Area Forecast Discussion
Issued by NWS Chicago, IL

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388
FXUS63 KLOT 160949
AFDLOT

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

.SHORT TERM...
336 AM CST

Today through Tonight...

Persistent light snow this morning, and heavier lake effect snow
in lake-adjacent counties of NW IL and NW IN today into tonight
are the short term weather concerns this morning. Reduced
visibilities and moderate snowfall rates at times in the lake
effect areas will likely have an impact on this morning`s commute,
and have elected to hoist a Winter Weather Advisory for Lake/Cook
in IL through midday, and for Lake IN through this afternoon.
Winter storm watch remains for Porter county, where a period of
heavier lake effect is possible later today and this evening,
though have pushed back start time to 9 am CST this morning.

Slow-moving closed upper level low was evident in GOES vapor
imagery early this morning, in the vicinity of the IL/WI border
region. Areas of light snow linger over northern IL and far
northwest IN as of 3 am in association with this feature,
particularly over portions of the central and southern Chicago
metro area. Visibilities in the 1-2 mile range have been noted,
likely producing a dusting to a tenth or two of an inch of
accumulations in some localized areas. This should persist in the
form of flurries or light snow through the morning hours, before
the upper low/trough axis begins to shift east of the forecast
area this afternoon. Of greater concern however, is the lake
effect snow showers which have been occurring along the western
shore of the lake (WI) overnight, where occasional 1/4-1/2 mile
moderate to locally heavy snow showers have been noted north of
the border. The low level convergence focus is expected to shift
south into far northeast IL this morning, before eventually
pivoting southeast into northwest IN this afternoon/tonight. It`s
important to note that there are some subtle but important
differences in the various high-res model guidance handling of the
location of the primary forcing, though it appears likely that
the IL lake shore area will see a period of lake effect snow
through the morning hours, as winds become north-northeast, with
northwest IL becoming more favored this afternoon and tonight.
Lake-H8 delta T`s increase to better than 20C this morning, with
equilibrium levels increasing to 10+ kft per NAM forecast
soundings. Thus, thermodynamically the potential for at least
localized moderate snowfall rates are supported. Given the
potential for perhaps 1-3" of additional accumulation (perhaps
greatest over eastern Lake Co IL this AM), and coinciding with the
Tuesday AM rush, have elected to issue a Winter Weather Advisory
through noon for Lake/Cook counties. With the band eventually
shifting southeast, have gone into this evening for Lake Co. IN,
with less confidence in how long it persists there based on
aforementioned model differences. Best fetch, thermodynamics and
potential for longer residence time of a generally single band
plume is expected to be across Porter Co later today and tonight.
Again, with somewhat low confidence in details of just where/when
this band sets up, have elected to maintain the going Winter Storm
Watch, in collaboration with WFO IWX. 6 inches of accumulation is
not out of the question there by the time snow shifts east of the
cwa late tonight.

Otherwise, temps have fallen off to near zero across the southern
half of the cwa early this morning, where stronger west-northwest
winds have pulled arctic air across the new snow cover. Closer to
the surface low/trough axis over southern WI, cold advection has
not been as pronounced. Have stayed close to the colder end of
guidance for today, generally upper teens to low 20`s for highs.
Decreasing winds and clouds tonight should again support temps
close to zero in some spots away from Chicago, and single digits
above in others.

Ratzer

&&

.LONG TERM...
348 AM CST

Wednesday through Monday...

A quieter stretch of weather, with moderating temperatures, is
anticipated through the mid-week period. Deepening low pressure is
progged to lift into the Upper Midwest by late in the weekend,
with a surge of Gulf moisture leading to melting snow cover, rain
and potentially some 45-50 degree weather. Cooler, but still above
normal weather looks to return early next week.

Deep upper trough over the region to start this week becomes
progressive and accelerates toward the East Coast on Wednesday,
allowing broad upper ridging to spread across the central parts of
the CONUS. At the surface, strong high pressure builds southeast
to the lower Mississippi Valley, with our flow becoming generally
from the west-southwest. Medium range guidance depicts a Clipper
passing well north of the cwa Thursday, before a largely zonal
upper flow pattern becomes established.

Upper troughing is progged to develop into the southwestern
states Saturday, inducing surface low development across the
Southern Plains. Increasing south low level flow develops in
advance of the low as it deepens and lifts northeast into early
Monday, with low level warmth and moisture pulled north from the
Gulf of Mexico. Increasing temps and dew points will likely get to
work at melting our snow cover as the warm front lifts across the
region Saturday night and Sunday, likely with the increasing
potential for fog and drizzle which eventually blossoms into rain
as the low passes northwest of us later Sunday and Sunday night.
The occluding cold front is expected to push across the area
early Monday morning, diminishing precipitation but also allowing
colder air to spill in and change lingering rain over snow or snow
showers.

Ratzer

&&

.AVIATION...
For the 06Z TAFs...

Main forecast concern is snow potential through Tuesday afternoon.

A band of moderate snow is moving into far northern IL with vis at
ugn as low as 1/2sm recently. This band of snow will spread south
across all the terminals over the next few hours but is expected
to slowly weaken. With prevailing vis already in the 2-3sm with
light snow...it is likely that vis will dip below a mile at
ord/dpa but if the band does weaken...confidence at mdw/gyy is
lower. Heaviest part of the band may remain east of rfd but vis at
or just below 1sm still possible there. Behind this band there
may be improvement to mvfr vis...or better. However...there still
appears to be the potential for continued very light snow. Lake
effect snow is expected to develop Tuesday morning and currently
is expected to remain east of ord/mdw. Confidence is too low to
include any mention at ord/mdw but trends will need to be
monitored for possible lake effect snow showers at ord/mdw during
the mid/late morning Tuesday. These lake effect snow showers are
expected to affect gyy but confidence on timing has decreased. Its
possible the lake effect snow persists into Tuesday afternoon at
gyy before shifting east.

West/southwest 10-15kts with a few higher gusts will shift to the
northwest by morning and then more north/northwest later Tuesday
morning. Prevailing mvfr cigs will continue into Tuesday afternoon
with brief ifr possible with any heavier bursts of snow. cms

&&

.MARINE...
324 AM CST

Low pressure is centered just east of Lake Michigan early this
morning with a baggy trough stretching west across the far south
end of the lake. The low will move east today and weaken allowing
northerly flow of 15 to 25 kt to overspread the entirety of Lake
Michigan by late this morning. Strong high pressure over the
northern Plains today will build across the lower Mississippi
Valley by tomorrow while low pressure moves southeast across
Hudson Bay. A westerly gradient will develop over Lake Michigan
between these two features and gales are a concern for Wednesday
through Wednesday night. GFS/NAM soundings both show a strong
inversion off the deck which will limit the depth of mixing and
casts some uncertainty on the strength and duration of any gales.
In addition, there are modest model difference and GFS soundings
do not support gales over northern Lake Michigan at all, and only
marginally so across the south. Meanwhile, NAM soundings show
marginal gales across the lake. Will hold off on any gale
headlines at this point and only mention occasional gales in the
GLF during those periods.

Another low will move across the Canadian Prairies this weekend
bringing another potential period of gales Friday night into
Saturday. Models continue to advertise a deep low late in the
weekend into early next week lifting from the central and southern
Great Plains into the Upper Midwest or Great Lakes. If this
scenario pans out, could be yet another window of gales,
possibly strong gales.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Weather Advisory...ILZ006-ILZ014 until noon Tuesday.

IN...Winter Weather Advisory...INZ001 until 9 PM Tuesday.

     Winter Storm Watch...INZ002...9 AM Tuesday to 6 AM Wednesday.

LM...None.
&&

$$

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