Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 032133

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
333 PM CST Sat Dec 3 2016

300 PM CST

Through Sunday...

Main concern is the first accumulating snow event of the season on
Sunday. In collaboration with NWS Quad Cities and Milwaukee, will
be issuing a Winter Weather Advisory for the I-80 and north
counties of Illinois. While heavily traveled roads, particularly
in the urban and suburban areas such as in downtown Chicago, may
end up with only slushy wet accumulations at most, confidence has
increased in snow rates and amounts being high enough to cause
issues on less traveled roads. Also took into consideration the
extra heads up for the first event of the season.

Snow has already broken out in western Iowa in response to
WAA/isentropic ascent and large scale lift associated with stout
northern stream shortwave. Latest observations already show
visibility under 1 mile in spots. The snow will continue to
progress eastward tonight and lingering lower level dry air over
the area should be overcome by the forcing and WAA pattern to
allow snow to break out in the pre-dawn hours in north central
Illinois. Given the magnitude of the forcing, the snow should
uptick in intensity fairly quickly after onset. With additional
energy rounding the base of the trough during Sunday morning,
there may even be an uptick in reflectivity in the precipitation
shield. Models appear to be keying on the stronger wave and
associated WAA/isentropic lift, as well as a seasonably moist air
mass with PWAT values approaching 0.6". As a result, the
pronounced uptick in guidance QPF continued with the 12z suite.

As the snow continues to overspread the rest of the area,
conditions will deteriorate fairly rapidly by the mid to late
morning. Have increasing confidence that there will be occasional
moderate (~1/2"+ per hour) snowfall rates enough to bring
accumulations on some surfaces and visibility down to ~1/2 mile
at times. The million dollar question is whether rates will be
high enough for snow to accumulate even on heavily traveled roads,
such as in Chicago and in nearby suburbs. Based on expectation
that was reported to us of runway temperatures remaining above
freezing at Chicago O`Hare, suspect that this will also be the
case on main roads. Furthermore, temperatures will be hovering in
the low-mid 30s while the snow will be falling. Therefore, the
message for the most heavily populated areas is that the impacts
are expected to be mitigated by the marginal road/ground and air
temperatures, with potentially only slushy accumulations on roads.

With all this being said, there has been the decided upward trend
in QPF in the guidance for reasons mentioned earlier, so do have
some concern that the official snow forecast could even be
underdone in some spots. The forecast features amounts of 2 to 4
inches in the advisory area, highest and closest to 4" near the
I-39 corridor, with amounts tapering southeast of the advisory to
2" or less. This is due to more marginal air temperatures and
potential for rain to mix in during the afternoon. The snow will
likely be heavy and wet, with collaborated snow ratios of
generally 10:1 or less, something to consider for shoveling.
Expecting the precipitation to taper off from west to east by the
mid to late afternoon.

Taking all the above considerations into account, opted to issue a
Winter Weather Advisory for the along and north of I-80 counties
in Illinois. While impacts may very well be marginal in some
areas, expect there to be high enough rates for travel issues from
low visibility alone, plus now a better chance for some road
accumulations, particularly in more rural areas. The Advisory is
in effect from 8am-6pm on Sunday, though it`s possible some tweaks
may be needed in these start and times with subsequent forecast




Sunday night through Saturday...

201 pm...Multiple forecast concerns and challenges continue for
early and middle next week with fairly low confidence.

Departing system Sunday night will leave some lingering flurries
or drizzle Sunday evening. Surface temperatures during this time
period likely to be at or above freezing but low temps will likely
dip below freezing by Monday morning so will need to monitor to
see if any lingering drizzle becomes freezing drizzle but no
mention with this forecast. High pressure will move across the
southern Great Lakes region Monday but low levels remain fairly
moist so its possible low clouds may hang on into Monday.

Low pressure is then expected to lift northeast across the Ohio
Valley Monday night. This system shifted a bit further to the
southeast in the past day and that general track continues as
another area of low pressure lifts north across the upper midwest.
A trailing cold front from this low then moves across the area on
Tuesday and this makes the wind direction forecast uncertain.
Thermally...the low levels appear warm enough that any precip that
does fall would likely be mainly rain...possibly mixed with snow
especially across the northwest cwa. Guidance also fairly
consistent with surface temps mainly steady in the mid 30s Monday
evening...dipping toward freezing by Tuesday morning across the
northwest. Confidence regarding surface temps is also low...
especially given the possibility of some snow cover across the
area from Sunday/s event...which could possibly allow temps to
drop cooler than guidance is showing. Exact position of the precip
shield is also uncertain as the 12Z nam is much further to the
northwest than the rest of the guidance. If precip does
materialize and current thermal profiles continue but surface
temps are a bit cooler than currently forecast...then some light
freezing rain may develop. Small changes in the track of this
system could lead to large changes to this forecast so trends
will need to be monitored.

The cold front noted above on Tuesday will begin to usher in
cooler air across the region...that continues through the end of
the week. But there remains uncertainty regarding any possible low
pressure development as well as its location and track. Current
operational runs do not support any significant surface low
development. But as the coldest air arrives Wednesday night into
Thursday morning...there remains a signal for light snow to
develop...possibly south of the area. Maintained low change pops
during this time for snow showers but confidence is low. High
temperatures for the end of the week likely only in the 20s
combined with at least breezy...perhaps windy conditions. Based
on current wind direction forecasts any significant lake effect
snow looks to remain east of the Porter County...but maintained
low chance pops for northeast Porter County for now. cms


For the 18Z TAFs...

Main concern is the snow on Sunday and associated impacts to CIGs,
VSBY and potentially runway conditions. MVFR deck looks to be
scoured out for a time this afternoon into this evening. Area of
snow associated with a strong upper level disturbance is already
starting to blossom over western Iowa. The snow will spread east
through the night and low level dry air should be quickly overcome
to result in a fairly rapid deterioration in conditions Sunday

Have the highest confidence in a more prolonged period of
moderate SN at RFD, where a tempo was introduced in the mid
morning. Farther east, moderate snow is certainly possible, but
with a bit lower confidence, included temporary 3/4 mile
visibility. CIGs should easily drop to low end IFR and
potentially LIFR, especially during the higher snow rates.
Confidence in snow accumulations on colder surfaces is high, with
2+ inches favored. However, confidence in impactful accumulations
on runways is lower due to the potential for runway surface
temperatures to remain above freezing through the event, along
with air temperatures at or slightly above freezing. The snow will
taper off during the mid to late afternoon. Expecting south winds
of generally less than 10 kt during the snow, with direction
ranging from 150-180 degrees.




333 pm...High pressure will move across the Great Lakes region
tonight as a trough of low pressure moves across the western lakes
Sunday afternoon into Sunday night. Another high will move across
the southern Lakes region Sunday night through Monday morning. The
gradient will tighten Sunday afternoon into Sunday evening and a
period of 15-25kts is likely.

The pressure pattern/gradient becomes somewhat complex and a bit
uncertain Tuesday into Wednesday as one low pressure moves from
the upper midwest Monday night into Ontario Tuesday. A trailing
cold front then moves across the western lakes Tuesday as a second
low moves northeast across the Ohio Valley Tuesday into Tuesday
night. As colder air spreads into the region...westerly winds will
likely increase into the 15-25kt range and possibly to 30kts.
Strong high pressure then builds across the northern/central
plains Thursday into Friday. The gradient tightens over the lakes
and still colder air spreads into the a period of
gale force winds is likely Thursday into Thursday night. While
there is consensus among the models with this solution...there
remains uncertainty regarding this forecast. cms


IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     ILZ022...7 AM Sunday TO 6 PM Sunday.




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