Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 152109

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
309 PM CST Mon Jan 15 2018

309 PM CST

Through Tuesday...

Have opted to let Winter Weather Advisory expire at 3pm CST as
planned. Made no changes to going Winter Storm Watch for Porter
County for heavy lake effect snow potential on Tuesday. Additional
details will be posted shortly.



144 PM CST

Wednesday through Monday...

The axis of the upper longwave trough clears the area late Tuesday
as the associated surface low finally starts to move off toward
the eastern Great Lakes. This will bring an end to any lingering
flurries or light snow for the remainder of the work week. The
most significant influence on local temperature and precipitation
trends then becomes a broad ridge building over the Plains.

Deterministic models are in reasonable agreement with the center
of this feature somewhere in the vicinity of Tulsa by early
Wednesday, with a cold start to the day followed by winds backing
from northwest to west locally on the northeast side of the high.
Winds continue backing to southwest by late Wednesday and persist
from this direction through Thursday, Friday, and Saturday as the
ridge moves into the Gulf Coast region and eventually into the

Along with a steady warming trend and late-week thaw, this would
set up an interesting open Gulf scenario for later in the weekend
across the Midwest, especially given the developing upper trough
across the Rockies and the surface low that models show forming in
the Southern Plains. The period from Sunday into Monday certainly
deserves attention as the time gets closer, and while it is far
too early to start clinging to any deterministic solution, there
is considerable and somewhat surprising agreement between the
latest ECM/GEM/GFS in bringing the deepening sfc low through
northern Illinois in the late Sunday timeframe.

Again, much could and likely will change over the next several
days, but very early signs suggest the potential for convection
ahead of this low and significant winter weather on its north
side. It is too early to say what this means for the local area,
as all depends on the ultimate track and strength of the low, and
whether it even develops as models presently advertise. For
example, a quick look at the GFS ensemble shows a much weaker and
more transient upper wave, as might be expected, rather than a
slower and deeper closed circulation as depicted in deterministic
solutions. Either way, there appears to be a good chance for
active weather late in the weekend, with the potential for both
liquid and frozen precipitation.



For the 18Z TAFs...

The ongoing snow event has been a bit slower to taper off than
had been expected, though the overnight period did indeed see the
heavier amounts as anticipated. Have had to extend lower cigs and
vsbys a couple of times this morning for moderate snowfall and
this will continue into the early afternoon. Regional radar
imagery and sfc observations suggest another couple of hours of
lower conditions in the 2-3 SM range or perhaps briefly worse with
around an inch of additional accumulation, then very gradual
improvement with flurries through the remainder of the afternoon
and evening.

Winds have turned westerly behind the passage of a cold front, and
gusts to around 20kt should start to pick up as per upstream
observations. Pockets of blowing snow could become an issue later
in the day given the fairly dry nature of the existing snow cover.

Overnight another shot of lake enhanced snow is possible, most
likely between midnight and dawn. This appears most likely to be
a transient band producing a quick shot of lower cigs/vsbys and
light accumulations of an inch or so.



144 PM CST

Low pressure moving across the south end of the lake will support
varying winds. Across the central and northern part of the lake,
easterly winds will gradually back to north by midday Tuesday
and then eventually to northwesterly by Tuesday night. Winds
across the entire lake then back to southwesterly by late
Wednesday and persist from this direction into the weekend.

Winds gusting well into the 20s will be common through midweek,
but a tighter gradient developing Wednesday likely will support
stronger southwest gusts to gale strength on Wednesday and
Wednesday night. Areas of freezing spray will be possible at times
from now through this midweek period, though heavy freezing spray
appears less likely.

Ice analyzed in the southern portion of the lake is inhibiting
handling of wave growth by our wave model during the long north
fetch late Tuesday into early Wednesday, but later updates may
have a different ice analysis and higher wave heights in addition
to the breezy onshore winds, espcially in the Indiana nearshore



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




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