Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 091741

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1141 AM CST Sat Dec 9 2017


One last surge of snow showers will shift from Cook County into
Northwest Indiana in short order. Visibility reductions down to
1-2 miles have been occurring acrosss far northeast Illinois, with
some minor accumuluations possible, though these showers are quick
moving. These may be a bit more potent across northwest Indiana
as we are starting to see 40+dbZ echoes on radar over the lake.
Deep unstable/mixed layer is expected to setup in lee of the lake
as colder air rushes southeast behind low pressure over
southeastern Lake Michigan. Signs of a convergent band have
appeared on radar over the center of the lake. RAP forecasts of
boundary layer convergence shifting this into Porter county late
this morning/early afternoon. There still appears a few hour
window in the 10am - 2 pm time frame where impressive
thermodynamics and wind convergence will support some brief heavy
snowfall rates. This will be in concert with very strong low level
wind field in the pressure rises behind the low. Scientifically
there are some factors coming together for brief though heavy lake
effect snow potential across portions of Porter county, though as
is always the case with lake effect, this could be a narrow area
and may be progressive. And hi res guidance is still mixed as to
how much of this will get into Porter county. The message remains
the same, those traveling across northwest Indiana from Porter
county eastward should use caution as conditions could deteriorate
quickly at times this morning into early afternoon.

Otherwise much lighter snow showers/flurries will linger for a
few more hours in Illinois with gusty northwest winds in place.




324 AM CST

Through tonight...

Bottom line up front: A band of moderate snow will continue east
this morning ahead of the cold front. A single band of lake effect
snow will bring moderate to heavy snow to portions of northwest
Indiana mid morning through this afternoon. I have issued a Winter
Storm Warning for Porter County, IN due to very gusty winds (psbly
up to 45 MPH), blowing snow creating white out conditions, and the
potential for high snowfall rates under the lake effect snow band.
Finally, high waves may create lake shore flooding in Lake and
Porter IN counties.

A band of moderate snow along the cold front stretches from
northwest Lake County IL to Kendall County to western Livingston
County, and it will continue east this morning. Visibilities under
the band will likely drop to 1 mile, and a quick half inch to inch
of snow is possible. Additional light snow showers are possible
behind this stronger band with minimal additional accumulation
through mid morning. The majority of the forecast area should be dry
from mid morning onward.

The surface low over southern Lake Michigan will move over Lake
Ontario this afternoon, and the pressure gradient over the forecast
area will tighten. Gusty northwest winds follow the cold front, and
max gusts will be 30-35 MPH away from the lake. Gusts to 45 MPH are
possible immediately along the Lake Michigan shore, but will quickly
diminish away from the shore. Technically, wind gusts this high
would require a Wind Advisory, but I will mention gusty winds in the
Porter County Winter Storm Warning. Blowing snow may create whiteout
conditions, so those traveling along I80/94 in NW Indiana should
prepare for hazardous travel conditions.

Overall my confidence is low in how lake effect snow will pan out
today. Conditions are very favorable for a strong single band of
snow. Delta T`s will be around 17 degrees, and modeled lake induced
EL extends to 500 mb.  Hi-res guidance has shifted the potential
meso-low further east with it no longer impacting northeast IL. A
strong single band of snow will form and likely impact Porter
County. The big question is how long will the band remain over
Porter County? Currently guidance has the band moving into LaPorte
County within 1-2 hours.  Snowfall rates could be as high as 2-3"
per hour within the band, and I cannot rule out a clap of thunder
given so much lake induced instability. I think the highest snow
fall amounts will be over northeast Porter County as that area
should see the longest lake effect snow band residence time.

I decided to issue a Winter Storm Warning for Porter County due to
the potential for a brief period of heavy snow and 45 MPH winds
creating whiteout conditions.  Hazardous travel will be the main
impact from the snow and wind.  High waves are also expected
immediately along the NW Indiana lake front, so I went ahead and
issued a Lake Shore Flood Advisory for Lake and Porter Counties as

Wind gusts diminish late this afternoon into this evening, and I am
very confident that lake effect snow will also remain east of the
forecast area at this time too. Snowfall totals will vary from less
than an inch west of a McHenry to Oak Lawn to Rensselaer, IN
line. 1- 2 inches are expected east of that line. The highest snow
totals will be across northeast Porter County where 3-6 inches
are possible. If the lake effect snow band is slower to shift
east, much higher snow totals are possible in Porter County.

A quieter and chilly night is expected tonight. Lows will be in the
teens and minimum wind chills will be in the single digits above



349 AM CST

Sunday through Friday...

Active weather pattern is expected to persist across the region
through much of the coming week, with a series of waves digging
through northwest flow into large scale eastern CONUS upper
trough. Model guidance does suggest a break down of the western
CONUS upper ridge toward the end of the week, which would flatten
the upper flow pattern somewhat and lead to a period of somewhat
milder weather next weekend.

In the meantime however, northwest flow will persist across the
region. While several minor amplitude mid-level disturbances will
likely ripple through the flow at times, guidance is in pretty
good agreement in depicting a stronger, more amplified short wave
digging out of central Canada and into the Midwest late Sunday
night into Monday. Models develop light qpf into mainly the
northern parts of the cwa by Monday morning, though forecast
soundings indicate only brief periods of better deep saturation,
somewhat similar to the current (Saturday morning) system. Still,
would expect some potential for light accumulating snow early
Monday, especially across far north/northeast parts of the
forecast area. A strong cold front then pushes through Monday
afternoon, with a slug of arctic air spreading into the area
Monday night and Tuesday. 850 mb temps around -16C, and blustery
winds look to support temps only in the low-mid 20`s, and wind
chills in the single digits on Tuesday behind the departing low
and cold front, on what looks to be the coldest day of the week.
While models do not indicate precip into Tuesday, deep adiabatic
lapse rates with the strong cold advection may very well support
some snow showers or flurries.

The cold moderates a bit Wednesday however, as another wave and
associated surface low dig into the region, with a warm front
moving across the area and bringing another chance of some light
snow. By Friday however, upper flow begins to flatten, and the
next low in the active pattern is progged to pass across the
northern Lakes. Temps in the 40`s may be in the cards by the



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.



317 AM CST

Gale warning is in effect for the southern 2/3 of Lake Michigan
from 6 am this morning into this evening.

Low pressure was moving across central Lake Michigan early this
morning, in association with a deep mid-level disturbance tracking
across the region. Winds will shift to the north-northwest and
quickly increase to gale force across much of the lake this
morning, as the low moves off to the east. Strong pressure rises
and a pressure gradient enhanced by mild lake waters may aid in
producing a period of 45 kt gales by midday/afternoon. Winds will
decrease quickly this evening as the low pulls away and a weak
ridge of high pressure spreads east across the lake overnight.

An active weather pattern will persist into next week, with a
series of low pressure troughs and cold fronts moving through the
region. A weak trough is expected to move across the lake Sunday
afternoon, with a stronger low moving through Monday. This low is
expected to continue to deepen as it moves east of the lake Monday
night, with strong pressure rises and a push of cold arctic air
leading to another period of northwest gales Monday night into
Tuesday. Winds eventually diminish by Wednesday as the next high
pressure ridge moves in, but with another low and potentially
gusty 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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