Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 110529
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1129 PM CST Sat Dec 10 2016

.UPDATE...
855 PM CST

Evening Update...

No changes planned to forecast or headlines at this time, though
will be making some minimal changes to near term pops based on
radar/obs trends.

Initial region of warm-advection forced light snow overspread the
region late this afternoon as anticipated, with heavier (mainly
moderate) snowfall within a region of frontogenetically-forced
northwest-southeast bands. Recent radar trends show this banding
has weakened and lifted northeast across southeast WI, Lake
Michigan and northern IN at this time. However, 00Z DVN sounding
depicts a fairly deeply saturated column with nice veering wind
profile signature associated with strong warm-advection. Within
this region of persistent upglide, latest high-res guidance
continues to support regeneration of transient f-gen banding which
should continue to result in periods of moderate or greater
intensity snowfall. While we`re currently in a lull in radar
returns especially across the western parts of the cwa, stronger
returns are blossoming across portions of eastern IA, and would
expect an uptick in coverage and intensity later this
evening/overnight as forcing and moisture advection persist. With
the first band working to moisten initially dry low-levels, will
likely see additional bands more efficient in lowering vis and
producing heavier snow, and latest RAP soundings suggest a decent
period of deeper dendritic growth after 05-06Z.

Initial bands of snow have produced 2+" of new snow in many spots
across the warning area, which is in line with going forecast
amounts through 06Z. Thus not inclined to make any big changes at
this time, with expected redevelopment and increase in snow
coverage and intensity later tonight.

Ratzer

&&

.SHORT TERM...
309 PM CST

Through Sunday Night...

The main forecast concerns with the winter storm continue to
revolve around total snow amounts, as well as the extent of the
mixing/changing to rain in my far south later Sunday into Sunday
evening.

Not much has changed in the overall thinking in terms of snow
amounts, with only minor tweaks made from the updated snow
amounts from this morning. Generally this will continue to result
in the highest totals of generally 8 to 12 inches north of I-80.
Amounts still look to tapper off into the 1 to 4 inch range over
my far southern counties (Livingston, Ford, Iroquois and Benton
counties). As a result, no changes are planned to the current
headlines.

An impressive band of moderate snow continues to develop east-
northeastward over eastern Iowa and western Illinois in
association with an initial band of frontogenesis along the
leading edge of the increasing isentropic upglide (warm air
advection). This area of snow should begin to result in a quick
onset of moderate snow from west to east over the area within the
next few hours. This could result in up to a quick inch of snow
through early this evening, especially over north central
Illinois. Once this snow onsets, it appears that it will continue
at times through the night and into Sunday morning over northern
Illinois and northwestern Indiana as warm air advection continues.
Mesoscale banding will also continue to be possible over northern
Illinois overnight as steep mid-level lapse rates remain in place
above 650 MB. As a result, snow amounts tonight alone could be in
the 3 to 6 inch range, especially north of I-80 where higher
ratios of around 15 to 1 will be likely.

Periods of moderate snow look to continue Sunday morning, though
there could end up being a break, or a period of lighter snow
during the morning, before another period of wetter snow onsets in
the afternoon in association with the approach of the mid-level
disturbance. This could again result in periods of moderate, to
possibly at times heavy snow over the area into early Sunday
evening. Temperatures still could get warm enough in my far south
for a mix or transition to a period of rain late Sunday into the
evening, though it appears areas farther north would remain
mainly snow. Snow ratios will likely be lower (around 10 to 1)
during the day, but another 3 to 6 inches will still be possible.

The snow should gradually come to an end from west to east during
the evening on Sunday. There could also be some patchy blowing
snow Sunday night as the winds pick up a bit on the back side of
the surface low. However, given the wet nature to the snow on
Sunday, blowing snow would likely be a minor impact.

KJB

&&

.LONG TERM...
249 PM CST

Monday through Saturday...

Concerns in the long-term portion of the forecast include the
potential for dangerous cold Tuesday night through Thursday night,
as well as a few opportunities for snow. Late next week, a more
significant storm system could take shape, with accompanying
accumulating wintry precipitation.

Anomalous mid and upper ridging across the Aleutians and
northeastern Pacific near Alaska will favor a very cold pattern
continuing through the upcoming week, as true Arctic air plunges
southward across the country. A piece of the polar vortex will
drop to near Hudson Bay, supporting 500 mb heights of near or
below 480 DM. Meanwhile, the fast progressive jet stream pattern
will remain as well, with low amplitude clipper type disturbances
possible Monday evening and again Wednesday. With the Arctic air
mass in place, any snow from these potential systems will be of
the fluffy/high-ratio variety. Thus, while the current operational
guidance would not suggest any significant events, snow will
easily accumulate on surfaces for at least some travel impacts.
Also, some previous model runs have offered potential for more
energetic disturbances and more notable accumulations, so cannot
rule this out either. Again, latest forecast suggests that period
of interest for potential fast moving snow producers are Monday
evening and Wednesday.

Returning to the potential for dangerous cold in the mid-late work
week period, behind the possible Wednesday system, shortwave
around impressive ~480 DM mid-level low near James Bay will send a
potent thermal trough our way. Models have exhibited run to run
variance in the magnitude of the cold air mass that reaches our
latitude, though there was consistency in today`s guidance in 850
mb temperatures of -20 Celsius or colder Wednesday afternoon
through early Thursday. With deep snow pack in place, this thermal
profile will easily support temperatures barely budging and then
falling Wednesday afternoon followed by widespread subzero
readings Wednesday night/early Thursday. Expansive high pressure
spreading toward area will also add a westerly wind component to
support wind chill values of -20F or colder (advisory criteria or
colder) for at least a portion of the area. Weak "warm" advection
will ensue aloft on Thursday, though frigid start and no mixing
under the high could keep temperatures in the single digits area
wide, if not colder in a few spots.

Finally, despite long lead time, concern is increasing for a
significant winter event later in the week. A very impressive
thermal gradient will exist across the Plains with Arctic high
pressure to the north. At the mid and upper levels, there is
decent guidance continuity in the idea of deep trough carving out
into the intermountain West, with a temporary surge of height
rises for the eastern 2/3 of the CONUS. Associated surface cyclone
will be working with aforementioned impressive thermal gradient,
as well as very strong WAA ahead of it, potentially supportive of
mixed precip for at least portions of the area. This will depend
on low pressure track and how far north warm surge makes it.
Cold northerly flow supplied by Arctic surface high over the
northern Plains, as well as deep snow pack locking in the cold
increase the concern. With it being 5-6 days out, however, have
maintained snow as precipitation type in the Friday-Saturday
period. There is enough confidence in precipitation occurring for
likely PoPs Friday night. Will need to watch this period closely
as it draws closer.

Castro

&&

.AVIATION...
For the 06Z TAFs...

Confidence in trends have increased some since the 00z tafs as a
lull in the snow has developed late this evening. Guidance shows
light to moderate snow developing back across the terminals in the
early morning hours with a period of moderate to perhaps briefly
heavy snow centered just before sunrise. Then trends continue to
suggest another lull in the precip mid morning Sunday. During
these lulls...light snow will still be falling but vis/cig will
improve to at least ifr and perhaps mvfr. Low pressure will then
move across the area Sunday afternoon and this period now looks to
be the time when the heaviest snow and lowest vis/cigs will occur.
Timing will need some adjustments/tweaks but appears snow will
once again develop in the late morning and continue through the
early evening with a period of heavy snow possible during the
afternoon. Tempo timing is centered on most likely timing for
heavy snow but its possible that once the snow begins...17z-19z...
it may quickly lower to 1/2sm or 1/4sm. Confidence only medium
from this distance so additional changes are possible with later
forecasts. As the surface low departs Sunday evening...should be
a sharp cutoff to the back edge of the snow Sunday evening.

South/southeast winds 10-15kts will continue through late Sunday
morning and turn more southerly for a time before turning more to
the south/southwest Sunday afternoon...then shifting westerly
Sunday evening. Gusts to 20 kts possible Sunday morning. cms

&&

.MARINE...
330 PM CST

An active pattern is in store on the lake through the upcoming
week. The pressure gradient between a weak surface low approaching
the lake from the west and high pressure departing to the east
will support south-southeast winds increasing tonight and topping
out at 30 kt on Sunday. The low will track northeast Sunday night,
with trailing cold front sweeping across the lake and resulting in
winds shifting to west-northwest up to 30 kt, though likely
remaining sub-gale. Westerly winds will diminish on Monday and
briefly shift southwest ahead of another cold front Monday night,
bringing another period of west-northwest winds to 30 kt. A brief
period of gales can`t be ruled out in this period given the very
cold air mass and deeply mixed unstable conditions.

Yet another Arctic front will cross the lake Tuesday evening, and
this frontal passage could set the stage for a fairly extended
period of westerly gales through early Thursday. An even colder
air mass will support very unstable conditions and deep mixing,
along with the pressure gradient remaining tight as high pressure
spreads eastward from the northern Plains. Despite the lake waters
still being mild for mid December, will need to watch for freezing
spray potential Wednesday and Thursday. Finally, to close out next
week, a stronger storm system could take shape and affect the
region as expansive high pressure spreads southeast across the
Plains.

Regarding headlines, have issued a Small Craft Advisory from early
Sunday morning into Monday, for the strong southerly winds
shifting to westerly. Hazardous conditions for small craft will
subside by mid to late Monday morning for the Illinois shore as
west winds diminish, and by mid to late afternoon for the Indiana
shore after winds and waves diminish.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Storm Warning...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022
     UNTIL MIDNIGHT Monday.

     Winter Weather Advisory...ILZ023 UNTIL MIDNIGHT Monday.

IN...Winter Storm Warning...INZ001-INZ002 UNTIL MIDNIGHT Monday.

     Winter Weather Advisory...INZ010-INZ011 UNTIL MIDNIGHT Monday.

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...4 AM Sunday TO 9
     AM Monday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...4 AM Sunday TO 3
     PM Monday.

&&

$$

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