Area Forecast Discussion
Issued by NWS Chicago, IL

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373
FXUS63 KLOT 222039
AFDLOT

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

.SHORT TERM...
239 PM CST

Through Monday...

The main concerns again tonight will with fog trends, especially
over northwestern Indiana and near Lake Michigan.

A powerhouse storm system over the Lower Mississippi Valley, will
shift towards the Central Atlantic Coast by late Monday. This
system will remain far enough south of the area to prevent much
more than a passing shower tonight into Monday. However, this
system will continue to support a northerly wind across the area
through Monday. It appears that with a bit more of a wind
component tonight that the fog may not end up being nearly as
widespread as it was this morning. However, fog will still be a
possibility. It appears the areas that would be best favored for
dense fog will be portions of northwestern Indiana, especially near
the lake. Area weather webcams show quite a bit of dense fog over
the lake. As as result, with the continued northerly wind tonight,
this fog will likely be pushed onshore over northwestern Indiana.
With this in mind, we have opted to reissue the dense fog advisory
for Lake and Porter Indiana through early Monday morning. I don`t
have a good feel for areas farther south, so I have opted to hold
off issuing, but if trends support it, an extension to the
advisory may be needed this evening. Additional areas of fog will
again be possible over portions of north central Illinois as well,
but the extent of this fog is still somewhat uncertain.

Otherwise, expect mainly cloudy skies through the day Monday. The
morning fog should again abate during the morning. Highs are
expected into the lower 40s.

KJB

&&

.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 UNTIL 9 AM Monday.

LM...Dense Fog Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-
     LMZ777-LMZ779 UNTIL 9 AM Monday.

&&

$$

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