Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221712

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1212 PM CDT Tue May 22 2018

258 AM CDT

Through tonight...

Main forecast concerns/challenges are with dense fog this morning
across much of northern IL and northwest IN.

Weak surface low and associated boundary continue to slowly
progress northeast across the CWA this morning, with much of the
CWA right along and north of this boundary. Very moist low levels
and light winds have allowed the dense fog to persist this
morning, and even expand south/southeast into other areas. Vis of
one quarter mile or less will continue, with vis near zero still
possible in some locations. Despite this low continuing northeast
of the area over the next several hours, moisture never really
scours out and with the gradient not increasing, won`t likely see
much of any improvement early this morning. May actually see
further south/southeast expansion of this dense fog into remaining
areas of the CWA on the backside of this low, south of the
Kankakee river valley. Will continue to monitor this possibility,
and add to current Advisory if needed. Current end time, mid
morning, of the Dense Fog Advisory looks to be on track at this

With weak ascent overhead, isolated showers will remain possible
over the next couple of hours. Do think any development will
remain rather isolated though, with much of the area remaining dry
early this morning. Low will depart this morning, however, weak
trough will remain situated on the backside of this low and this
could support some weak isolated showers later today mainly over
the far southern CWA. Have not included any mention in the
forecast, given how low chances appear at this time. This trough
quickly shifts out of the area today though, with high pressure
building across the region. Cloud cover will be a challenge later
today into tonight. Although latest guidance would suggest
clearing skies later this afternoon into tonight, not completely
confident with this solution. Did trend this way in the grids, but
could see clouds lingering tonight and fog once again developing.
Building high pressure would support a setup for any cloud cover
that does not scatter today, to likely remain and expand tonight.
Will need to monitor trends and latest guidance and adjust as



319 AM CDT

Wednesday through Monday...

High pressure will remain in place across the region at the start
of the period with Wednesday expected to be dry. A warming trend
is expected Wednesday, but will monitor any remaining cloud cover
tonight into Wednesday that could possibly impact high temps.
Overall trend into the end of the work week will be upper level
ridging with a continued warming trend. Some guidance indicating
the possibility for some weak mid level energy to ride along this
ridge Wednesday night into Thursday. If this were to occur, some
isolated precip development could clip areas in far northern IL.
Have maintained slight chance pops, but think the higher chances
for precip will remain north of the CWA. By late Friday into the
start of the weekend, the ridge breaks down as upstream energy
dips down across the region. Pattern going into the weekend would
then support periodic and scattered shower/thunderstorm
development with highest chances likely during the
afternoons/evenings, and above normal warmth.



For the 18Z TAFs...

Conditions continue to gradually improve across the area early
this afternoon. Overall, expect clouds to begin to scatter out
across the area over the next couple hours with a period of VFR
possible by mid to late this afternoon. However, forecast concerns
quickly focus on the threat of low conditions in fog again tonight
into early Wednesday morning.

A surface ridge of high pressure will build over the area for
tonight. While winds will initially be northeasterly this
afternoon, expect winds to abate quickly during the evening before
becoming light and variable overnight. With clear skies this
evening and ample low-level moisture remaining in place, expect
fog to develop across the area again sometime mid to late this
evening and continue overnight. There remains uncertainty in how
low conditions will get at the main Chicago terminals, but the
potential is there for another period of dense fog before
conditions quickly improve Wednesday morning. At the present I
have gone as low as 1/2 mile at DPA, but only down into the 1 to 2
mile range at the main Chicago terminals. We will have to watch
for a period of lower visibilities late tonight.

VFR conditions will return to the area by late Wednesday morning.
Otherwise, expect light easterly winds during the afternoon.



241 AM CDT

A weak low will move from northern Illinois to the eastern Great
Lakes today while a weak ridge will remain over the northern Great
Lakes region and consolidate over the western Great Lakes behind
the departing low later today. This high will then gradually shift
east to the eastern Great Lakes by Thursday. Weak flow will be in
place across Lake Michigan through this time frame. Fog will also
continue at times over the next 24-48 hours. A dense fog advisory
remains in effect for southern half of Lake Michigan through mid
morning. It`s possible it may need to be extended later into the
day. Late in the week, low pressure will form over the Canadian
Prairies and dig into the Upper Midwest, though is progged to
weaken during this time. Fresh southwest flow will develop,
especially over the north end of the lake. Winds are expected to
drop off again over the weekend as an expansive high builds over
Hudson Bay with ridging extending into portions of the Great



LM...Dense Fog Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-
     LMZ777-LMZ779 until 4 PM Tuesday.




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