Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 072113
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
313 PM CST Thu Dec 7 2017

.SHORT TERM...
1203 PM CST

Through Friday...

A band of snow flurries will continue east through northwest
Indiana. Clouds accompany this feature, but will scatter out
pretty quickly from west to east. Mid and higher clouds will
rotate southeastward tonight, gradually lowering through the night
and into Friday, though they will likely remain scattered enough
to allow temps to dip into the teens tonight.  High pressure
across the plains will shift south of the area and then into the
Ohio valley by Friday. This will allow winds to shift from west-
northwesterly today to southwesterly Friday. Breezy southwest
winds will usher in slightly warmer temperatures, but they will
still hold in the low to mid 30s. Low pressure will drop
southeast through Wisconsin Friday, but this clipper will remain
largely north of the area through the bulk of the evening
commute.

KMD

&&

.LONG TERM...
259 PM CST

Friday night through Thursday...

A fairly active weather pattern is still expected for the next
several days.

In the longer term forecast period, the first concern continues to
be a measurable snow event beginning late Friday afternoon or early
evening and continuing through the night.  Focus will transition
from a synoptic snow Friday night to lake effect snow on Saturday.
Have actually made little change to the going forecast for the
snow, with a swath of 1.5 to 2.5 inches with the synoptic snow
roughly north of a line from Rockford, IL to Wheatfield, IN.  South
of this line an inch or less is expected.  Confidence in lake effect
snow amounts is a bit lower due to the more mesoscale nature of lake
effect banding.  Thermal profiles over nwrn Indiana where air
temperatures in the middle 20s combined with Lake Michigan water
temperatures in the low 40s will be, at least, moderately unstable,
so locally heavier snow bands will be likely.  Latest thinking is
that there should be around 4 to 6 inches total snow in the favored
lake effect area of nwrn Indiana, generally over Porter County,
where the nwly fetch will be most favored for heavier lake effect
snow amounts.  Should any more stationary banding develop, which is
still not out of the question at this time, there may be locally
higher accumulations.  Blowing snow will also likely be a concern on
Saturday over nwrn Indiana as as strong nwly-nly develop over the
southern portions of Lake Michigan and nrn IL/IN as the sfc low
lifts away from the region and strong high pressure builds across
the Upper and Middle Mississippi Valley.  Winds gusting up to 30 mph
and relatively high snow-to-liquid ratios, around 20:1, could bring
periods of reduced visibility and snow blowing across roads.

Wind direction will back to more westerly and southwesterly Saturday
night as the sfc low lifts through sern Canada and a weak ridge of
high pressure moves across the midwest, the lake effect potential
will come to an end.  Sunday should be relatively quiet, but the
next system will bring another round of light snow on Monday.  the
models are in relatively good agreement on a fast-moving clipper-
type system dropping through the Upper Mississippi Valley late
sunday night and tracking across the lower Great Lakes region on
Monday.  latest guidance suggests that the main shortwave will
roughly track along the IL/WI border.  This would suggest that the
corridor of heaviest snow should remain north of the border, with
the northern tier counties seeing at least a small amount of
measurable snow.  At this point, it looks like locations between I-
80 and I-88 may see a little light snow and locations south of I-80
seeing little or no snow on Monday.  Snow chances for the region
will not end here as winds will, once again turn more northerly as
the clipper tracks off to the east, setting up a more extended
period of lake effect snow potential for nwrn Indiana. Beginning
Monday night and continuing into the middle of next week, a series
of shortwaves dropping out of Canada carves out a deepening trough
over the ern CONUS and strong upper ridging building over the west.
This will keep the local area under fast nwly flow aloft.  Each
successive shortwave will help reinforce the cold air that will
already be in place and bring periodic chances for snow and lake
effect snow.  Given the timing differences among the models with
respect to the speed, exact track and intensity of the individual
waves, confidence in the finer scale details of the exact impacts is
relatively low.  However, the main trend of the forecast for the
early to mid portions of next week should be for below normal
temperatures with periodic chances for pcpn.

&&

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

A band of snow flurries will continue east through northwest
Indiana. MVFR clouds accompany this feature, but will scatter out
pretty quickly from west to east. Mid and higher clouds will
rotate southeastward tonight, gradually lowering through the TAF
period. High pressure across the plains will shift south of the
terminals and then into the Ohio valley by Friday. This will allow
winds to shift from west-northwesterly today to southwesterly
Friday. Gusts to 20 kt remain possible. Low pressure will drop
southeast through Wisconsin Friday. Snow will remain confined
north of the Wisconsin border through the 30 hr TAF cycle.

KMD

&&

.MARINE...
315 PM CST

A series of low pressure systems will move across the region
through the weekend and into the middle of next week, keeping
winds elevated through much of the period. Low pressure lifting
across northern Quebec and high pressure building into the Ohio
Valley will bring a relative lull in the winds. Low will form
over the western Great Lakes Friday and Friday night and as it
lifts through the eastern Great Lakes Saturday, strong high
pressure will build over the plains, setting up a period of strong
north winds, with Gales possible on Saturday, particularly for the
southern portions of the lake where the gradient will be
strongest. Reinforcing shots of colder air will move into the
region as a series of lows track across the Great Lakes through
the middle of next week. This will keep the pattern very active,
with periodic chances for gales or near-gale force winds.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...Gary to Michigan City IN until 4 PM Friday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...9 PM
     Thursday to 4 PM Friday.

&&

$$

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