Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 290151

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
751 PM CST Mon Nov 28 2016

751 PM CST

For evening update:

Had bumped temps a bit this evening ahead of approaching cold
front, and adjusted hourly pops/wx and thunder potential a bit
based on observational trends. Also bumped winds up (especially
gusts) based on trends through early evening and forecast

Vigorous mid-level short wave was noted propagating across the
mid-Mississippi Valley this evening per GOES vapor imagery. Strong
mid-level height falls on the order of 150 meters (in past 12
hours) were overspreading low-level warm sector air mass
characterized by temps and dew points in the lower 50s ahead of an
occluding cold frontal boundary moving east across the Mississippi
River region. While a narrow (mostly) dry slot had developed over
parts of northern/northeastern IL late in the afternoon, the
approach of the mid-level vorticity center and steep mid-level
lapse rates in excess of 7 deg C/km were aiding renewed
enhancement of rain and embedded convective elements over north
central IL and southern WI over the past couple of hours. This
rain will spread northeast across much of the IL portion of the
cwa in the next few hours, with a few rumbles of thunder and a
period of heavier rain/showers through midnight. Precip looks to
quickly diminish from the west after midnight as the mid-level
vort and surface occluded front move through the forecast area.

South-southeast winds had increased late this afternoon across the
region in response to surface pressure falls with the approaching
strong short-wave, with several sites gusting around 30 mph at
times. Winds should decrease a bit late this evening with the
approach of the frontal trough, though forecast soundings suggest
a period of gusty southwest winds will develop again behind the
front as slight cold advection steepens lapse rates within a
deeper surface-based mixed layer during the pre-dawn hours. The
south winds ahead of the front have allowed temperatures to rise
slightly into the 50-52 degree range in most locations through the
early evening, and should remain above 50 until cold fropa



257 PM CST

Through Tuesday evening...

A broad area of showers has spread across Illinois and Indiana
today ahead of an approaching frontal zone that is expected to
move through late this evening. These showers are associated with
a rich plume of moisture being advected off the Gulf between the
approaching front and a center of high pressure located just off
the mid Atlantic coast. Upper level support for the showers is
seen in several bands of shortwave energy, depicted by multiple
deterministic models. There has been a question whether the gap
between shortwaves aloft as seen in the models would be reflected
in a precip gap at the surface, and this seems to be what has
evolved from the area between RFD and UIN. Additional showers and
thunderstorms have been trying to develop across eastern Iowa in
closer proximity to the front, but these have been dissipating as
they approach the local area. Will thus continue the trend of a
short break in the precip for some areas this evening, then
increasing shower chances as the front moves through.

Rapid clearing behind the front, but persistent southerly flow,
will support very mild temperatures tomorrow before the next
frontal zone approaches later in the evening.



350 PM CST

Tuesday night through Monday...

Model guidance is largely dry Tuesday night, but there is a strong
vorticity maximum rotating ahead of the large upper trough to our
west, and the NAM and a few SREF ensemble members are breaking out
some light QPF with some steep elevated mid level lapse rates.
While not expecting much, could not argue against the slight
chance pops for a brief shower or storm in spite of the global
guidance and model blend largely dry with good upper support and a
pronounced surface boundary.

This leading cold front will move through the area Wednesday
leading to a largely dry though cooler day. The upper low will
pinwheel closer to the area late Wednesday and Wednesday night,
and there will likely be some shower activity with it. This is
usually favored during the day time and the low will be coming
through in the evening and overnight. Rain/snow shower mix would
be expected with profiles largely supportive of snow falling
overnight into the morning, though surface temps are above
freezing. There could be some flurries as well Thursday morning.
The upper low will pull away Thursday night. The coolest air mass
arrives Friday/Saturday but little support for precip. Highs
struggle for 40.

Models are not in good agreement Saturday night into Sunday, but
all do suggest some precipitation and active weather, but are
vastly different with any possible phasing of the southern and
northern stream energy. At this point the best bet is to put some
chance pops in for snow.



For the 00Z TAFs...

Any diminishing trend in the current showers across the area has
since ended, with showers continuing to impact all the terminals
through late this evening. Any thunder chances in the near term
will remain well west of the terminals in Iowa but as a front
approaches later this evening, there may be a few isolated rumbles
of thunder across northern Illinois. Given the low confidence,
have continued to exclude any mention of thunder in the tafs. All
precip will end by the 3-5z time frame, with dry conditions then
expected into Tuesday. Ceilings are hovering right around 1KFT,
but anticipate them to lower to IFR over the next hour. With lower
confidence of LIFR ceilings occurring and with the brief window
of possibility, now only mention LIFR ceilings in a tempo. It`s
possible that LIFR does not even occur, before FROPA late this
evening quickly scours out any low clouds. Winds will continue to
be just east of south in the 150-170 range this evening, and then
slowly become more south southwest. A brief increase in gusts may
occur with FROPA tonight with gusts possibly getting into the low
30 KT range. However, winds should settle back down soon there



350 PM CST

Left the headlines alone, in spite of the fact that there will winds
will likely waver between gale and sub gale levels through
tonight and into Tuesday. Strong low pressure across the upper
Midwest will begin to weaken tonight. A secondary low will develop
ahead of this stronger low and pass over the lake tonight.
Occasional SE gales will slacken as the low passes over the lake,
but expect impressive pressure rises behind this secondary low and
a shift to gusty southwest winds overnight into Tuesday,
slackening from southwest to northeast.

The low across the upper Midwest will slowly drift east through
midweek and maintain southwest winds. A leading cold front will
pass over Lake Michigan Wednesday but with little wind change, but
a secondary cold front will come through Thursday as the low will
move east to Quebec and will maintain 15-25 kt winds but shift to
more of a WNW direction. Strong high pressure will move south of
the lake this weekend. Another low is expected on Sunday which
could have adverse impacts on Lake Michigan, but at this point
models are not in very good agreement as to the strength and
location of this system.



     AM Tuesday.

     Gale Warning...LMZ777-LMZ779 UNTIL 6 AM Tuesday.




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