Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 031138

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

337 AM CST

Through Sunday night...

Main forecast concerns/challenges are with the expected
accumulating snow over much of northern Illinois and northwest
Indiana late tonight into Sunday afternoon, as well as additional
snow chances later in the week.

In the near term, as was expected, cloud cover holding in place
over much of the region and don`t really see any reason why
anything would chance today into tonight. So have increased cloud
cover to reflect the continued cloudy skies. High pressure
overhead looks to remain situated today and then slowly work east
later this afternoon into this evening. This will limit any precip
chances today, with dry conditions expected into much of this

Accumulating snow still expected over much of the CWA later
tonight into Sunday with confidence high for most locations
observing snow. Upper level trough still on track to push through
the central CONUS later tonight into Sunday, with the mid level
flow anticipated to be rather energetic as it approaches the area.
Despite no real surface reflection to note, forcing appearing to
supportive of periods of light to moderate snow on Sunday. This is
due to WAA ahead of this trough, vigorous shortwave energy, as
well as some negative tilt to this trough. Guidance has come in
more aggressive with forcing and associated QPF and have trended
the forecast this way. Also, trends have continued to shift
towards more of an all snow event for most locations. This would
be for much of northern Illinois, however, still have some slight
concerns for a rain snow mix by midday for areas southeast of a
Piper City IL to Wheatfield IN line. So do have snow for all areas
in the CWA with the exception of the noted area in east central
Illinois and northwest Indiana, where a rain snow mix could occur
Sunday afternoon. However, will need to continue to monitor this
as its quite possible for these areas to still see snow in the
afternoon, as the stronger forcing helps to keep the column cold
enough for snow.

Have continued the trend of increasing pops with this system with
fairly strong agreement among model guidance. There is some
variability with the start of the snow ranging from late tonight,
to right around sunrise. This is likely due to guidance struggling
with the anticipated drier air in place. However, with the cloud
cover still expected to be in place tonight which guidance is not
quite capturing, think the column will saturate sooner rather than
the slower guidance is advertising. So trended the forecast
towards the quicker guidance and increased pops significantly late
tonight, in the 9-12z for areas along and west of I-39. Onset of
snowfall expected to be right just before sunrise over north
central Illinois, and then continue to spread east/northeast
through mid morning. As forcing quickly increases during this
time, and expected to increase in the snow growth zone, do think
that light to at times moderate snow will be possible over north
central Illinois. Confidence does lower slightly with regards to
the possible moderate snow over northeast Illinois and northwest
Indiana, but if trends continue, could see this occurring in those
locations as well. Continued forcing into the afternoon will
support continued light snow, likely diminishing to the northeast
around 21z.

Still have the highest snowfall totals over north central
Illinois and with the snow now expected to arrive earlier and with
slightly higher intensity, do think this snow will have strong
chance to accumulate. This is includes over pavement, with
pavement forecasts showing road temps likely right around or
slightly lower than freezing in that I39 corridor. So do have some
concerns for a period of snow covered roads at least through mid
morning in this location. So have increased snowfall totals to 1-3
inches with the possibility for isolated amounts around 4 inches
in the I39 corridor. Areas in northeast Illinois still expecting
1-2 inches, and then around 1 inch in east central Illinois and
northwest Indiana.



348 AM CST

Monday through Friday...

Still keeping an eye on two possible systems to affect the area
during the week. The first system still expected to lift northeast
out of the southern Plains Monday night into Tuesday. However,
guidance indicating a more easterly track with this wave and
surface reflection. Despite this easterly shift, areas south of
I80 still have a possibility to see some precip by early Tuesday.
Overall confidence is low with this system, so have limited pops.
Given low confidence with the extent of any colder air,
confidence is also low with precip type and have continued to
mention both snow and rain. Will need to monitor, as the
potentially strong ascent and high QPF field could still impact a
portion of the CWA. Guidance continues to struggle with the next
possible system by Wednesday, mainly with the strength of the
system. Cold air will likely be in place with all snow expected,
however, guidance would suggest varying amounts and strength to
the wind field. Have maintained chance wording over the CWA
though, as its appearing snow will occur. Much colder air and lake
effect snow potential will then be concerns through the remainder
of the forecast period.



For the 12Z TAFs...

Status quo appears to be the rule today with high end MVFR or low
end VFR stratus hanging strong through the daytime hours.
NAM/HRRR/RAP forecast soundings all keep the stratus locked below
an inversion. Expect the MVFR to trend toward VFR today, with model
guidance finally breaking up the stratus layer below the
inversion later today and tonight as mid clouds overspread the
area. High pressure will be in place most of the night making for
relatively quiet conditions with light winds and VFR cigs, but
significant changes will be in store on Sunday.

Expect a significant ramp up in precipitation to our west and
southwest after midnight as a potent yet compact low approaches
and warm advection kicks in for top down saturation to begin.
Expect snow to spread into RFD before 12z, initially lighter,
before more significant impacts after this current TAF cycle.
There will be a period of more moderate snowfall as the combined
forcing arrives, with the higher rates and more efficient snows
just west of the main Chicagoland TAF sites during the 12z-15z
time frame, but expect the 15z-22z time frame will be the period
of concern over the remainder of the area. Expect reduced
visibility to IFR/LIFR in the snow for a time with a quick
deterioration once deeper saturation and lift arrives. Precip may
eventually transition in the evening to a drizzle with the loss of
deeper saturation. More details on specifics in future TAF cycles.
Winds will be SSE initially transitioning to SSW during this

For the upcoming snow event:

Snow type: WET

Most likely start time: RFD 9-11z, ORD/MDW 12-14z.

Highest confidence of heavier snow: RFD 13-18z, ORD/MDW 17-22z.

Accums: RFD 1-3", ORD 1-2". Highest north central Illinois and
away from the urban core where surface/pavement temperatures are
more borderline and will support accums more so on grassy



337 AM CST

The pressure gradient over the lake will relax further today as
high pressure transits the region. A trough axis and modest cold
air surge will cross the lake late Sunday afternoon and evening
which will bump southwest winds to 15-25 kt and then shift winds
to westerly overnight. High pressure will pass across the Ohio
valley early Monday before merging with another high over Quebec
Monday night.

Model guidance is in pretty good agreement in lifting another low
from the Gulf of Mexico Monday and into the eastern Ohio Valley
later Tuesday with the better pressure gradient just grazing the
southern tip of the lake while a stronger low will lift from the
upper Midwest into Manitoba/eastern Ontario during this time.
Impacts from these systems appears low, but model agreement then
decays Wednesday with regards to the potential for another low to
move across the Ohio Valley later in the week and how close to the
lake the impacts of the winds will be from this low initially.
Fortunately the message is that a strong cold front will pass
across the lake resulting in near to possibly gale force wnw winds
for an extended period of time late in the week as strong high
pressure builds across the plains.






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