Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 011742
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1142 AM CST Wed Mar 1 2017

.SHORT TERM...
346 AM CST

Through Today...

February went out like a lion and March thankfully will enter more
peacefully, although not completely quiet, as today will be a
windy and turning cooler day, with rain showers transitioning to
snow showers before ending. Some of these snow showers could come
down at a modest clip, mainly across the northern Chicago metro
during the mid to late afternoon.

The upper low across southern Minnesota early this morning will
dig east-southeast today and further strengthen the surface low
over the central/eastern Great Lakes about 8 mb over 9 hr. This
pattern will strengthen low-level cold advection over the area
with sustained west-northwest wind speeds increasing to 20 to 25
mph by noon. Gusts will reach 40 mph at times this afternoon
across a fairly large part of the CWA, and maybe even higher in
some locations. Temperatures will be falling through the day area-
wide.

Forecast soundings show the steepened low-level lapse rates and
boundary layer moisture favoring low-level CAPE for light to
moderate showers, especially with the vorticity maximum moving in
across northern Illinois early to mid afternoon. The trajectory
of the best lift/CAA are juxtaposed in northeast Illinois, but
feel the whole area has a decent shot of seeing light showers.
Profiles support a change over to snow showers north of I-88
through the afternoon and then moving south after, although
forcing for precipitation will begin to quickly shift east early
this evening. Snow shower accumulation north could produce a
dusting to possibly near an inch, especially in Boone, McHenry,
and Lake Counties.

MTF

&&

.LONG TERM...
412 AM CST

Tonight through Tuesday...

Upper level trough still likely pushing through the region this
evening, while the stronger more defined shortwave energy shifts
east of the area. However, cyclonic flow still in place with some
vorticity maxima traversing the flow may support some additional
showers early in the evening mainly for the eastern half of the
CWA. Boundary layer cooling early on should allow any lingering
rain snow mix to become all snow. However, don`t anticipate much
of any snow accumulation, with the exception in northwest Indiana
where flow off the lake may support some additional snow showers.
If this were to occur, accumulation will still be real minor.
Conditions should remain dry for the remainder of the night, but
with this cyclonic flow still in place, scattered flurries will
remain a possibility over the entire CWA.

Turn attention to fast moving system dropping southeast through
the region on Thursday. This system has been advertised among
varying model guidance for some time now, however, guidance this
morning is still varying to the placement and intensity of the
attendant weak surface reflection. Associated model QPF also
really varying at this time too. Although I do think it will
precipitate Thursday, with confidence low on placement and even
exact timing, have maintained chance pops. Will need to continue
monitoring this system though as forcing does appear to be strong.
This forcing along with potentially very steep lapse rates could
support a period of some higher intense showers, especially in the
afternoon. Boundary layer warming will likely allow for some
varying precip type, but if this stronger forcing does occur,
could see snow showers be the more dominant precip type. Something
else to monitor, is the potential for some type of deformation
precip, which guidance is advertising as well. Precip chances
linger early on Thursday evening, but a drying trend should occur
for the remainder of the night into Friday as high pressure builds
across the region. Another wave may drop southeast across the CWA
Friday night into early Saturday morning and with the cooler air
still in place, snow would be the precip type. Warming trend is
then expected into the weekend.

Rodriguez

&&

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

Cigs/vis have improves to mvfr at the terminals, though winds have
becm wnwly at arnd 15g25 kt. Expect that the gustiness to persist
through the afternoon in strong cold advection behind the exiting
cold front. Winds are anticipated to remain arnd 290-310 degrees
at the terminals, though there is a chance that direction could
trends toward 320-330 at times this afternoon as high pressure
builds eastward across the plains and the low lifts out to the
east. Gusty wnwly-nwly winds should continue through the evening
as the gradient and cold advection, as well as pressure rises
behind the exiting low should remain strong enough to generate
gusts to arnd 25 kt through the evening hours. Still anticipate a
period of light pcpn moving in this afternoon, with the going
timing still looking reasonable. Sfc temps are high enough such
that pcpn should start out as rain or a rain/snow mix, but should
quickly transition to all snow as colder air continues to filter
into the region. Some accumulating snow is possible, but pavement
temperatures should be high enough such that any snow will melt on
contact. Any accumulation will likely only be on grassy surfaces.
There should be a general improving trend to cigs/vis as pcpn
potential ends early this evening. mvfr cigs could hang in
through the evening hours, but as high pressure continues to
build east though the night and into tomorrow, cigs/vis should
improve to vfr late tonight. There will be another chance for
some snow showers tomorrow afternoon as a weak clipper-type system
drops through the northern plains to nrn IL/IN tomorrow
afternoon. Moisture should be limited with this system, so major
impacts are not expected at this time, but have introduced a
prob30 for mvfr cigs in shsn, and as confidence in timing and
strength increases closer to the time of onset, will likely be
able to refine any possible vis impacts.

&&

.MARINE...
420 AM CST

Winds are quickly changing across the entire lake including the
nearshore as low pressure departs to the east, with northerly
winds spilling south on the backside of this system. Winds are
quickly increasing to gales over the north half this morning and
expect this trend to continue over the south half by mid morning
today. Do have gales to 40 KT occurring today, which will likely
continue into this evening. Winds are quickly turning to the north
along the IL nearshore with speeds getting into the 20-25 KT
range. With this quick change have started the small craft
advisory early this morning for the IL side and then start the IN
side here later this morning. Winds will then trend more to the
northwest later this morning and increase to 30 KT over the
nearshore. Do think occasional gale force gusts will be possible,
and have included this in the forecast. These higher speeds may
linger into the evening, but a diminishing trend is likely tonight
into Thursday. Higher waves will likely continue into Thursday
though.

Rodriguez

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Gale Warning...LMZ777-LMZ779 UNTIL MIDNIGHT Thursday.

     Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 10 AM Thursday.

     Small Craft Advisory...ILLINOIS NEARSHORE WATERS UNTIL 4 AM Thursday.

&&

$$

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