Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 121031

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
431 AM CST Thu Jan 12 2017

339 AM CST

Through tonight...

The week of mixed precip events continues. Freezing drizzle has
been reported at various locations overnight with trace to very
light accumulations.

The freezing rain advisory remains in effect into mid morning,
and likely will need to be expanded a row of counties south.
Thunderstorms also are possible as supported by considerable
lightning presently being observed, together with models showing a
couple hundred J/kg of CAPE. Even so, the area of thunderstorms is
moving fairly quickly and anticipated ice amounts of around 0.10
inches over 2-3 hours should mainly be problematic for roads
rather than trees and power lines.

After these showers move through this morning, precip chances
diminish and north winds will support steady to slowly falling
temperatures through the afternoon.



404 AM CST

Friday through Wednesday...

After a brief break on Friday, precip-type challenges continue
this weekend. For both Saturday and Sunday the POPs do not climb
above the chance category, and those mainly across the southern
part of the forecast area away from the center of the cold surface
ridge that will be positioned to our north. However, during
periods when the atmosphere does trend toward saturation the west
winds aloft will support an above freezing layer over a much
colder surface layer. Sleet and freezing rain are both reasonable
possibilities, with a bit of snow on the north edge and some rain
chances in the far south. Fortunately the QPF values at this point
are fairly low, suggesting most of the precip this weekend could
be of the nusiance variety rather than high impact. This of course
will continue to be monitored closely.

By Sunday night a low pressure system lifting out of the Southern
Plains will bring increased precip chances along with rapidly
increasing surface temperatures. The existing FZRA in the forecast
for Sunday night reflects the fact that there could be a period
where the cold surface air remains in place long enough to support
a few hours of freezing rain, but there is great uncertainty with
the timing of precip chances and temperature trends. By Monday
afternoon we should be seeing mostly just rain.

Beyond Monday, models are suggesting a front could remain stalled
across or just south of the area, so we have maintained precip
chances along this feature, but a different placement could bring
significant changes. Overall there is very low confidence for
Tuesday and Wednesday at this time.



For the 06Z TAFs...

Main forecast concerns are potential for freezing rain or a mix of
wintry precip centered on 12z Thursday morning. An upper wave will
move across central IL Thursday morning bringing a period of
precipitation that has been trending further north and toward the
terminals centered around 12z. Precip looks to potentially develop
right over the area during the overnight hours and quickly move
northeast. Still some uncertainty regarding how far north this
precip extends but close enough given most recent guidance to
include a tempo for freezing rain at all sites except for rfd. Its
possible there may be a mix of snow but freezing rain looks to be
the main precip type. Duration also looks fairly short...likely no
more than 2-4 hours at any one location. Prior to this precip...
there remains the potential for freezing drizzle.

North/northeast winds 10-15kts with higher gusts will turn more
northerly by morning and may diminish some. Once the precip ends
by mid morning...winds will shift northwesterly and remain in the
10-15kt range. Generally ifr cigs through mid morning Thursday
that will quickly lift through mvfr late Thursday morning and then
scatter early Thursday afternoon. cms


334 AM CST

Headlines...Gales are expected over the far northern end of the lake
this evening as the weakening low passes just north of the lake.
The wave model believes there is more ice than there actually is
over the nearshore waters, so raised wave heights across the NSH
waters. 15Z/10AM CST still seem like a good time for the IL small
craft to end. Winds will be borderline hazardous through most of
tonight for the IN NSH waters. In addition, hazardous waves should
be advected into NSH waters. So decided to extend the IN SCY
through Friday afternoon. Waves will be lower if ice returns.

The trough of low pressure that stretches from southeast Ontario
through Oklahoma will continue northeast today.  North to northeast
winds diminish and become northwest this morning. A low over
southern Manitoba weakens as it across southern Ontario. The
pressure gradient tightens over the far northern end of the lake
this evening and west gales are expected. Winds become northwest to
30 kt across the rest of the lake.

A large high builds over the plains tonight and shifts over the lake
Friday. Winds weaken and become easterly and southeasterly over the
southern two thirds of the lake while the northern third backs to
southwest through Friday evening. The center of the high will
continue east, but the larger ridge will remain over the lake
through the weekend.  Winds become southwest across the lake
Saturday with the highest speeds of 20 kt over the northern end of
the lake.  The theme is similar Sunday where strong southwest winds
will once again be over the northern end of the lake, but winds
become southeast over the southern half.

The next low forms over the plains Sunday night and the low is
progged to reach the lake Tuesday. High pressure follows the low.



IL...Freezing Rain Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     ILZ022-ILZ023-ILZ032-ILZ033-ILZ039 UNTIL 9 AM Thursday.

IN...Freezing Rain Advisory...INZ001-INZ002-INZ010-INZ011 UNTIL 9 AM

LM...Small Craft Advisory...NEARSHORE WATERS
     UNTIL 9 AM Thursday.




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