Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 032001

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


1045 AM CST

No significant changes to the forecast for today. Aside from a
temporary hole in the overcast over parts of Lake and Porter
County Indiana and possibly some small breaks in north central
Illinois this afternoon, the cloudy skies will continue.
Temperatures will not rise much from their current levels, topping
out in the mid to upper 30s. Looking ahead to the snow producing
system on Sunday, based off 12z model data, forecast appears to be
in good shape, with snow starting in the pre-dawn hours in north
central and northwest Illinois and then spreading east through the
morning. Not yet considering any headlines for this event.



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.




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.



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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