Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 110022

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
622 PM CST Mon Dec 10 2018

622 PM CST

Doing the dense freezing fog a bit earlier this evening. Area of
very low stratus appears to be building back to the surface as
dense freezing fog again early this evening over east central IL.
GOES-16 nighttime microphysics RGB shows this very low stratus and
dense fog beginning to advect north into southern Ford and
southern Iroquois counties.

While off an hour or two on the timing, the RAP is depicting this
scenario and then shows the same dense freeze fog moving east
into northern IN and clearing out southeastern CWA in a few hours.
This scenario is quite plausible and strongly considered waiting a
bit and watching trends before issuing a dense fog advisory, but
thus far GOES 1-min imagery shows area expanding and back edge not
moving much, so think the course of least regret is to issue the
dense fog advisory now and if things clear out, cancel the
advisory early. Given how dense the fog is and the freezing/icy
potential (KCMI down to M1/4SM with an RVR of 1200ft plus 0.01"
of ice accretion) feel the best course of action is to get
headline out now.

Updated forecast products and the advisory will be out shortly.

- Izzi


239 PM CST

Through Tuesday...

Tonight and tomorrow appear to bring a continued stretch of quiet
conditions with southwest flow between a surface low moving across
the Great Lakes and a ridge stretching from the western Gulf across
most of the eastern CONUS.  The only question and concern would be
possible redevelopment of fog and low stratus.  The most favorable
areas for this would appear to be in the some locations that took so
long to clear today.  This would be especially true farther south
nearer the ridge axis where winds are lighter, high temperatures did
not get as warm today, and remnant boundary layer moisture is more
abundant.  Opted not to include mention of this in the afternoon
update but this potential will continue to be monitored.

If conditions manage to stay clear, most locations should reach the
30s tomorrow with southern portions of the forecast area approaching
40 degrees.



205 PM CST

Tuesday night through Monday...

Tuesday night into Wednesday, an elongated shortwave trough is
progged to move from the northern Great Plains across the Upper
Midwest. Models have come into better agreement with the wave
closing off at 500mb by Wednesday morning and tracking across far
southern Wisconsin. Models indicate the axis of highest QPF will
occur north of I-80, if not north of the IL/WI state line.
Forecast soundings continue to indicate additional concerns that
will need to be worked out. Anticipate a nearly isothermal layer
right around the 0C isotherm from the surface to around 925mb
early in the day Wednesday which will result in some p-type
concerns and uncertainty. In addition, there are several periods
during the event where saturation is not sufficiently deep to
introduce ice crystals into the column and may result in drizzle.
Will continue to mention the possibility of a wintry mix on
Wednesday. Some areas may see a dusting of snow, and will have to
keep a close eye on pavement temps during any periods of liquid

Further out, guidance continues to advertise a deep upper low
developing across the southern Great Plains late Thursday, though
models continue to have very large differences in how this low
evolves heading into the weekend. By late Friday evening, for
example, guidance ranges from New Orleans to Cleveland with
respect to the 500mb closed low position. The ECMWF has had a
consistent southerly track for several days now, however, both
the GFS and ECMWF have trended towards each other as of the latest
12Z cycle. Regardless of continued tracks differences, it appears
there will be a decent chance for warm advection driven rainfall
later in the day Thursday into Friday. Warm air wraps around the
low and is cut off from the colder air over Canada, so event may
stay primarily rainfall throughout. There remains uncertainty in
how far north precip will continue through the remainder of the
day Friday into Saturday, and much of that precip may stay to our
south if the southerly tracks verify. Expansive area of high
pressure is expected to build in behind the low early through the
middle of next week bringing a stretch of more quiet weather to
the region.



For the 00Z TAFs...

Stratus and fog is once again present this evening, however, is
mainly to the southeast of most of the terminals and should
remain that way tonight into Tuesday morning. Am monitoring a
small area of IFR ceilings though, which may briefly impact GYY
over the next couple of hours. At this time, don`t anticipate this
area to expand and impact the other terminals but should mainly
continue to drift to the northeast. The steady southwest winds in
place this evening will persist through the period.






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