Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 030939

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
339 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.




Sunday night through Friday...

143 pm...Forecast concerns include precip timing Monday night/
Tuesday and again Wednesday/Wednesday night...along with precip
type. Much colder air...and below normal...expected Thursday and
Friday next week.

High pressure will be moving across the Ohio Valley Sunday night
into Monday. Any lingering precip Sunday evening will likely be
just flurries/drizzle and at that time surface temps appear to
remain above freezing.

The models have come into better agreement with each other for the
12z runs but there remains uncertainty with one surface low
moving northeast across the plains Monday night into Tuesday and a
second low lifting across the Ohio Valley/Mid Atlantic Monday
night through Tuesday night...and whether these systems eventually
merge into a larger system as previous runs have suggested. Gfs
has been consistent with the current trend for the past day or so
with the Gem/Ecmwf now trending that direction. As a result...low
temps Monday night are expected to be slightly cooler...perhaps
lower 30s but precip may only reach into the far southern cwa
through Tuesday morning. Thus there appears to be more potential
for mixed precip or at least a rain/snow mix but now affecting
much less of the cwa. Temps should warm enough Tuesday to change
any precip over to mainly light rain but location/timing is
uncertain enough that much of the area could end up being dry and
confidence for dry conditions does look better for Tuesday night
into Wednesday morning.

Colder air now spreads into the area Wednesday as a weak wave
quickly lifts northeast along the transition to much colder air.
Ecmwf develops a weak low and both the ecmwf and gem show light
qpf into Wednesday evening. Given current trends...looks cold
enough then for any precip to be mainly snow but confidence in
this solution is low. As colder air spreads across the region
Wednesday night through the end of the week...highs may stay in
the 20s with lows in the teens. Add strong/gusty westerly
winds...min wind chills Thursday and Friday mornings may fall to
around zero. cms


For the 06Z TAFs...

Limited impacts in the TAFs through Saturday night with just some
MVFR overnight and likely into early-mid Saturday morning.

Cyclonic flow continues across the region and will into early
Saturday. Cloud depths remain around 1,500-2,000 ft thick on
recent aircraft soundings in the region and almost all sites are
reporting one overcast layer, often a good sign it is solid and
not going anywhere without any notable advection. Any cloud bases
below 2,000 ft are a low likelihood and probably only possible at
RFD near daybreak. There could be a few holes in the cloud cover
late Saturday into Saturday evening, but that is low confidence.

Beyond the current TAF period...there is likely to be snow during
the day Sunday at the airports with IFR visibility at times.
Temperatures look to be 33 to 36 during the snow. Around an inch
is forecast at ORD, MDW, and GYY, with one to two at DPA and RFD,
though accumulation on pavement probably will be difficult.



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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