Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 112111

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
311 PM CST Wed Jan 11 2017

311 PM CST

Through Thursday...

It is a very busy and complex forecast for the area this
afternoon through Thursday morning. The main hazards and
challenges, however, will be focused on the dense fog currently
ongoing across much of the area, and will then shift to the threat
of freezing precip this evening and tonight.

Dense fog very quickly overspread much of the area this afternoon,
and this prompted the issuance of a dense fog advisory, for all
but the far northern tier of counties in Illinois. While dense
fog will likely linger over my southern CWA through much of the
evening, before improving overnight, it appears that areas along
the I-80 corridor will see improvement during the early to mid-
evening as a cold front gradually sags southward over the area.
For this reason, I will allow the fog advisory to end for all but
my far southern counties by 03Z this evening. The fog advisory
will end in my far south by 06Z.

The next issue then continues to be the threat of freezing
precipitation tonight into Thursday morning. The surface cold
front is currently shifting into far northern Illinois. This is
allowing the winds to shift northerly, and with subfreezing
already shifting over southern Wisconsin, temperatures will likely
drop below freezing by early this evening over far northern
Illinois. This will set the stage for a period of freezing rain
this evening as a band of frontogenesis strengthens over the area
in response to the approaching weather disturbance. It still
appears the main threat for freezing rain through 9 PM will be
north of I-88. In this area some minor ice accumulations up to a
tenth of an inch will be possible.

As the freezing temperatures sag southward through the evening,
areas in and around the main Chicago metro area will likely begin
to transition over to either some light freezing rain or freezing
drizzle. Drier air in the mid-levels is still expected to
overspread the area by late evening, and as this occurs, the rain
should begin to lighten or be reduced to freezing drizzle. As a
result, lighter ice accumulations are expected south of I-88,
however, even a small amount of ice can cause significant issues.
For this reason, I opted to expand the freezing rain advisory to
include Lee county in north Central Illinois and also Cook and
DuPage counties in northeast Illinois. I also extended the
advisory through early Thursday morning. This was done to cover
the threat for periods of freezing drizzle over the area
overnight. I decided to hold off on expanding the advisory south
of I-80 to let the evening shift get a look at the latest
observational trends. However, they may end up having to include
much the rest of the area for the overnight period as freezing
drizzle could become the precip type there late tonight into early

Another mid-level disturbance is expected to push across the area
Thursday morning. This could result in a period of light snow or
sleet as some deeper moisture moves over the area. This could
result in some minor accumulations. Otherwise conditions should
improve Thursday afternoon.



236 PM CST

Thursday night through Wednesday...

An active pattern is shaping up for the extended forecast period
with main concerns focusing on pcpn type and timing.

For the early portions of the extended period, from Thursday night
and into the weekend, a fast, zonal pattern aloft will develop over
the upper midwest and Great Lakes region between a strong upper
ridge parked over the sern CONUS and ern glfmex, a strong upper low
moving through the Canadian Maritimes and a deep closed low dropping
south along the west coast toward the desert southwest.  The main
weather concerns will come from a series of weak shortwaves moving
through the fast upper level flow.  Consequently, timing of weather
impacts from these waves is low confidence given the inherent
differences among guidance sources, especially with the speed and
exact track of the individual waves.  Expect conditions across the
CWA will remain dry through Friday night and into Saturday, though a
fast moving nrn stream shortwave could bring a little light snow to
locations along the WI border.  Into Saturday, a weak srn stream
shortwave is expected to track along the nrn periphery of the upper
high, tracking across srn IL/Ohio Valley.  This particular system is
tricky because sfc temperatures will start out the day below
freezing and then rise into the lower 30s through the day.  The
marginal temperatures would suggest that pcpn could range from light
snow over the nrn portions of the CWA to a rain/freezing rain mix
over the south, and then tapering off to some light freezing rain
overnight.  Again, this is a low confidence forecast as moisture
will be very limited, low level temperatures will be marginal,
hovering right around the freezing mark and timing of the short wave
energy lifting out of the desert southwest as the closed upper low
drops into nrn Mexico.  This trend will continue through the weekend
and into Sunday with the local area remaining on the nrn periphery
of a series of weak srn stream shortwaves tracking across the srn
portions of the CWA or just south of the CWA.  Will generally
maintain low chance or slight chance PoPs for Sunday into Sunday
night, with temperature profiles still suggesting a chance a wintry
mix of pcpn.

Anticipate increasing pcpn chances into Monday and early next week
as the closed upper low begins to lift newd, possibly reaching the
cntrl plains as early as Monday morning.  While have some concern
over the timing of the increasing PoPs as the models have a tendency
to be too quick in lifting these swrn closed lows, feel relatively
confident in the increasing PoPs, just have some concern that the
forecast may be a bit too fast.  Regardless of any minor timing
issues among the longer range models, the general trend shaping up
for early next week is for wet, with periods of rain, a rain/snow
mix/or short periods of all snow.  With a building upper ridge
expected to develop over the cntrl plains/MS valley in advance of
the low, temperatures should be relatively moderate at above
seasonal normal levels.  P-type trends will likely be strongly
diurnal with daytime highs in the lower to middle 30s on Sunday,
rising generally into the the 40s for Monday through Wednesday.
Overnight lows should generally be above normal as well.  The most
troublesome temperature trend will be late Monday night into Tuesday
as strong warm advection should set up in advance of the upper low
kicking out of the southwest with temperatures overnight remaining
well above freezing.


For the 18Z TAFs...

A messy weather period is on tap for the area this afternoon
through Thursday. The main aviation weather concerns are:
1.) Low CIGS this afternoon, likely to fall into or near LIFR by
late afternoon.
2.) Lowering visibilities in fog and develop showers this
3.) Small potential for some thunderstorms...mainly south of the
terminals late this afternoon into the evening.
4.) Periods of freezing Rain and/or Drizzle developing from north
to south this evening.

A cold frontal trough will set up over the area through the
afternoon. This will allow winds to switch northerly over far
northern Illinois later in the day. As this occurs, much colder
surface temperatures will undercut some warmer temperatures aloft,
and set the stage for some freezing rain this evening as
precipitation develops over the area. The main question is how
quickly the subfreezing temperatures get to each terminal for a
change over to freezing rain at the surface. Currently it appears
KRFD will change over the freezing precip by 00Z as the front
passes. However, it will take some time for the freezing
temperatures to reach the eastern terminals this evening. Because
of this, the freezing rain or freezing drizzle potential may hold
off for a few hours this evening. It also appears that drier air
aloft will begin to filter in over the area by later this evening.
As a result, the precipitation could end up being more of a
freezing drizzle over many of the eastern terminals. Either way,
it looks messy. The freezing drizzle could continue at times
through the night. Another disturbance could then produce another
short period of light snow or sleet over the area Thursday
morning, but confidence on this is low at this time.

Some afternoon and early evening thunderstorms will also be
possible today, but the best chance of this should remain south of
the terminals. Otherwise, expect very low CIGs and lowering
visibilities to develop north over the area within the next couple



219 AM CST

Headlines...Kept all hazards as is, and expecting to let the small
craft and gale headlines to be expired on time this morning. Issued
another small craft advisory for mid morning through mid afternoon
for hazardous wind gusts. May need another small craft tonight for
waves, but am not confident enough to issue the SCY just yet.

Southwest winds quickly become southerly this morning and increase
to 30 kt. A low pressure trough sets up over the far southern end of
the lake this evening. Winds become north to the north of the trough
and north winds are expected across the lake overnight. Another
small craft may be needed as waves build over the southern end of
the lake.

The low pressure trough shifts east as a weakening low moves over
Ontario Thursday.  Winds back to west, and speeds increase to 30 kt
over the northern half of the lake as the low passes by.  The low`s
cold front swings through in the evening. A large high builds over
the plains Thursday and then spreads over the lake Friday. Wind
become east under the high except for southwest winds over the
northern end of the lake.  A low forms over the plains Sunday night
and it is expected to reach the upper Great Lake region Tuesday.



IL...Dense Fog Advisory...ILZ010-ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-
     ILZ020-ILZ021-ILZ022-ILZ023-ILZ032 UNTIL 9 PM Wednesday.

     Freezing Rain Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     ILZ011-ILZ012 UNTIL 9 AM Thursday.

     Freezing Rain Advisory...ILZ010-ILZ013-ILZ014...9 PM Wednesday
     TO 9 AM Thursday.

     Dense Fog Advisory...ILZ033-ILZ039 UNTIL MIDNIGHT Thursday.

IN...Dense Fog Advisory...INZ001-INZ002 UNTIL 9 PM Wednesday.

     Dense Fog Advisory...INZ010-INZ011-INZ019 UNTIL MIDNIGHT




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