Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 132116

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
316 PM CST Fri Jan 13 2017

230 PM CST

Through Saturday...

The forecast message remains similar for northern Illinois and
northwest Indiana tonight. There is a chance of light freezing
precipitation in the southern third, or basically along and south
of U.S. Highway 24 (Pontiac to Rensselaer). If this materializes
it would seem to be spotty between mainly midnight and 9 a.m., and
impacts look limited enough to just handle with a Special Weather
Statement for the far southern three counties noting potential
light freezing precipitation.

Modified arctic high pressure of 1045 mb centered across Wisconsin
will slowly weaken through Saturday but hold its own, with
northeast winds supplying some dry lower level air (most
pronounced between 875-950mb). This will limit some virga/flurries
along the WI/IL border late this afternoon, and also the northern
extent of light precipitation tonight as isentropic ascent and
expands across central Illinois. Lift and saturation are limited
in the mid-levels/ice presence zone, and should rule out snow for
a weather type tonight. Also, a fair amount of precipitation in
the upstream moisture feed could cut off a better moist fetch too.
So if precipitation can reach into the southern forecast area
tonight and Saturday morning, it likely would be light spotty
freezing rain, with possibly ice pellets at first due to wet bulb
cooling in the lower levels.

Clouds will continue further north with a dry forecast north of
I-80. Temperatures may drop a few degrees initially this evening
but with thicker cloud cover they should hold steady the rest of
the night. Saturday should see highs around 30 for most of the



314 PM CST

Saturday night through Friday...

Main concern for the long term forecast period will be the timing,
coverage and duration of a wintry mix of precipitation shaping up
for Sunday and into early next week.

Following a brief period of quiet weather, conditions will become
unsettled again, with a period of wintry pcpn setting up.  By
Saturday night, the first round of wintry mix pcpn will have come to
an end as the weak nrn stream shortwave energy moves off to the
east.  The trends for longwave pattern aloft have remained
consistent in the idea of building upper ridging over the cntrl and
ern conus with a closed low over nwrn Mexico beginning to kick out
to the northeast.  While the general trends of the models have
remained consistent, they are showing diminishing agreement as the
GFS wants to weaken and shear out the upper low and track it more
quickly into the midwest by Monday night and early Tuesday, while
the ECMWF has remained more consistent with previous runs, with a
slower, more nnely track of the upper low.  What is even more
troubling is that the GFS now wants to phase the srn stream system
with a nrn stream shortwave dropping out of Canada and merging into
a stronger upper low over the upper Midwest on Tuesday.  This is a
significant departure from yesterday`s runs for the GFS.  The ECMWF
is also trying to bring some nrn steam energy into the cntrl plains,
but exhibits far less phasing of the nrn and srn streams, and is
initially much slower with the system lifting out of the southwest,
but does accelerate the system as nrn stream shortwave energy digs
into the nrn plains.  Generally tend to prefer the ECMWF solution as
the GFS is such a departure from previous runs and the tendency for
the models to be too fast in kicking out southwestern closed lows.
So, even with some decreasing forecast confidence in the timing of
pcpn moving into the area, have trended the extended forecast
closer to the slower ECMWF previous guidance that would suggest a
slower solution with the 12z GFS looking a bit suspicious.

So, for the long term forecast period, the general trend for the
pattern will still be favorable for a wintry mix of pcpn to move
into the swrn portions of the CWA through the day on Sunday, with
pcpn potential ramping up quickly Sunday afternoon and evening.
With fast swly flow aloft developing, strong warm/moist advection
will develop.  The air mass should be cold enough initially to
support all snow, but warmer air should quickly overrun the cold
air the sfc, setting up a more favorable scenario for freezing
pcpn.  Exact p-type is a concern, with any light snow quickly
transitioning over to freezing rain, sleet or a mix.  Monday
morning is still expected to start out sloppy with freezing rain or a
rain/freezing rain mix, but with very strong warm advection through
a deep layer, and even sfc temps likely to rise well into the middle
to upper 30s to near 40 F by noon, pcpn will quickly change over to
all rain across the CWA. It`s a bit too early to entertain the
idea of issuing any type of headline product, a Winter Storm
Warning or Winter Weather Advisory will likely be needed.  Given
the strength of the warm advection and the unseasonably warm air
being tapped to move in, have some concern that the trend of the
change- over from freezing to liquid pcpn may be too slow and that
pcpn could transition to all liquid durg the morning hours. Expect
that pcpn will begin to taper odd by Tuesday afternoon or early

Temperatures will trend to well above normal by Monday, with highs
expected to be in the upper 30s to middle 40s.  Highs Tuesday should
be in the middle 40s to lower 50s.  A slight cooling trend is
expected for Wednesday as a series of weak nrn stream shortwaves
cross the midwest, but temps should trend higher into late next week
with much of the CWA seeing temps reaching the upper 40s to lower
50s again on Thursday and possible even higher on Friday as upper
ridging crosses the midwest.  Some guidance suggests that temps
could approach 60 F over the srn portions of the CWA, with highs in
the low 50s all the way up to the Wisconsin border.


For the 18Z TAFs...

Little for concern in this TAF period, with precipitation expected
to remain south of the TAF sites through Saturday.

Strong high pressure centered across Wisconsin will maintain a
north-northeast wind this afternoon that will become more
northeast by late in the day. Sporadic gusts to 17-20 kt are
possible this afternoon, namely at MDW and GYY based on
observational trends. The northeast winds had steered lake effect
MVFR clouds into GYY and MDW and near ORD, but these will be
fading this afternoon. Otherwise just mid-high clouds into

Wintry precipitation will lift northward across central Illinois
later tonight, but should remain well south of the TAF sites
thanks to low-level dry air. Areas of 4000-6000 ft cigs are
expected to develop over the airports late tonight and persist
into Saturday morning, with light and variable winds at the



230 PM CST

As high pressure moves from Wisconsin over the lake later tonight,
the northeast winds will lighten and become east. Waves should
slowly continue to ease along the Illinois and Indiana nearshores.
A quick-moving low across Canada will tighten the pressure
gradient across the northern part of the lake Saturday, with a
brief increase in gusts to around 25 kt near the northern
straits. A prolonged period of southerly flow is forecast Sunday
afternoon through most of next week (at times southwest and others
southeast). While gales are not presently forecast, there does
look to be possible 30 kt winds during midweek as low pressure
passes the western/central Lakes.






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