Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221729
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1129 AM CST Sun Jan 22 2017

.UPDATE...

1055 AM CST

I ended the dense fog advisory for all of my Illinois counties,
but have continued it through 18Z over Northwestern Indiana. Most
sites have visibilities above 1 SM. Conditions should continue to
improve this afternoon. However, some fog, possibly dense may
continue this afternoon, especially near the shores of Lake
Michigan into Northwestern IN.

KJB

&&

.SHORT TERM...
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
daybreak.

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
materializes.

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.

MTF

&&

.LONG TERM...
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.

&&

.AVIATION...
For the 18Z TAFs...

Visibilities continue to improve across the area late this
morning. The only exception is areas along the lakeshore in
Northwestern Indiana, including KGYY. Light flow off the lake
will likely keep the visibility lower at KGYY this afternoon as a
result. Otherwise, CIGS should gradually improve into the 1,000 to
1,500 FT AGL range this afternoon. A rather potent area of low
pressure will track well south of the area this afternoon. The
main effect this system will have over the area is to produce a
continued north-northeasterly wind through the forecast period.

CIGs will likely not improve much tonight into Monday. In fact,
CIGs could drop back into the IFR category tonight. There could
also be some additional areas of fog that develop again tonight.
However, with more of a wind component tonight, it appears the
threat of dense fog at the terminals will be low.

KJB

&&

.MARINE...
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
evening.

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
nearshore.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
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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