Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 091957

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
157 PM CST Sat Dec 9 2017

111 PM CST

Through Sunday...

A convergent band of lake effect snow is moving into northwest
Indiana. Latest observational trends upstream across Lake Michigan
follow the conceptual model that the focus of heavy lake effect
will generally shift east of the area through the afternoon. RAP
boundary layer convergence fields have captured the evolution
fairly well, and it keeps at least a few hour period into early
evening of strong convergence across LaPorte county eastward.
There is still some lingering though weaker convergence in
northern Porter county, and the strong signal is still awfully
close to northeast Porter such that we do not plan any changes to
the winter storm warning for lake effect snow and blowing snow.
And the winds in northeast Porter and southwest LaPorte are
howling, with gusts of 38-45 kt being reported right at the
shoreline. Rapid changes in visibility are expected, though the
strong winds may preclude accumulations from getting out of hand
and keep the snow progressive, at least in our area.

Deep low pressure over southeast Lake Michigan will shift east
and weaken through the afternoon and more so this evening. The
gradient will be strong enough to maintain wind gusts near 30-35
mph through the afternoon even away from northwest Indiana. And
the lake snow belt can continue to expect at least occasional
moderate to briefly heavy snow shower bands into early evening.

Elsewhere, lower clouds will fade this evening. High pressure
will pass well south of the area tonight while a secondary low
pressure trough will develop from the upper Midwest into the
plains. Winds will shift back to southwesterly overnight ahead of
this low that will dive southeast ahead of another sheared vort
max in northwest flow. Model guidance paints just some mid level
clouds and scattered lower clouds. But, the mid level forcing and
warm advection ahead of this low may be able to offset somewhat
drier lower levels for a few flurries overnight with this
feature. The southwest winds will allow temps to bounce back into
the 30s Sunday. The trough axis will pass through in the early
afternoon and a send a weak cold front through, again with most
likely just an increase in clouds.



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 18Z TAFs...

Gusty northwest winds and MVFR clouds are the concern in the near
term, otherwise concerns for the terminals will remain fairly low
following the afternoon period today. Deep low pressure over
southeast Lake Michigan will shift east and weaken through the
afternoon. The gradient will be strong enough to maintain wind
gusts in the 20s through the afternoon with a 320-340 direction.
In the 22z-0z time frame speeds will relax some to ease 22L
departure concerns for ORD. There could still be a few flurries
across NE IL, but the main snow band will be across northwest

MVFR clouds will fade this evening. High pressure will pass well
south of the area tonight while a secondary low pressure trough
will develop from Lake Superior southeast to Iowa. Winds will
shift back to southwesterly overnight ahead of the weak low with
just some mid level clouds and scattered lower clouds, and maybe
a few flurries overnight. The trough axis will pass through in the
early afternoon, and winds will head back to the northwest with a
lowering VFR deck in place.



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.


IN...Winter Storm Warning...INZ002 until 9 PM Saturday.

     Lakeshore Flood Advisory...INZ001-INZ002 until midnight Sunday.

LM...Gale Warning...IL nearshore waters until 6 PM Saturday.

     Gale Warning...LMZ743-LMZ744-LMZ745-LMZ777-LMZ779 until 9 PM




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