Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 040925
AFDLOT

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

.SHORT TERM...
324 AM CST

Through tonight...

Main forecast concerns/challenges are with the expected
accumulating snow over much of northern Illinois and northwest
Indiana this morning into the late afternoon. Main forecast
changes this morning were to increase snowfall amounts and expand
the advisory southeast to now include Livingston/Kankakee counties
in Illinois and Lake/Porter counties in northwest Indiana.

Everything still appears to be on track for the first
accumulating snowfall today. Upper level trough noted on satellite
imagery, with several embedded impulses quickly lifting northeast
into the mid Mississippi Valley. Ahead of this feature, isentropic
ascent is also helping to saturate the column. Although returns
noted on radar, this saturation has not occurred quite yet, with
snow not occurring at this time. Adjusted pops in the near term to
reflect latest trends with upper level support providing the onset
of snow across the far western CWA right around 12Z. This would
include all areas in the west, along the I-39 corridor. Remaining
areas to the east in northern Illinois still on track for right
around a 15z time frame, and then soon there after for areas in
northwest Indiana. Given this noted time line, have adjusted
timing of the advisory, mainly to delay the eastern half until
15z.

As this upper level trough swings through the area later this
morning into the afternoon, expect a transition from light snow to
more moderate intensity snow as large scale ascent really ramps up
ahead of this feature. Most guidance in agreement with the
increase in forcing due to many features coming together, which
include a stout negatively tilted trough and more potential for
instability aloft. Would anticipate many locations across much of
northern Illinois and northwest Indiana to see this ramp up in
snow intensity by midday, and even more so by early to mid
afternoon. This not only includes areas along the I-39 corridor
which was previously noted, but now also includes many areas in
the northern two thirds of the CWA likely centered from just north
of I-88 south to along/south of I-80. In these locations, strong
forcing and more efficient snow production will likely allow for
an increase in snow amounts even into the afternoon when the more
intense snow will be able to offset melting. In the height of the
more intense snow, 18-23Z, am a little concerned that the snow
will come down hard enough to accumulate more on the roadways with
at least slushy conditions possible. All of this and the increase
in snow amounts, feel the advisory is still warranted and worthy
of expansion. Now have snow amounts of 3-5 inches along the I-39
corridor, 2-4 inches elsewhere in the advisory in northern
Illinois and northwest Indiana, and then 1-3 inches across the far
southeast CWA. In this location, rain may briefly mix with snow
but do think forcing will be strong enough even in this location
to help the column/boundary layer to cool to support all snow in
the afternoon. Snow will quickly taper off right around the 00z
time and although some light snow may still be ongoing across the
eastern CWA in the 00-03z period, it will likely be light enough
to not warrant extension of the advisory.

Rodriguez

&&

.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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