Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 100935

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

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



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.



For the 06Z TAFs...

High pressure over the region shifts east this morning. Light
winds back to southwest under VFR cloud cover. A band of light
snow is on radar over western Wisconsin. Guidance brings the band
over RFD around 09Z, and the band may clip the eastern terminals
early this morning. I think the band will weaken as it shifts
east, so I will keep the eastern terminals dry for now. Winds
become northwest at 10 kt this afternoon.

Another clipper system will arrive early Monday morning bringing
snow and MVFR cigs to the region. It looks like the more
significant snowfall will be after 12Z.



200 PM CST

Gale warning is in effect for the southern 2/3 of Lake Michigan
into this evening.

Low pressure that moved across cntrl/srn Lake Michigan is now
lifting across srn Lower Michigan toward the ern Great Lakes.
Strong north to northwest winds have reached close to storm force
over the far srn tip of the lake. The pressure gradient and cold
advection should remain strong enough to support gales continuing
into this evening. Winds should begin to diminish through this
evening as high pressure begins to build in from the west. Winds
will also back to swly overnight tonight and tomorrow morning as a
weak low tracks through the upper Great Lakes.

The active weather pattern will persist through next week, with a
series of low pressure troughs and cold fronts moving through the
region. A strong low is expected to drop out of the Upper
Mississippi Valley Monday, bringing increasing swly winds.
Following the passage of this low, a reinforcing shot of cold air
and a strengthening pressure gradient between the deepening low
over the ern Great lakes and high pressure building over the wrn
plains will support another round of northwest gales Monday night
into Tuesday evening. Winds will eventually diminish by Wednesday
as the next high pressure ridge moves in, but with another low
and strong northwest winds again Thursday.


LM...Small Craft Advisory...IN nearshore waters until 3 PM Sunday.




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