Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 132353

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
553 PM CST Wed Dec 13 2017

158 PM CST

Through Thursday...

This afternoon, low pressure is centered near just east of
Milwaukee, WI while a cold front stretches south-southwest across
DeKalb, IL down to near Peru. Along and ahead of the cold front,
there are a few showers moving east across the area. Temperatures
north of the I-88 corridor favor mainly snow while south of I-80
where surface temps have warmed into the upper 30s and low to mid
40s, any precip should be rain. This leaves a corridor of a rain
snow mix roughly between the I-80 and I-88 corridors this
afternoon. Dry low levels ahead of the front does appear to be
eroding some of the precip, so some of the echoes noted on radar
currently not reaching the surface while snow in some of the
heavier echoes is reducing visibility down to a mile at times.
Given the fairly short-lived nature of the precip with the fast
moving front and dry antecedent conditions, any snow accumulation
north of I-88 this afternoon should be limited to a dusting at
best but widespread accumulations are unlikely.

This evening, the low will shift into southwest Michigan with cold
air wrapping into the region late this evening and overnight. Lake
induced instability really begins to ramp up across southern Lake
Michigan after 03Z this evening as 850mb temps cool below -12C.
Guidance has been in reasonable agreement thus far indicating
scattered snow showers moving inland off of Lake Michigan around
this time over far northeastern IL and northwest IN. Some areas
could see a dusting of snowfall mid to late this evening with
these initially scattered snow showers. Meanwhile, LES parameters
should become increasingly favorable overnight into Thursday
morning as 850mb temps continue to cool. A relaxing gradient
behind the departing low should allow convergence over the lake to
strengthen with a single band of snow developing down the lake.
Trajectories initially favor this band to move onshore over SE WI
and far NE IL late this evening then transition south across the
Chicago metro into NW IN overnight. Depending on the residence
time over any given area, would expect snow totals in the half to
one inch range on the Illinois side overnight with the potential
for some very localized higher amounts. Instability appears to
peak late tonight into early Thursday with a due north fetch
favoring Lake and Porter Counties in Indiana receiving modest
accumulations. Strong subsidence behind the departing upper wave
will suppress inversion heights between 4000-5000ft and delta-Ts
peak around 14-15C. This will favor accumulations on the Indiana
side in the 1-3 inch range with again localized higher amounts
possible. This snow will likely impact the morning commute in NW
IN and plan to issue an SPS in the meantime to highlight the
potential impacts.



144 PM CST

Thursday night through Wednesday...

For the early portion of the long term forecast period,
Thursday Night through Saturday Night, model guidance is in
relatively good agreement on the longwave pattern, with high
amplitude ridging over the wrn CONUS and broad troughing east of the
Rockies.  A series of nrn stream shortwaves dropping through the
fast nwly flow aloft will bring some pcpn chances for much of the
CWA.  Thermal profiles will be cold enough to support snow as the
prevailing p-type, with the main concern being location and amount
of any possible accumulation.  The general shortwave track will be
through srn WI/Lake Michigan/Lower Michigan with the strongest
forcing north and east of the CWA.  However, there should be enough
upper forcing dipping into nrn IL/IN to support some light
accumulating snow, mainly for the nrn and ern portions of the CWA,
flurries more likely for the swrn portions of the CWA.  With weak
forcing and low deep layer moisture in place, any accumulation would
likely be nothing more than a dusting.

For the latter portion of the long term forecast period, from Sunday
through Wednesday, the various longer range models quickly diverge
in how they handle southern stream systems.  The various model begin
to significantly diverge as early as Friday night and Saturday for
the wrn CONUS, but these issues will likely not impact forecast
confidence and concerns until Sunday or Sunday night.  So, given the
model differences, have opted not to make any significant changes to
the going forecast, with a general trend of near to slightly above
normal temperatures.  While confidence in finer scale details is
low, the source of systems impacting the region are of pacific-
origin, thus the trend for generally higher than normal
temperatures. P-type and coverage will be most problematic on
Sunday. While the GFS and ECMWF both indicate srn stream shortwave
energy lifting out of the southwest, which would support mentionable
pcpn, but given the timing differences, will limit PoPs to slight
chance to low chance levels.  There is a chance that much or all of
the pcpn could be rain or a rain/snow mix with short periods of all
snow at more diurnally favored times around time of min temp.


For the 00Z TAFs...

553 pm...Forecast concerns include possible light snow this
evening...lake effect snow later tonight and winds this evening.

Northwest winds have been gusting into the 30-35kt range late this
afternoon and will continue for the next few hours before slowly
diminishing during the mid/late evening. Wind directions will
gradually turn more northerly through this evening and likely
remain northerly into the early morning hours. The gradient will
begin to relax by sunrise with prevailing speeds in the 10-15kt
range. As high pressure moves across the area Thursday...Winds
will slowly turn westerly by afternoon then southwesterly Thursday
evening with speeds under 10kts.

An area of light snow across southern WI early this evening is
expected to gradually shift southeast as it weakens. Confidence of
how much light snow makes it into the Chicago area terminals is
low. Its possible this may dissipate to just flurries by the time
it moves across the area in the 02z-03z time frame. Scattered
flurries will be possible for most of this evening. A band of lake
effect snow is expected to develop later this evening into
southeast WI and the quickly move across northeast IL during the
early morning hours and into northwest Indiana toward sunrise.
Confidence for where this band moves and how strong it becomes is
low. It may remain east of ord and possibly mdw as well. Opted to
maintain tempo mention with this forecast but shorten the duration
and adjust timing later based on latest short term guidance.
Further changes can be expected with later forecasts as trends
emerge. Best chance for a prolonged period of ifr or lifr
conditions with snow appears to be at gyy. This band is expected
to shift east of gyy by mid morning. cms


144 PM CST

The wind pattern over Lake Michigan will be very complicated for
the remainder of this afternoon and evening as strong low pressure
tracks across sern Wisconsin and srn Lake Michigan and then
tracks south of Lake Erie this evening. A small Craft Advisory
remains in effect for the IL/IN nearshore waters with swly winds
to 30 kt early this afternoon then shifting to nwly by late
afternoon. Winds should remain strong and nwly through the night.
While prevailing gales are not expected, there is a chance for a
short period of Gale Force gusts early this evening when cold
advection and pressure rises behind the exiting low are strongest
and would most likely be confined to the far southern end of the
Lake. Winds should diminish through the morning on Thursday as
high pressure builds across the Middle Mississippi Valley and into
the Ohio Valley. Conditions through the weekend should be
relatively quiet. The next chance for impactful weather is not
expected until early to the middle of next week as a strong cold
front pushes across the Great Lakes region. However, the longer
range model guidance is in very poor agreement on how the pattern
evolves next week, so confidence in timing of this next system is


LM...Small Craft Advisory...nearshore waters until noon Thursday.




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