FXUS63 KLOT 172325

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

300 PM CST

Through Sunday...

As the stubborn f-gen driven banded snow and rain north of I-80
finally winds down, the next round is expanding into areas along
and south of Interstate 80 with the approach of another low
amplitude wave. Based off radar extrapolation, the focus for
tonight's banded precip will be primarily along and south of I-80.
Fairly strong lower and mid level frontogenesis is already
yielding some pockets of near 30 dBZ radar echoes in far western
IL. With models indicating the stout fgen centering at around 700
mb continuing much of the night, expect steady precip rates mostly
light to moderate but at times can't rule out brief heavier

The main limiting factor for snow accumulations tonight
will be the temps and wet bulb temps starting solidly above
freezing. Higher precip intensity should help cool temps along
with colder air gradually filtering in from the north. However,
with no strong cold advection, expecting temperatures to only
slowly fall to around freezing through the night. Snow amounts of
up to an inch or so are possible, likely mostly on
grassy/colder/elevated surfaces. Can't rule out some isolated
amounts perhaps up to 2". Primary impact should be visibility
under 1 mile at times, with any roadway impacts less certain and
more likely confined to rural county roads and bridges/overpasses.
The precip will diminish and shift off to the east Sunday morning.

Only other minor issue is lake effect showers developing this
evening with convergent north-northeast flow down Lake Michigan.
The showers should start in far northwest Indiana and then
gradually shift west into northeast Illinois overnight. Marginal
therodynamics will result in inversion heights of 5000 feet or
less. In addition, a dry layer from about 850-700 mb casts doubt
on whether snow will even mix in with the spotty lake effect
precip or if it will mostly be sprinkles/drizzle. Temperatures
remaining above freezing near the lake will preclude any travel

Clouds will diminish from north to south on Sunday as high
pressure ridge axis overspreads the area. High temperatures will
top out in the mid to upper 30s most areas, with only low 30s in
parts of far northern IL well inland. Winds will be light due to
the approaching high pressure.



140 PM CST

Sunday night through Saturday...

Sunday night through Wednesday: Deep troughing over Hudson Bay will
only gradually shift eastward to northern Quebec during this time,
with multiple embedded shortwave troughs rotating across the Great
Lakes. The first shortwave and associated surface trough will cross
the CWA late Monday afternoon into Monday evening. Isolated to
scattered snow showers will be possible for a 2-3 hr window as the
trough passes.

NNW winds behind the front will support LES for NW Indiana Monday
night into Tuesday morning, with a window for LES backing into
eastern Cook County late Monday night into Tuesday morning. With
H8/SST delta temps of ~14C, only isolated to scattered showers are

A second shortwave rounding the broader trough will pass NE of the
area Tuesday night. Enough dry air should be present to prevent
precip across the NE CWA, but cannot rule out a few showers.

Thanksgiving Day through Saturday: Guidance has come into better
agreement that the eastern Canadian trough will move eastward as a
broad mid-level trough shifts from the Great Plains to the Great
Lakes region. Though light WAA precip should remain north of the
area on Thanksgiving, the CWA may end up in a persistent shield of
stratus through much of the day. Warmer conditions arrive on Friday,
with temps at or possibly slightly above normal for late November.
Another trough axis will track eastward across the area and bring a
period of rain Friday night into Saturday morning.



For the 00Z TAFs...

525 PM...Forecast concerns include the potential for lake effect
rain/snow showers at mdw/gyy and mvfr cigs through Sunday morning.

A band of rain/snow...possibly trending to all snow overnight will
continue into Sunday morning generally south of Interstate 80 and
the terminals. This precipitation may continue off and on at gyy
this evening. In addition...lake effect rain or snow showers are
expected to develop this evening...move across gyy and then
possibly move toward mdw overnight. Confidence for lake effect
precipitation is medium/high but low for timing/duration and
precip type. This activity should weaken and dissipate after

Some holes are developing in the mvfr cigs along the IL/WI state
line and these may continue tonight but ifr cigs or lower can be
expected south of the terminals with prevailing rain/snow. With
winds off the lake...expect mvfr cigs to continue for the Chicago
area terminals with some potential for ifr especially at mdw/gyy
overnight. Cigs should quickly scatter by mid/late Sunday morning
with increasing high clouds Sunday night.

North/northeast winds generally under 10kts will slowly turn
northerly tonight...northwest Sunday morning and then more
westerly Sunday afternoon. cms





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