Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 162008

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

1045 AM CST

Decided to issue a Winter Weather Advisory for Porter County,
replacing the Winter Storm Watch that was in effect. Heavy lake
effect showers/squalls have already been affecting Porter County
this morning and continue to do so, so the Advisory is effective
immediately, with an end time of 06z/12 AM CST tonight. Did not
yet make any changes to the existing headlines for Lake IL, Cook
and Lake IN counties, but may need to extend Cook County for a
few hours pending radar trends.

Have been closely monitoring lake effect snow trends this
morning. A well defined meso-low can be seen on GOES-16 imagery
over the far southeastern portion of the lake. The progression of
this will help drive convergence trends through the afternoon.
In watching observational and radar trends this morning, have
noted a gradual eastward shift of echoes due to northwest
boundary layer flow near the Wisconsin and Illinois shores, and
this will continue on the Illinois side with focus shifting into
NW Indiana this afternoon. The initial activity has been showery
in nature, with brief heavy showers/squalls. Have noticed an
uptick in more concentrated activity off the eastern Cook shore.
Expect any additional accums for Lake IL to be light. Locally up
to 2" or so are possible from downtown Chicago and south, where
currently more concentrated activity resides and also the single
band off far northeast Illinois and eastern WI that could skirt
the Illinois shore through mid day. It`s also possible that the
band off far NE IL and WI could merge with developing activity off
eastern Cook and into NW IN.

Trends with the expected single band are the biggest concern and
still rather uncertain with respect to residence time over any
given area in northwest Indiana. The thermodynamic environment is
quite favorable today along with inversion heights near/around
10kft, already seen in action with the heavy snow showers/squalls
that have affected the area this morning. Thus heavy snow will be
more than likely in this band that will pivot into Lake County
Indiana and then Porter County before exiting eastward this
evening. Radar trends and high-res guidance suggests that the
heavier single band could have a few hour residence time over Lake
County, which on top of what has fallen already, could result in
local amounts up to 5" in the northern part of Lake IN. Will
continue to need to watch radar closely for signs of any longer
residence time of the single band and heavier amounts than
expected. For Porter County, the latest thinking is that residence
time of heavier single banding or concentrated activity will be
long enough in any given location to attain warning criteria
amounts of 6"+. Therefore, did not have enough confidence to issue
a Winter Storm Warning and opted for a Winter Weather Advisory.

Webcams from during the heavy snow showers/squalls have shown very
low visibility and snow covered roads, so this is the main message
today, expected continued difficult to hazardous travel near the
lake. Periods of significant impacts are likely to the I-80/94



336 AM CST

Today through Tonight...

Persistent light snow this morning, and heavier lake effect snow
in lake-adjacent counties of NE 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 IN 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.



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.



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.


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.



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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