Area Forecast Discussion
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FXUS63 KLOT 221204 AAA

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
604 AM CST Sun Jan 22 2017

600 AM CST

Dense fog remains thick across most of the CWA, including multiple
reports of the visibility of 100 ft or less. This includes along
Interstate 88 in DuPage County and I-55 in northern Will County.
There is however a patch of improved visibility centered from
Waukegan southward along I-294, including ORD and MDW. That area
which is under low stratus has dense fog on both sides (more of a
radiation fog to the west, more of an advection fog to the east).
So would expect some of that area to fill back in with fog
shortly after sunrise. Given that it is a small area have made no
changes to the advisory early this morning.

Air temperatures of 28 to 32 at airports across north central
Illinois into the far west metro indicate some possibility of
slick spots, though pavement temperatures at a couple sites
available to us from McHenry County are mainly at mid to upper



324 AM CST

Through Today...

The Dense Fog Advisory has been extended until noon across the
CWA, with some locations seeing visibility of less than 100 yards
based on reports and webcams.

The dense fog is being observed areawide this morning within a
broad trough left from a dissipating low across the Upper
Peninsula of Michigan, and dew points at or above normal high
temperatures this time of year. The thicker depth of the stratus
on the ground is indicated via satellite from Waukegan to Peru
and southeast as of 3 a.m. To the northwest of that line,
visibility has dropped back down with more of a radiation fog
component. This caused temperatures to drop to 30-32 in some
locations of north central Illinois, and have highlighted
scattered slick spots on elevated roadways in that portion of the
advisory and in messaging. Some of these temperatures near
freezing could reach the western and northern Chicago suburbs by

The wind flow in the low-levels will start to become light
northeast shortly after daybreak in response to a deepening potent
surface low across the lower Mississippi Valley. Synoptically,
this is not favorable to dissipate a very strong inversion to
clear the fog out quickly, and climatologically, dense fog at
daybreak this time of year tends to stick around through the
morning. So have felt comfortable extending the advisory, though
it is likely some parts of the CWA will lose the widespread nature
of the dense fog prior to noon. Increasing north-northeast flow
off of Lake Michigan this afternoon may keep advecting marine fog
into shore areas. Will allow day shift to see if that

Given the fog and stratus, have undercut guidance temperatures
some for today and may not have enough given what should be a
minimal temperature climb at best during the morning. Highs will
still be above normal for this time of year.

A 700-850mb baroclinic zone stretched from southwest-to-northeast
over the CWA today will strengthen due to the system to our south.
Isolated rain showers are possible within the frontogenetic
circulation by late morning, mainly along/south of a Peru to
Chicago line. Any activity should start shifting southeast during
the mid to late afternoon.



358 AM CST

Sunday Night through Saturday...
Main forecast concern for the long term forecast period will
focus on the details of the system expected to bring the next
significant chance of pcpn Tuesday night into Wednesday.

However, for the beginning of the period beginning Sunday night,
the overall pattern will be undergoing some major changes from
the high amplitude, short wavelength, quickly progressive pattern
to a more broad, high amplitude, longer wavelength, slowly
progressive pattern. By Monday night, the deep upper low and sfc
reflection will quickly move through the lower Mississippi Valley
to the mid atlantic region by Monday night. A weaker, nrn stream
shortwave will lift through the western Great Lakes. As the
northern stream system progresses, temperatures will drop off some
from the recent unseasonable warmth, but temperatures into early
next week will still remain above normal, with highs generally in
the lower 40s. Some lingering showers will be possible over the
sern portions of the CWA as the sfc low associated with the srn
system lifts through the Tennessee Valley, with an inverted trough
extending north across the IL/IN border and Lake Michigan. This
inverted trough will provide enough forcing, though weak, to focus
some shra. While the environment will still remain relatively
moist, with pwats expected to still be 1/2 to 3/4 inch, the
forcing will be weak, so pcpn amounts should be minimal and not
aggravate the ongoing river flooding issues which have been caused
by localized ice jams and moderate rainfall this past week.

Forecast focus will quickly shift to the potential for the first
accumulating snowfall in quite some time.  The system in question
will be a northern stream shortwave expected to drop out of the
northern Rockies Monday night and track across the plains Tuesday.
The longer range models are in general agreement on the
development of this system, however, there are significant
timing and intensity differences among the various models. The NAM
has been a major outlier in handling this system, keeping only a
weak open wave, so has been generally disregarded. The SREF, GFS
and ECMWF are clustering around a reasonably similar solution
while the GEM has a much more southern track and is considerably
weaker. So, for the latest forecast update, have generally
followed the more consistent GFS/ECMWF/SREF idea of tracking the
upper low across the upper Mississippi Valley Tuesday night and
Wednesday. The associated sfc reflections from these models are
not quite as consistent as the advertised upper level patters
would suggest. The GFS is trending much quicker than the
ECMWF/SREF solutions Tuesday night, which will have an impact on
timing of the onset of pcpn as well as p-type in how quickly deep
layer cold air can move into the region. Have trended the latest
forecast on the slower ECMWF/SREF sfc low, with the GFS seemingly
too fast considering the consistency among the models with the
upper level feature. So, the net impact should be to spread some
light rain into ncntrl IL by Tuesday afternoon. Into the evening,
pcpn should quickly change over to a rain/snow mix for locations
north of the I-80 corridor Tuesday night, while locations to the
south should see all rain while the colder air slowly filters
across the area. Through the day on Wednesday, p-type should still
be mainly liquid, as sfc temps will still be too high to support
anything much more than some snow mixing in with the rain. Current
indicators would suggest that the best opportunity for any snow
accumulation will likely be closer to the IL/WI border as the
preferred solutions track the sfc low invof the border, which is
not particularly conducive to snow for much of the CWA. While the
nrn tier counties will be most likely to see accumulating snow,
amounts are still highly uncertain as temperatures should still be
marginal and much of the snow generation will have to rely on
upper level forcing and dynamic cooling.

For the latter portions of the long term forecast period, the trend
should be back to more seasonable temperatures as a series of nrn
stream shortwaves help carve out a broad upper trough over the nrn
2/3 of the CONUS with upper ridging building over the west coast.
This will set up a an extended period of deep layer cold advection
in nwly flow aloft.  Max temperatures should be back in the lower to
middle 30s by Thursday and the upper 20s to lower 30s for Friday and
into next weekend.


For the 12Z TAFs...

A very challenging early morning TAF with dense fog and LIFR
conditions areawide as of 1130Z, with the exception in a swath
centered on I-294, or basically ORD to MDW, where visibility is
MVFR. This "bubble" of improved conditions for ORD and MDW is
between light northwest winds to the west, and light east winds to
the east, so do see fog trying to inch back in at or just after
sunrise. However, it is appearing more likely that it will not be
as dense of fog as seen earlier this morning. For that reason,
have gradually improved the TAFs, and may send an AMD closer to
13Z if conditions continue to indicate that trend sustaining

Otherwise, 1/4SM or less visibility is being observed at the other
TAF sites and especially for RFD expect that to continue for 3 or
even 4 more hours with high confidence. Gradual improvement should
happen across the area later this morning as a north to northeast
wind starts to increase. The ceiling improvement is likely to be
slower than the visibility in that time. The northeast winds
could continue to steer marine fog into GYY and possibly keep MDW
and ORD at some reduced visibility into the afternoon. Confidence
is low on how far inland that would extend.

Isolated light showers are possible between 16Z and 22Z, but
better chances are south of the TAF sites.

Northerly winds should hold up at 7-10 kt tonight and
climatologically that should prevent LIFR conditions, although
models are indicating low conditions returning. Don`t have the
greatest feel, but kept conditions elevated.



324 AM CST

Fog, much of it likely dense, continues over the lake this
morning. Northerly flow will steadily increase from late morning
into the afternoon, and should at least push/dissipate some of the
fog from north to south into the afternoon. The northerly flow
will further increase tonight into Monday, and a Small Craft
Advisory is likely for the Indiana shore for Monday and Monday

After a weak ridge passes over the lake on Tuesday during the day,
strong low pressure passing over or just south of Lake Michigan
will bring increasing northeast winds shifting northwest during
midweek. While this event presently does not have a gale
magnitude with it, it does look it will have 30 kt winds at times
and Small Craft Advisory criteria in the Illinois and Indiana



IL...Dense Fog Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ023-ILZ032-ILZ033-ILZ039 UNTIL NOON Sunday.

IN...Dense Fog Advisory...INZ001-INZ002-INZ010-INZ011-INZ019 UNTIL
     NOON Sunday.

LM...Dense Fog Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-
     LMZ777-LMZ779 UNTIL 3 PM Sunday.




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