Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 101132
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
532 AM CST Sun Dec 10 2017

.SHORT TERM...
314 AM CST

Through tonight...

Low pressure over Minnesota will weaken as it moves across the
western Great Lakes today. A band of light snow rapidly weakened
as it reached north central IL. Some areas may see flurries early
this morning, but no accumulation or impact is expected.

Southwest set up warm air advection. High temps will be in the mid
30s. Winds shift briefly to northwest behind a weak cold front
associated with the weakening low.  A high pressure ridge then
spreads over the region tonight, and winds back to southwest.
Clouds increase ahead of the next clipper system which should
limit cooling. Low temperatures will be in the low 20s.

Snow ahead of the clipper may spread north of I-80 early Monday
morning. I think chances for snow are better along and north of I-
88. Models differ on the speed of clipper and how quickly the
atmosphere will saturate.  The GFS has the quickest and strongest
snow signal, while the NAM keeps snow to our northwest through 12Z
Monday.  I went with a middle ground approach with no accumulation
tonight.

JEE

&&

.LONG TERM...
334 AM CST

Monday through Saturday...

Main focus in the longer term portion of the forecast is on a
strong digging short wave progged to impact the region Monday into
Monday night, with a secondary push of arctic air bringing colder
temps and a lingering threat of lake effect snow for parts of
northwest Indiana through Tuesday night or early Wednesday.

Guidance is in pretty good agreement in depicting a strong
Alberta Clipper digging southeast from the Northern Plains by
early Monday morning, and propagating across the local area Monday
afternoon and evening. Warm advection/isentropic ascent develops
in response to the advancing wave early Monday morning, with top-
down saturation on the nose of the warm advection expected to
produce a period of light snow during the morning, largely focused
across WI and far northern IL along/north of the I-90 corridor.
The main mid-level vort and a surface low pressure reflection then
move into northern IL Monday afternoon, with stronger deep-layer
forcing for ascent and saturation, likely to be aided by
ageostrophic ascent associated with the tightening baroclinic zone
behind the surface cold front pushing through the region. A
little too far out to put much faith in model depictions of
mesoscale features, especially with respect to location, though
some of the high-res guidance does suggest some banding across the
area Monday afternoon. Have focused accumulations through the day
along/north of the I-90 corridor as mentioned previously, with a
couple inches looking possible. Depending on the ultimate track of
the vort, southern parts of the may be far enough south to see a
rain/snow mix along the warmer side of the tight baroclinic zone.

Surface low pressure begins to deepen just east of the forecast
area Monday night, as additional strong short wave energy digs
into the amplifying upper trough over the Great Lakes region.
Gusty northwest winds strengthen as strong low-level cold
advection takes hold, and the gradient tightens on the back side
of the deepening low. Scattered snow showers may persist across
the area, while parts of northwest Indiana, especially east of
Gary, see the potential for another round of accumulating lake
effect snow showers which will likely persist through Tuesday and
Tuesday night before low level flow weakens and backs more
westerly on Wednesday. Cold air floods into the area Monday night,
with temps on Tuesday only in the low-mid 20`s and lows by
Tuesday night perhaps only in the single digits to mid-teens.
Combined with the gusty northwest winds, wind chills in the single
digits are likely during the day, dipping near zero in spots
Tuesday night.

With the deep eastern NOAM upper trough lingering mid-week, the
potential exists for a few other minor amplitude disturbances to
bring some clouds and possibly light snow/snow showers at times
through Thursday/Thursday night, though by Friday the upper ridge
across the western CONUS breaks down and allows the upper flow to
flatten a bit, and a moderating trend to develop into the weekend.
Guidance remains in good agreement in developing southerly low
level flow across the region Friday night into Saturday, with
temperatures progged to reach 40 or better by that time.

Ratzer

&&

.AVIATION...
For the 12Z TAFs...

VFR conditions are expected today. A weakening low pressure system
will pass over the region, and winds will veer to northwest behind
the low`s weak cold front. Winds then back to southwest by early
Monday morning as a high pressure ridge quickly passes overhead.

Attention then turns to a clipper system and its associated snow
Monday. Guidance differs on when snow will reach the terminals,
and I`ve opted for an earlier onset for flurries/light snow.
Accumulating IFR snow is more likely around or after mid Monday
morning. MVFR cigs will likely accompany the snow. An average snow
type is forecast, and snowfall rate should be a quarter inch an
hour or less.

JEE

&&

.MARINE...
356 AM CST

Primary marine forecast concern is the likelihood of northwest
gales developing across the lake Monday evening, and persisting
into Tuesday before easing.

A strong digging upper level disturbance will bring deepening
surface low pressure to the western Lakes region by Monday
afternoon and evening. The low will continue to deepen Monday
night as it moves east of the region, while cold arctic air
spreads in as high pressure builds south across the Plains. Winds
will shift to the northwest by Monday evening as the low and
associated cold front move through, with winds quickly increasing
to 40 kt gales by late Monday evening. Cold air, with temperatures
in the teens to lower 20`s, will be drawn across the lake by
these strong winds, with the potential for freezing spray
increasing by Tuesday morning, and persisting into Tuesday night
before the winds gradually ease. Gale warnings will most likely be
needed for all of the open waters of Lake Michigan for the Monday
night into Tuesday afternoon.

Winds will then diminish fairly quickly late Tuesday night into
Wednesday, as weak high pressure ridging spreads across the lake.
Northwest flow aloft may bring another low or two across the area
Wednesday night and Thursday.

For the Illinois and Indiana nearshore waters, northwest winds by
Monday night will likely require a gale warning for the Indiana
waters, with at least small craft advisory conditions expected for
the IL waters.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...IN nearshore waters until 3 PM Sunday.

&&

$$

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