Area Forecast Discussion
Issued by NWS Chicago, IL

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827
FXUS63 KLOT 081650
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1050 AM CST Thu Dec 8 2016

.UPDATE...
1049 AM CST

Radar returns across the CWA contain a few isolated bands of snow
where visibilities have dropped to 4 or 5 miles. For the most part,
the area will continue to see insignificant snow flurries with no
visibility restrictions. The threat for light snow showers continues
into the evening as low level moisture indicated by forecast model
soundings suggests a small layer of super-saturation with respect to
ice in the lowest 3KFT. The lack of depth to this layer or negative
omega values will keep the snow contained to just flurries for most
of the area.

Wind gusts over 25 mph coupled with surface temperatures in the
upper teens combine to create brisk conditions. Wind chill values of
-2 to +7 are observed currently with only slight improvement through
the day and evening.

MM

&&

.SHORT TERM...

Today Through Friday night...

Concern in the short term is the ongoing Round 1 of cold, which is
about to get a bit colder here in the next few days, inland snow
shower chances, and lake effect snow for Porter/Lake counties of
NW Indiana.

Aloft...a deep upper low continues to churn east though southern
Canada, currently spinning near James Bay. A sharp upper trough
axis will rotate through the area this morning along with
additional shortwave energy diving southeast through Minnesota.
These forcing mechanisms are already leading to some
flurries/light generally non accumulating snow showers early this
morning. Low level thermal profiles today continue to show pretty
good low level lapse rates and the top of the mixed layer into the
snow growth region. This will be the case at least through early
afternoon. Upstream obs in MN depict visibility occasionally
dropping to the 3-5sm range. Part of this the fluffier snow, and
part is the wind, as enhanced mixing in the continued cold
advection (driven by strong high pressure along the plains) has
supported the uptick in wind early this morning as well and expect
these to continue today. These combined factors will make today
feel more uncomfortable than Wednesday. High temperatures will
hold in the mid 20s for most locations. That means current wind
chills in the single digits to the mid teens will remain in place
through the day.

Later this afternoon a subtle surface trough axis will cross Lake
Michigan and allow a lower level wind direction shift more
northwesterly during the evening and overnight hours. This coupled
with a favorable lake thermodynamic environment both with a large
850-lake T difference and deep lake inversion heights and
sufficient RH, would expect some snow showers to creep into Porter
and possibly Lake IN. Any stronger activity could produce an inch
or more of snow. Currently no advisory is in place, but advisories
will be in effect across LaPorte and points northeast. Will need
to watch if activity shifts in more aggressively.

Lake effect re-focuses east of the area on Friday. Moisture slowly
eases and high pressure will be shifting eastward. That will leave
our area with another cold day but with a little less wind. Clouds
may be a bit more prevalent like today with the high still off to
the west. Highs will again be in the lower to mid 20s. The ridge
will shift overhead Friday night. Lows fall back to the teens with
higher cloud cover increasing overnight.

KMD

&&

.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 12Z TAFs...

No significant changes expected this forecast period with MVFR
ceilings, flurries or light snow showers, and westerly winds
expected to persist. Flurries are occurring at this time,
periodically becoming more widespread. However, don`t anticipate
much of any impacts, with this snow remaining light with
little/no vis restriction. Monitoring upstream mid level
disturbance dropping south out of Wisconsin that should arrive by
around the 18-20z period this afternoon. This could bring an
increase in intensity of the current flurries/light snow with vis
possibly falling to the 3-5sm range. Have included this in a tempo
for later this afternoon but will need to monitor for this higher
intense snow to be more prevailing, as well as an outside chance
for this to be slightly even more intense. Increasing low level
lapse rates with this disturbance could possibly bring vis in this
snow even lower maybe to around 2SM, and possibly to 1-2SM.
However, with confidence lower on this possibility, have not
included this low of vis. Snow intensity decreases later this
evening, but with periodic flurries still likely tonight.

Rodriguez

&&

.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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