Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 170944

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
344 AM CST Sat Nov 17 2018

344 AM CST

Through Today...

Main concern for today will be snow trends associated with a weak
clipper tracking across the region today.

Latest radar imagery indicates that the heaviest and most widespread
snow associated with the clipper is over ern IA/nwrn IL, as of
330am, with an area of more broken light snow extending across nrn
IL and into nwrn IN.  As the main vort max and mid-level forcing
tracks ewd, expect that snowfall rates should increase, with the
main period of accumulating snow expected to be from arnd the
current time through arnd noon for the Rockford area and arnd 5am-
1pm for the chicago metro area.  Have not made any significant
changes to the going forecast, with the greatest accumulations still
expected to be along and north of the I-80 corridor.  There is still
some concern for locally higher amounts due to localized banding,
even at this point, confidence in where any enhanced banding may set
up is still low and radar trends will need to be closely monitored
as this type of banding is difficult to pin down.  Still expecting 1
to 2 inches of widespread snowfall north of the I-80 corridor.
Should any mesoscale banding develop, localized areas of 2 to 4
inches may be possible.  There is a chance that pcpn could mix with
or change over to rain for far nern IL/nwrn IN by early afternoon
ans the pcpn begins to wind down as temperatures climb into the
middle 30s.  Temperatures across ncntrl/nwrn IL will likely remain
low enough to support all snow through the duration of the pcpn.
Also, the models continue to indicate that an elevated frontal zone
will develop this afternoon and then become stationary across the
southern portions of the CWA later this afternoon and into tonight.
This could bring some light snow accumulation to areas south of the
I-80 corridor.  The elevated forcing is expected to be relatively
weak and sfc forcing should be minimal, so any accumulation
over the southern portions of the CWA should be light.


253 AM CST

Tonight through Friday...

For this period the items of interest are snow returning tonight,
mainly for south of I-80, with light accumulation probable through
early Sunday morning. Temperatures continue below normal for the
first half of the holiday week, reinforced with a cold front
Monday evening possibly bringing snow showers. The signal for
moderation late in the week continues, increasing confidence of
near normal temperatures for Thanksgiving and the following day.

An active upper and mid level jet flow will remain over the area
tonight, along with an associated 600-850mb thermal gradient,
especially across the southern half of the CWA. Guidance is in
fairly good agreement with a low amplitude, sheared wave
traversing slowly along this gradient late this evening through
early Sunday morning. In response, the frontogenetic (f-gen)
circulation does increase again, though not as strong as this
morning. Nonetheless the response of lift with saturated EPV (less
stability) will likely lead to light snow becoming more
widespread. While guidance has a lot of agreement in this
portrayal, there is modest spread in how far north the stronger
west- southwest to east-northeast f-gen axis orients. Some models
keep it across the far southern CWA as opposed to a couple
outlying northern solutions having it over I-80, which would
probably mean another dusting across the southern Chicago metro.
While have leaned middle to south solutions still feel that some
brief light snow could reach just north of I-80. As the main wave
shifts east Sunday morning, chances over the southern CWA will
diminish from west to east.

This will be a high likelihood of precipitation but low QPF
event. Temperatures in the boundary layer will remain around
freezing due to the clouds, which combined with the thermal
profiles aloft should provide ratios a bit lower than climo. All
in all tonight`s event looks like around a one inch event in the
maximum area, but if any stronger f-gen were to be more
persistent, some isolated two inch totals would be possible in
east central Illinois to northwest Indiana.

Otherwise for tonight, there is a consistent signal on high-
resolution guidance of a convergent axis across the southern part
of the lake that could serve as a focus for some light showers
into northwest Indiana and possibly Cook County/Chicago late this
evening into overnight. Low inversion heights, due to a limited
degree of cold advection, look to keep anything light and likely
below the ice nucleation zone. So if these showers materialize it
would be light rain showers with surface temperatures probably a
couple degrees above freezing.

It should take some time on Sunday pushing clouds eastward, which
lends some lower confidence on highs. If clearing does occur for
Sunday night, temperatures could certainly dip into the teens if
it remains clear. Some guidance brings in high level moisture
later in the night. This is associated with an upper short wave
that will drop over the Great Lakes Monday afternoon into the
evening, with another wave diving southeastward over the Missouri
River Valley. If these are shifted a little bit either way for
better focused forcing over the CWA, could realize a period of
snow showers later Monday.

After cool conditions Tuesday, moderation then looks to begin,
likely slowly on Wednesday but then more so into the holiday.
Right now guidance is in good agreement with a dry Thanksgiving.
Southerly winds should be on the increase too given the forecast
pressure fields of a lee-side low and a strong high over the
Northeast. The forecast near due south wind direction will
probably mean low clouds at some point, followed by rain chances
when the next wave approaches, at this time and likely to change
is later Friday (slight slowing trend the past 24 hours in



For the 06Z TAFs...

Primary forecast concern remains snow timing/chances and
associated vis/cigs. significant changes to the forecast. A band of very
light snow/flurries has formed over northern IL late this evening
and this may fill in a bit more over the next few hours. This
band is expected to be rather narrow but there remains uncertainty
for how long it may persist. A larger area of light snow is
expected to spread across northwest IL overnight and reach the
Chicago area terminals toward daybreak and some timing tweaks may
still be needed with later forecasts as the most recent guidance
has slowed the arrival of the snow just a bit. End time may also
need some adjustments as light snow may linger into the early
afternoon as it slowly moves south. Prevailing visibilities are
likely to be in the 1-2sm range with lower vis possible...mainly
across northwest IL.

There may be some lake effect snow showers Saturday night but
confidence remains too low to include in the forecast at this
time. The most favored areas would likely be near/southeast of mdw
into northwest IN and at gyy.

Winds are expected to become variable over the next few hours and
then turn northerly by morning though confidence is low on
specific wind directions. Eventually winds should become north/
northeast by afternoon with speeds increasing toward 10kts.

Mvfr cigs are slowly scattering out from west to east across the
area late this evening. This clearing is expected to move across
the rest of the terminals by early morning but mvfr cigs will fill
back in as the snow arrives toward daybreak. Ifr cigs are also
possible with the snow. Mvfr cigs will be slow to lift Saturday
afternoon and may not lift to low vfr until Saturday evening. Mvfr
cigs will still be possible near Lake Michigan Saturday night. cms





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