Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 011944

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

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.



229 AM CST

Friday through Wednesday...

Upper trough over the region will slowly ease its grip heading
into the weekend, but continued cyclonic low level flow and lack
of dry air advection should be enough to keep stratus locked in
through Friday night. Given the low sun angle and high moving
overhead Saturday, strongly suspect that stratus could be a fairly
prominent player Saturday as well. MAV suggests clearing Saturday,
but MET guidance keep us mostly OVC, have trended init grids more
pessimistic, but confidence in the cloudy forecast is not as high
for Saturday.

If the sun make an appearance Saturday, it will be short lived as
next shortwave begins to approach the region Saturday night and
likely brings some light precipitation to the area Sunday. Thermal
profiles still look pretty borderline with regards to p-type and
currently expected rain or wet snow trending to light rain or
drizzle Sunday as we lose ice in the clouds.

Forecast confidence heading into next week remains a bit below
average. ECMWF/GFS/GEM have come into better agreement early in
the week in showing northern jet buckling north with upper ridging
over area. Simultaneously, shortwave energy cuts off into a closed
low mid/upper level low over northern old Mexico. Guidance kicks
that shearing upper low out early in the week and operational
models are showing a weaker and weakening system with little
phasing with the northern stream as it moves east. Depending on
the track and intensity it could bring a bout of precipitation to
our area in the Tuesday-ish time frame, which at this time looks
like it`d be all rain.

Midweek and beyond, medium range models and respective ensembles
are in good agreement on showing a substantial pattern change over
North America. Common theme is significant negative 500mb height
anomaly over Great Lakes region, which would support coldest air
mass of the season arriving late next week. The million dollar
question, which has had almost as many answers in the past several
days of model runs, is will there be cyclogenesis on the leading
edge of this arctic air mass, and if so, how strong and where.
Medium range guidance continues to offer the full spectrum of
answers to that question, so the potential for any precipitation
(wintry or otherwise) has much higher uncertainty than does the
expected significant downward trend in temps.



For the 18Z TAFs...

Ceilings the main forecast challenge over the next day or so
across northern Illinois and northwest Indiana. MVFR conditions
are in place early this afternoon, with a modest upward trend
expected this afternoon. Could be a brief window of low end VFR
but not confident enough on that to include in TAF. Also have
lower confidence in trends overnight as there are conflicting
signals between MOS and raw model soundings. Leaning towards the
more pessimistic end of the spectrum following NAM/GFS soundings
which suggest low MVFR or possibly even IFR as a weak secondary
trough moves across the region early Friday morning. Winds will
veer slightly from the west to northwest with this trough as well,
then should see a gradual improving trend again behind the



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.



LM...Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 3 AM Friday.




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