Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 122012

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

212 PM CST

Through Friday...

Quiet weather will start the forecast into the day Friday. A strong
northern stream trough across Ontario will shift rapidly east
tonight with quick height rises and associated building of the
arctic surface high. This high center is forecast to strengthen
to near 1050mb near Minneapolis later tonight, with a tightening
pressure gradient on its front side sustaining northwest winds
over the area. In fact, gustiness should become a little more
prevalent this evening, ushering in wind chills into the single

While the low clouds will shift slowly southeast this evening,
transient mid and high clouds are expected tonight. This may help
offset the cooling somewhat. If not though, lows could end up
further down into the single digits across more of northern

Winds will turn northeast on Friday as the high center passes to
our north. Mid-level clouds should be expansive possibly with
some virga in northern locations. Highs should again be in the 20s
for most of the area.



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 nuisance 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 18Z TAFs...

The concerns with the aviation forecast for this TAF period are
MVFR cigs through mid-afternoon, with patchy 900 ft cigs drifting
past too, and winds bending back to the north by daybreak Friday
and to the northeast shortly after.

Northwest flow is continuing to supply MVFR cigs over the area
with a back edge in these noted along a line from Milwaukee to
Moline. This should continue southeast through the afternoon.
Confidence is fairly high in cloud bases remaining between 900 ft
and 2000 ft until the low clouds scour.

Northwest winds should inch up a tad after dark as the pressure
gradient tightens due to the magnitude of the arctic high to the
north. There could be some occasional gusts. As the high builds
southeast from the Upper Midwest, the surface winds will turn
toward due north by sunrise Friday with speeds likely around 10
kt. Confidence in these winds is high. The direction will inch
northeast through the morning and specific hourly details in that
will be refined into tonight.

Some lake effect MVFR clouds may be drawn back over the Chicago
TAF sites during the day, but confidence in this is low.



212 PM CST

A quick-moving surface low across southern Ontario this evening
will be follow by a big-time arctic high building into the Upper
Midwest. The brief but sharp pressure gradient increase in-between
these features should sustain west to northwesterly gales across
the far northern part of the lake and freezing spray. Moderate
freezing spray is probable further south as well.

After the high center moves across the northern lake Friday
evening, another fairly stout high is predicted to build across
the western Great Lakes by Saturday afternoon and night.
Eventually this gives way to a deepening low pressure system
across the southern Plains on Monday night. This low should track
to near Lake Michigan by Tuesday night or Wednesday.



     3 PM Friday.




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