I-80 behind the trough, and temps
similar to today farther south. High pressure builds in on
Saturday. Quiet weather will be in place under the high but expect
clouds to remain locked in under a strong inversion.

Deubelbeiss

&&

.LONG TERM...

Saturday night through Thursday...

236 pm...Multiple forecast concerns and challenges including
light snow changing to light rain Sunday...Light rain Monday night
into Tuesday...then much colder/windy at the end of the period
with possible rain/snow during transition.

High pressure will be moving east of the area Saturday night into
Sunday morning as flow turns back light southerly Sunday ahead of
a weak clipper-like system moving across the upper midwest.
Precip amounts look to be rather light with this event...perhaps
no more than a tenth of an inch of qpf. And initially...the low
levels will be rather dry...so precip will take time to reach the
surface while the low levels saturate. Nam/gfs thermal profiles
all would support precip beginning as light snow Sunday morning as
it spreads northeast across the area. As low levels begin to
moisten and warm during the morning...precip is expected to
transition to a mix midday and then eventually to light rain.
Forecast soundings show mid level drying by mid/late Sunday
afternoon so precip may end up just liquid drizzle by afternoon
and possibly lingering into the evening. How fast precip
transitions to a mix or to all light rain as well as how early in
the morning light snow begins in the west...will determine how
much...if any...snow accumulation occurs. Current expectations
would be minor snow accumulation where/if snow begins before
sunrise and then just wet minor accumulations further
northeast...including the metro area...and mainly on grassy
surfaces. Again...qpf amounts are expected to light so any snow
accumulation is expected to be minor.

Next system arrives later Monday night into Tuesday morning as low
pressure moves northeast across the southern Great Lakes region.
There remains quite a bit of uncertainty as to how this system
eventually evolves into the midweek system with the gfs now
showing more northerly winds Tuesday with the cold air then just
spreading across the region. Gem would suggest a strong low
developing over the northern plains/upper midwest with more
southerly flow Tuesday. Wind direction may or may not be important
for precip type Monday night into Tuesday...but confidence is low
and this will need to be monitored as this time period approaches.
Thermally...it currently looks warm enough that precip type will
be all rain with surface temps holding steady in the mid 30s
Monday night.

There appears to be good consensus of much colder air spreading
into the region Wednesday night/Thursday next week. How cold and
what happens during the transition remains uncertain with low
confidence. Various runs in recent days have shown various low
tracks with varying strengthens across the lakes...west into the
plains. As noted above...current gfs suggests the Tuesday system
will pull then allow the colder air to spread into the region.
With such a big change in the pattern...some storm system moving
northeast through the center part of the country seems most
plausible even if there are no large impacts locally. Maintained
chance pops for a longer duration then probably will be needed
with precip transitioning to snow Wednesday afternoon into
Wednesday evening. Winds also will be increasing during this time
and a bit early for details...but could be a fairly windy period
into Thursday. cms

&&

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

Ceilings and pcpn chances remain the primary forecast concerns for
the overnight hours across northern Illinois and northwest
Indiana. Cigs improved to lower end vfr for a few hours this
evening, but expect a trend back into lower end MVFR conditions
for the remainder of the night and into the morning hours. Latest
radar imagery continues to show sct weak returns with some light
showers or drizzle. High res model guidance suggests that the sct
shra/drizzle may continue overnight as a series of sheared out
shortwaves rotate around the swrn periphery of the main upper
low, which goes along with the latest radar/satellite trends. So,
will extend the vcsh mention for the remainder of the night.
However, do not expect much, if any, impact at the terminals and
the chances for any wetting rain is low. Sfc temps should remain
high enough that any pcpn should remain liquid. Expect little
change to general conditions through the day tomorrow, with cigs
gradually lifting to vfr levels by afternoon. Winds should remain
generally westerly through much of the period, but gradually
veering from wly to nwly by tomorrow morning as a weak sfc trough
moves through the region.

&&

.MARINE...
141 PM CST

Moderate north to northwest flow will continue across Lake
Michigan rest of today into tonight behind a departing low over
Quebec. A small craft advisory remains in effect for portions of
the Indiana nearshore water due to moderate wind speeds and
shore- parallel flow allowing for moderate waves. Winds will
gradually diminish on Friday as the gradient relaxes, then high
pressure will move overhead on Saturday. Fresh southwest winds
develop Sunday behind the ridge. Details in the forecast become a
little murkier next week with models struggling on the evolution
of a potentially strong low pressure system and powerful cold
front. In advance of the low through the early to middle part of
next week, expect strengthening easterly flow with gusts to around
30 kt possible. Westerly gales appear probable behind the low
though not confident on specific timing and duration given
differences in the models.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 3 AM Friday.

&&

$$

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