Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 162049
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
249 PM CST Tue Jan 16 2018

.SHORT TERM...
247 PM CST

Through Wednesday...

For the remainder in the afternoon and into early evening, the main
concern will continue to be the lake effect snowfall over nwrn
Indiana.

Latest radar trends indicate that the lake effect plume that had
developed earlier today over southern Lake Michigan is slowly, but
steadily moving east across nwrn IL and srn Lake Michigan.  The back
edge of the snow is along the Lake/Porter County line and snow has
ended over nern Illinois and Lake and Newton Counties in nwrn
Indiana.  There is still the potential for moderate to heavy lake
effect snow, mainly over Porter County, with some lighter snow
extending into nrn Jasper County.  Northern Lake County could
possibly see an additional 2 to 3 inches of snow through earlier
this evening.  The higher of these amounts should be over portions
of the county that have not yet seen the heavier snow just yet and
snowfall rates of 1 to 2 inches per hour are possible. While the
Winter Weather Advisory has been allowed to expire over Lake and
Cook counties in Illinois, the Advisory for Lake and Porter
Counties in Indiana will continue.

Through the evening and into the overnight hours, high pressure over
the Upper Missouri Valley will slide to the southeast, and will be
centered over the Lower Ohio Valley by Wednesday morning.  Winds
across the region will gradually back from nwly to swly, shutting
off the lake effect snow potential through the evening.  Under
decreasing cloud cover, diminishing winds and fresh snowfall,
temperatures should drop into the single digits above 0 F overnight.
With winds arnd 5 to 10 mph, wind chill readings should drop to -10
to -15 F overnight.

Winds will strengthen again through the day tomorrow as the center
of the strong high remains parked over the Lower Ohio Valley and low
pressure tracks across south-central Canada.  With a strengthening
pressure gradient, swly winds should gust to 25 mph by late morning,
with some gusts possibly up to 30 mph in the afternoon.  The upper
level pattern is expected to remain high amplitude and slowly
progressive, with deep long-wave upper troughing slowly moving east
of the region while upper ridging remains over the Rockies. With
deep layer cold advect and nly flow aloft, temperatures tomorrow
should remain in the upper teens to lower 20s.

&&

.LONG TERM...
207 PM CST

Wednesday night through Tuesday...

Pleasant conditions and moderating temps will be the rule for late
week before precipitation chances arrive over the weekend. The
upper trough that brought the current cold snap to the area will
be departing to the east with upper ridging across much of the
western U.S. spreading eastward. Low/mid level temps will rebound
as the ridge moves eastward allowing for a noticeably milder day
for Thursday as highs warm into the lower 30s. A shortwave will
traverse the ridge Thursday but have little influence on our
weather. Once the wave passes, the ridge broadens allowing warmer
air to push toward the region. The snow pack will likely have some
influence on temps by Friday. Dewpoints will also increase Friday
which will bring the potential for some fog development,
especially Friday night. Saturday may be the more likely day for
fog development as a bigger push of moisture arrives but some
erosion of the snow pack will have occurred by then, though it may
be minimal overall. A deep upper trough will be moving across the
southwest U.S. into Saturday then lift northeast into the central
and southern Plains by Sunday. A surface low will track northeast
from the southern Plains with guidance pretty consistent on taking
the low north of the local area. Given this is a ways off, details
are subject to change but would expect to see precipitation
chances increase later Saturday or early Sunday as a warm frontal
boundary develops near or over the area. Mixed precip may be a
possibility, especially across northern areas, Saturday night
before transitioning to all rain. Temps still look to surge into
the 40s or lower 50s for Sunday. The surface low and upper trough
pass Monday which would likely bring a change to snow as mid and
upper levels cool. Cooler air will move in with medium range
guidance differing somewhat on the magnitude.

MDB

&&

.AVIATION...
For the 18Z TAFs...

Main concern for the period will be continuing Lake Effect Snow
at GYY while ending at the other terminals.

With winds shifting to nwly over land, the lake plume is setting
up over the lake and the fetch will take lake effect snow into nrn
Indiana. There may be an hour or 2 of light snow showers or
flurries at ORD/MDW, but the impactful snowfall is quickly ending
over nern Illinois. Conditions will continue to gradually improve
through the afternoon and evening hours as a ridge of high
pressure approaches the region. Ceilings will lift to VFR and
eventually scatter out by late afternoon or early evening.
Northwest winds in place much of the day today will back to the
west overnight and southwest early Wednesday as the center of high
pressure slides into the lower Ohio Valley tomorrow.

&&

.MARINE...
247 PM CST

An active period of weather is setting up for Lake Michigan.

As high pressure settles into the Lower Ohio Valley tomorrow morning
and low pressure tracks across south-central Canada, winds over the
lake will back from generally northerly to southwesterly tomorrow.
With the high expected to very slowly sag south toward the northern
Gulf of Mexico and a series of low pressure systems track across
south-central Canada and the Upper Great Lakes, winds are expected
to increase to Gale Force by early Wednesday morning.  The surface
pressure pattern is expected to change little from Wednesday morning
through Thursday morning, so an extended period of gales is
possible, with winds initially reaching Gale Force over the northern
portions of the lake Wednesday morning, and then strengthening to
Gale force over the southern portions of the lake in the afternoon.
The gradient over the lake is expected to weaken through the day on
thursday, with winds diminishing below Gale Force from north to
south through the day.  With wind direction expected to be
southwesterly, winds may not reach gale force over the nearshore
waters, so, have gone with a Small Craft Advisory for the nearshore
waters and a Gale Watch for the open waters.  The Gale Watch will
likely be replace with a Gale Warning as exact timing becomes a bit
more certain.

Another low will track across the Canadian Prairies over the
weekend, bringing another potential period of gales Friday night
into Saturday. The longer range guidance continue to advertise a
deepening low developing over the South High Plains on Sunday and
lifting through the Middle Mississippi Valley on Monday and
continuing across the northern portions of the lake Monday night.
Another round of southwest gales is possible in advance of the
system, but the extent of any potential gales will depend on the
exact track of the system, which is still somewhat uncertain at this
time.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Winter Weather Advisory...INZ001 until 9 PM Tuesday.

     Winter Weather Advisory...INZ002 until midnight Wednesday.

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...1 PM Wednesday to 4 PM Thursday.

     Gale Watch...LMZ777-LMZ779...1 PM Wednesday to 3 PM Thursday.

&&

$$

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