Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 161907

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
107 PM CST Fri Feb 16 2018

106 PM CST

Through Saturday...

Quiet and cold conditions will be theme for tonight, followed by
at least some scattered snow showers Saturday afternoon.

High pressure is centered across the central plains this
afternoon, and this has maintained a cool but dry northwest flow
area wide. The high will shift across central Illinois tonight.
While winds do not appear to completely subside, the high will
be close enough to allow for decent decoupling. With initial
clear skies and warm advection not kicking in immediately, expect
a decent drop off in temperatures this evening, with teens
spreading in fairly quickly west to east through the evening.

Our next weather maker is out over the northern rockies/plains. We
get into modest warm advection ahead of this system. The main
moisture transport remains focused well south of the region, but
the northern stream shortwave seems potent enough to produce some
showers Saturday afternoon. Wet bulb thermal profiles suggest the
main precipitation type will be snow, with a mix possible farther
south. It is a pattern that suggests some areas don`t get much,
but some areas see a burst of moderate snow showers, given the
presence of fairly steep lapse rates aloft ahead of the incoming
shortwave. Surface temperatures will be initially marginal for
accumulation, and therefore even the higher end QPF on the EC
(around 0.05") would struggle to produce much more than a few
tenths. Grassy surfaces appear favored given the marginal surface
temperatures, but pavement temps may not warm a ton given the
colder night tonight and clouds arriving tomorrow, so some road
impacts are possible.



259 AM CST

Sunday through Thursday...

Broad, flat upper ridging develops Sunday behind late Saturday`s
quick-moving short wave. Weak surface high pressure is across the
area early in the day, before winds turn southerly in response to
surface pressure falls across the Plains and upper Midwest in
response to an amplifying long-wave trough across the western
CONUS. A series of minor amplitude waves are progged to ripple
northeast out of the base of the western trough, and across the
upper Midwest Sunday night through Tuesday, with guidance in good
agreement in maintaining a slow-moving frontal zone just northwest
of the forecast area during the period. This keeps the LOT cwa in
a region of warm, moist advection Sunday night into Tuesday, with
ECMWF and GFS both depicting 50+ degree temperatures across much
of the forecast area. Rain is depicted developing Sunday night,
with some low probability potential for some light freezing rain
along the IL/WI border as temps will be close to freezing mark
initially there. Rain potential looks fairly high Monday, Monday
night and Tuesday as strong southerly low level flow pulls Gulf
moisture northward. Have maintained a mention of thunder Monday
and Monday night, when modestly steep mid-level lapse rates
coincide with the strongest low level theta-E advection and
moisture transport. The cold front is progged to slowly move
southeast across the forecast area late Tuesday into Wednesday,
with lingering precipitation likely mixing/changing to snow from
the northwest.

Upper pattern remains westerly through the end of next week, with
no strong surge of cold air noted behind the early/mid week
system. Models give some hint of another quick-moving wave which
may affect the area with some rain/snow Friday. Otherwise, temps
look to remain around or above average for late February.



For the 18Z TAFs...

-High End MVFR dissipating
-Snow showers timing and coverage Saturday afternoon
-MVFR/IFR Visibility occurrence in snow showers

- High on MVFR clearing times
- Medium on exact timing for snow showers. High on there being
  snow showers around during the 18z-0z time frame Saturday.
- Medium on Visibility impacts. High on MVFR where showers occur.
  Low-Medium on IFR lasting any significant period of time.

The remnants of MVFR clouds will shift southeast this afternoon,
lingering longest along and east of I-57, though expect VFR at
most sites by mid afternoon. NW winds will shift to SW and weaken
this afternoon into tonight. High pressure will move east of our
area toward daybreak. A band of snow showers will develop across
the area Saturday afternoon from west to east. While not all areas
may have snow, this is the setup that can produce brief though
modest snow showers. Surface temperatures will be borderline, but
a quick dusting will be possible in the stronger showers and
MVFR/brief IFR. Winds will remain SW in the showers, which could
be mixed with rain mainly south of the terminals.



325 AM CST

Strong high pressure is building into the northern Plains early
this morning with north winds to around 25 kt in place over Lake
Michigan. Winds will ease to 10 to 20 kt this afternoon and
evening as the high center builds towards the region, passing
just south of Lake Michigan tonight. Winds back to southwest
late this evening and tonight behind the ridge axis and will
increase to 25 to 30 kt ahead of an approaching cold front. Winds
may briefly tag gale force across northern Lake Michigan Saturday
morning. Winds will drop off late Saturday night into early
Sunday morning for a few hours as another ridge axis quickly moves
across the region, then moderately strong south winds gusting to
around 30 kt will return by midday Sunday as another low takes
shape over the Great Plains.






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