Area Forecast Discussion
Issued by NWS Chicago, IL

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670
FXUS63 KLOT 040905
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
305 AM CST Sun Dec 4 2016

.UPDATE...900 PM CST

The going forecast for Sunday`s first accumulating snow of the
season remains on track, with no significant changes this evening
and none made to the Winter Weather Advisory. A Special Weather
Statement was issued for the southern CWA for just increased
awareness of the season`s first minor snow accumulation in that
area.

A pronounced channel of return moisture is evident on evening
water vapor imagery and upper air analysis, with high clouds
streaming back over the area. Lower clouds had eroded in many
places allowing temperatures to cool with readings 30 to 33 across
the area and am expecting those to just hold steady through
daybreak. Radar indicates quite a bit of precipitation ahead of
the now positively-tilted trough from Minnesota back into the
central Plains, with virga expanding into northern Illinois. The
00Z DVN sounding indicated a not-so-overly-dry layer between 700
and 850mb that should gradually be overcome overnight and more
quickly early Sunday morning as the wave becomes neutrally tilted
and forcing increases. The going start time in the forecast from
west-to-east looked good and just added a little more temporal
resolution. The Advisory start time will not match the snow start
time exactly (likely a little late for Rockford and a little early
for Chicago), but there was no reason to get overly fancy and
possibly add layers of confusion with the start/end times.

While this is a progressive and low amplitude wave, the going
factors for snow of two plus inches and moderate rates at times
over the northern half of the CWA still are on track...that is
1.) a well placed upper jet and associated lift for moderate
rates, 2.) deep moisture (3-3.5 g/kg on key isentropic surfaces)
for about nine hours, 3.) modest replenishment to that moisture
(20-25 kt 850-925mb) during that time, 4.) saturation to at times
supersaturation with respect to ice through the -12C to -18C
layer, and 5.) profiles somewhat favorable for good moist
aggregation of flakes below that. Thermal profiles look to keep
the snow wet and the low levels remain marginal south of the
advisory and likely in the heart of Chicago as well. As noted
below, paved surfaces should initially struggle to accumulate
until rates pick up, which should occur by mid morning in north
central Illinois and toward noon in the Chicago metro. Even with
that, some treated roads just may not see any accumulation at all,
and that is always a tough thing to gauge with the first event of
the season. Overall though, the forecast message is in good shape
with observational trends and incoming 00Z NAM QPF, and have thus
made no changes to forecast snowfall amounts.

MTF

&&

.SHORT TERM...
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 7am-6pm on Sunday, though it`s possible some
tweaks may be needed in these start and times with subsequent
forecast updates.

Castro

&&

.LONG TERM...

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

&&

.AVIATION...
6z TAFS...9z TAF AMDs...

Snow is set to impact the terminals today with a period of
moderate to briefly heavy snow today. Model agreement is still
fairly good at this point in terms of timing of snow onset and
peak snowfall such that the trends in the TAF are overall fairly
good, with minor tweaks in timing plausible. Expect light snow and
MVFR vsbys at onset to quickly transition to IFR vsbys. LIFR vsbys and
even periods of LIFR cigs are expected in the heavier snow. Model
time height forecasts depict a narrow snow growth region but
strong omega in that region ahead of a potent shortwave. As this
wave approaches, later this morning and early afternoon, this will
combine with impressive isentropic lift to bring a period of
moderate to heavy snowfall rates. Feel that 1SM vsby is an
accurate representation given that we do not have a deeper snow
growth region, but there is enough of signal to support occasional
3/4sm vsby in the prime period of 18z-23z this afternoon. Tried
to tighten up the timing of vsby reduction and cig changes
corresponding with the latest high res expect snow onset
times...and TAF AMDs are forthcoming.

Snow will generally be of a wet nature with ratios of 10-12:1. It
is always challenging to tell the impacts of the snow on warm
runways (per metro observations and reports from ORD/MDW)this
early in the season with melting snow certainly expected, but
with 2-3" inches forecast in many locations including the urban
metro and higher amounts west, expect that there will be some
slushy accums on pavements as well in the heavier snowfall
periods. Winds will be out of the southeast and generally on the
lighter side, transitioning to southwest as the snow tapers from
west to east this evening.

KMD

&&

.MARINE...
301 AM CST

Somewhat benign conditions will be found across the lake to start
the week, with gusty west-northwest taking hold for the latter
half.

High pressure across the Great Lakes and Ohio Valley will move
east today. Meanwhile, a surface pressure trough across the Upper
Midwest will spread east across northern Lake Michigan tonight
into Monday. Another low pressure system will move into the upper
Midwest on Monday as weak high pressure spreads northeast through
the Ohio Valley. Quickly on the heels of the high, another low
will move in a similar fashion from the southern plains to the
Ohio Valley Tuesday. The result of these pressure patterns will be
a shift to west winds tonight and then a return to southwest
Monday and a portion of Tuesday.

Things get more active mid to late week as strong high pressure
will move into the northern plains Wednesday and into the nation`s
heartland by early Friday, while low pressure across Ontario will
shift east through Canada and merge with low pressure off the
Canadian maritimes by the weekend. These features allow a strong
cold front to shift across the lake late Tuesday and continue
under gusty wnw winds through at least Friday. Winds to 30 kt
seem a lock, and given the strong cold advection mixing should not
be a problem, but model winds above the surface are not much
higher. Will continue with a mention of gales in the GLF, with the
most likely period late Thursday into early Friday.

KMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     ILZ010-ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-
     ILZ022...7 AM Sunday TO 6 PM Sunday.

IN...NONE.
LM...NONE.
&&

$$

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