Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 161727
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1127 AM CST Fri Feb 16 2018

.SHORT TERM...
259 AM CST

Today through Saturday Night...

Rain has cleared the southeastern portion of the forecast area
early this morning, in association with the cold front which was
pushing away to the southeast across the Ohio Valley. Clouds and
perhaps a few flurries will linger early this morning however, as
a mid-level short wave trough propagates across the region. This
wave should clear the area by midday however, with clearing skies
expected this afternoon as subsidence develops in the wake of the
short wave. Strengthening subsidence will allow surface high
pressure to spread southeast from the central and northern Plains,
reaching the mid-Mississippi Valley by evening. Afternoon
sunshine will likely offset low level cold advection, on modestly
blustery north-northwest winds, though readings will be closer to
average or a little below.

With the surface high moving across the region this evening,
clear skies and light winds will likely allow for a quick drop-off
of temps this evening. Single digit lows are likely across much
of north central Illinois, with teens expected farther south/east.
High clouds will then likely develop and thicken after midnight,
in response to another quick-moving short wave trough digging
southeast from the Canadian Rockies. Increasing cloud cover, and
winds becoming light south as the high moves off to our east will
allow temps to become steady or slowly rise a bit prior to
morning.

Precipitation chances increase Saturday, as the digging short
wave approaches the area. South-southwesterly low level flow will
induce isentropic ascent, though a rather dry low level air mass
will likely inhibit any precipitation until the afternoon. Even at
that, model time-height depictions and soundings indicate a
fairly short window for light precipitation during the late
afternoon and early evening hours. Precip type is somewhat
questionable due to diurnal timing, with mid-upper 30`s surface
temps across the southern half of the cwa suggesting primarily
rain or rain/snow, while low-mid 30`s north would be more
favorable for snow. QPF amounts are generally less than 0.10" in
any case, with modest snow to liquid ratios likely limiting any
snow accumulation to less than an inch and largely north of I-80
across northern IL. Cold front quickly moves through around
midnight or so, with decreasing clouds during the night.

Ratzer

&&

.LONG TERM...
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.

Ratzer

&&

.AVIATION...
For the 18Z TAFs...

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

Confidence:
- 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.

KMD

&&

.MARINE...
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.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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