Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 100915

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
315 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



145 PM CST

Sunday night through Saturday...

A fairly active weather pattern is still expected for the next
several days, with periodic chances for pcpn.

The upper level pattern will continue to trend toward high amplitude
and slow progression with upper ridging building over the wrn 1/3 of
the CONUS and broad troughing over the ern 2/3 CONUS.  The slowly
progressive, high amplitude long-wave pattern lends relatively
higher confidence in the overall trends of the long term forecast
period, with persistent nwly flow aloft with a series of northern
stream shortwaves bringing period chances for pcpn, mainly due to
fast-hitting clipper-type systems.  While temperature trends will
generally be for below normal, a couple brief warm-ups should occur
in advance of the associated sfc lows dropping through the upper
midwest.  The longer range guidance remains in relatively good
agreement on the first clipper tracking across the region on Monday
and another on Wednesday night, which will take a track most
favorable for pcpn.  There will be other shortwaves dropping through
the fast nwly flow aloft that will track either too north or too
south to bring pcpn to the CWA, but will bring reinforcing shots of
cold air that will keep the upper trough over the ern 2/3 of the
CONUS.  The longer range guidance is trending toward more
progression and decreasing amplitude to the upper level pattern
toward the end of next week as Pacific-sourced shortwave energy
breaks down the upper ridge over the west with weak upper ridging
building ewd.  This should allow for a moderating trend for
temperatures toward the end of next week.t week.


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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