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FXUS63 KLOT 110442

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1042 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 06Z TAFs...

Stratus and fog will remain to the southeast of the terminals this
morning with mainly clear skies expected for much of today. Some
haze or fog will remain this morning, but don't anticipate vis to
fall too low. The steady southwest winds in place will persist 
through today, but then become more south southeast tonight. The 
end of the period will remain dry, though some precip chances 
return after the period, early Wednesday morning. VFR ceilings 
return late in the period, and once again, will likely see some 
lowering right after the current forecast period.



IL...Dense Fog Advisory...ILZ033-ILZ039 until 3 AM Tuesday.

IN...Dense Fog Advisory...INZ010-INZ011-INZ019 until 3 AM Tuesday.




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