Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 011739

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1139 AM CST Thu Dec 1 2016

229 AM CST

Through tonight...

Not too much to talk about in the short term. Pretty much what you
see out is what you get with temps only expected to rebound a few
degrees today for highs and only drop a few degrees tonight with
stratus deck helping lock temps in the 30s. Could be some spits of
rain or snow grains, mainly this morning, but not expecting
anything measurable or of any real significance.



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



238 AM CST

Headlines...Have high confidence in hazardous winds and waves across
the IN nearshore waters into this evening.  Waves are forecast to
subside slower than originally thought so extended the small craft
advisory to late tonight. Decided to not issue a small craft advsy
for the IL waters as hazardous wind gusts once again look marginal.

Weak low pressure is over southeast Quebec and it will slowly fill
and then merge with a stronger low over Nova Scotia.  High pressure
builds over the plains tonight.  West to northwest winds to 30 kt at
times diminish to 15-25 kt tonight, and then become southwest over
the weekend behind the high.  A low forms over the northern plains
early next week with a secondary low forming over the southern
Mississippi Valley.  Winds become southeast in response to the lows.
Prolonged southeast winds may lead to hazardous waves in the IL
nearshore waters early/mid next week.  The larger low moves over
southern Canada mid-week while the secondary low lifts north along
the Mississippi River valley and reaches the lake mid-week.



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




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