Area Forecast Discussion
Issued by NWS Chicago, IL

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311
FXUS63 KLOT 082117
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
317 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...
315 PM CST

Friday night through Thursday...

A very active period of weather, with possible periods of high
impact weather is appearing likely, at least at times this weekend
and into next week. The main concerns will be:
1.) The increasing potential for 2 to 3 shots of accumulating
snow between Saturday and Wednesday, with several inches possible
with each wave.
2.) A good potential for a period of very cold weather around the
middle of next week, with the possibility of wind chill
headlines.

The main features of interest, expected to produce snow across the
area later this weekend are still over the Gulf of Alaska and the
Aleutians, so the forecast certainty on the specifics with this
weekends systems is still not 100%. However, aside from the 12
UTC NAM solution, which is not favored, model guidance suggests
that a period to moderate snow will develop from the Dakotas east-
southeastward across northern Illinois and northwestern Indiana
through the day Saturday. This is expected to occur as isentropic
upglide (warm air advection) increases across the the region in
response to our first disturbance tracking eastward across the
Upper Midwest. This will likely lead to a quick moistening of the
column and the development of snow over the area by mid to late
Saturday afternoon. Mesoscale forcing for ascent also looks
likely over the region in response to a developing area of
frontogenesis over northern Illinois into at least Saturday
evening. This should result in some areas of heavier snow at
times.

As far as snow amounts through early Sunday morning, it appears
that a good 3 to 6 inches of snow could fall over portions of the
area, especially north of I-80. This will be possible especially
considering that a fairly deep dendritic growth zone, with good
forcing is expected to set up over the area, likely supporting a
higher ratio snow of 14 to 15 to 1.

Following this first wave of snow, it is possible that the snow
could come to an end, or at least ease in intensity for a period
on Sunday before our next wave of snow approaches later Sunday
and Sunday night. Confidence on specifics with this second wave of
snow continues to be on the lower side, especially give this
system is still well out over the Northern Pacific. However, in
spite of these uncertainties, the relatively low amplitude upper-
level flow expected over the country, should again favor steering
this next disturbance eastward across the central portion of the
country later Sunday into Sunday night. As such, this would tendto
favor at least another good threat of accumulating snowfall
developing over the area later Sunday into early Monday. The
actual strength of this system is also highly uncertain at this
time, though there is the possibility of a stronger system, which
has been hinted at by some model runs and ensembles. If this does
occur, it could result in significant snow accumulations over
portions of the region, as well as the possibility for some
precipitation type issues over southern portions of the area. For
now, I have remained somewhat conservative on snow amounts for
this second system, and have mentioned rain or snow across my far
south. The evolution of this storm system will have to be watched
closely in the coming days.

If that were not enough, more snow is possible late Tuesday into
Tuesday night ahead of an approaching arctic push. This could
again result in some light accumulations, though the very cold
conditions will become the main story over the area for Wednesday
and Thursday. Highs look to only be either in the single digits or
the teens, with lows possible dropping below zero in some areas at
night. Winds with the arrival of the arctic airmass could also
result in wind chills in excess of -20 Tuesday and Wednesday
nights, so wind chill headlines could be needed.

Beyond the current forecast period there are signs of more snow,
so the larger scale weather pattern looks to remain favorable for
active and cold for at least the next week.

KJB

&&

.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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