Area Forecast Discussion
Issued by NWS Chicago, IL

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605
FXUS63 KLOT 082011
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
211 PM CST Thu Dec 8 2016

.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...
323 AM CST

Saturday through Wednesday...

There is some slightly better agreement with the 0z suite of
guidance. Still a lot in the air regards to specifics, but we
have enough to make some insight.

Confidence is still fairly high in a band of snow developing
Saturday afternoon and evening as warm advection ramps up ahead of
a weakly amplified mid level flow and a pacific shortwave that
will be moving southeast across the plains. Compared to guidance
last night, the upper pattern is less amplified on the GFS/GEM
and more in line with what the EC and GFS ensembles have featured
for the upper pattern, but the EC is on the weaker envelope of
guidance for the wave itself. What this feature has going for is
a band of elevated frontogenesis that appears will be in its
strongest state just to our west Saturday afternoon, but then
glide on through at least the northern half our area later
Saturday afternoon and evening. The elevated warm advection will
push thermal profiles aloft into a favorable snow growth region.
While the band of fgen is weakening somewhat per the GFS, fairly
strong omega within this fairly deep growth zone should support a
band of fairly heavy, though possibly not super long lasting
snowfall. This axis has to this point been fairly consistent in
focus along the I-80 corridor northward. Confidence though is not
high yet on this as the NAM/SREF are even farther north (though
the 6z NAM is shifting southward), and both of these get the i-88
corridor north in on the action. Confidence at least on this point
is pretty high on timing, though maybe not for the onset given
these factors. PW values are not too high and mixing ratios hover
around 2g/kg, but dendritic growth could certainly lead to higher
snow ratios with this go around. Even the EC with its lighter QPF
has several inches of snow. WPC favors a 4-6 inch stripe somewhere
in the region, and away from the and which for the moment is on
the higher end of the ensemble guidance. But this is certainly in
the ensemble envelope with the GFS favored a bit with this initial
wave. Mesoscale processes will likely come into play here too.

Then things get a bit more interesting and convoluted moving into
Sunday. After this band it appears there will be somewhat of a
break in precipitation as we await another likely more amplified
fast moving shortwave emanating from the Pacific. Both the GFS/EC
handle the timing of this wave fairly well, coming Sunday
afternoon and evening. The differences on impact come from how
much of a surface low will be able to spin up in this regime. The
GEM is super excited, the EC in the middle, the GFS less so though
it is at least trying to spin up a low as is its ensemble mean.
This wave does still have very good lift with it, and will bring
another round of snow and mixed precip with it. This system also
has a bit more moisture availability. Right now this wave has a
bit more uncertainty with the northern extent of impacts still a
bit muddled.

We have yet another chance for accumulating snowfall Tuesday on
the leading edge of the truly arctic air as a deep cold upper low
will move into the upper Midwest. There could be another brief
snow again Wednesday as several waves pivot through the fast flow
south of the low.

If that were not enough, we will have to deal with some bitterly
cold air more typical of January Tuesday night through Thursday as
both the GFS/EC have 850 temperatures dipping down to at least a
few degrees colder than our current run, the GFS currently is much
colder than that. If the GFS were to pan out, we would be in
record territory for cold, but at this point wind chill readings
of -10 to -20 appear plausible, possibly colder. And if you have
not gotten your hats and gloves and scarves out yet (hope so),
this is time to do it. With the active weather in the short term
did not make too many tweaks to temps (other than some slight
downward shifts from the blends). For those of you who don`t want
to hear about winter after this, it is suggested to not look at
the longer range guidance that greets us with another deep trough
moving east through the plains that could bring another round of
messy weather following the cold spell.

KMD

&&

.AVIATION...
For the 18Z TAFs...

Northwest flow across the area lends to overcast conditions at
the TAF sites, with most areas experiencing flurries.
Visibility restrictions are minimal with any slight banding that
occurs. Satellite imagery shows that forcing and banding is not
coming to fruition as previously anticipated, so an optimistic
outlook was put into place for the afternoon and evening. Wind
gusts continue to register in the 22-25kt range, with high west
winds continuing into the evening hours. Non accumulating snow
showers may remain into early Friday, not likely to create any
impact.

MM

&&

.MARINE...
351 AM CST

Windy conditions persisting over the lake this morning, with these
windier conditions continuing today and into tonight. However,
speeds will observe a diminishing trend on Friday. In the near
term, west gales are continuing over the north half but with
these gales expected to diminish through mid morning. Don`t
anticipate any prevailing gales to occur over the north half later,
but as a secondary surface trough swings through the region later
today, could see occasional gale force gusts. These similar gale
force gusts will likely be observed over the south half today,
however, possibly starting earlier today. So have made mention of
gale force winds/gusts in the south half today, but don`t
anticipate these conditions to warrant a gale warning at this
time. Strong westerly winds to 30 KT also expected for the
nearshore, with similar conditions of gale force gusts also
expected later today.

Rodriguez

&&

.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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