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FXUS63 KLOT 221130

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
630 AM 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 12Z TAFs...

Fog and low ceilings continue to be the main challenge this
morning. Weak area of low pressure is shifting east of the
terminals this morning leaving a very weak northerly gradient in  
its wake. Fog is expected to linger over the next couple hours 
bottoming out in the 1/4-3/4SM range for much of the area. Fog 
should lift with visibility quickly improving by mid morning but 
low ceilings holding on with only a very gradual improvement 
through the day. VFR becomes increasingly likely by late afternoon 
or evening though there is low confidence in exact timing. Winds 
should favor a NNW direction this morning behind the departing 
low, then will turn northeast this afternoon and evening behind a 
lake breeze. As high pressure builds across the region overnight, 
winds will drop off and become light and variable or calm. Fog 
becomes a concern again for tonight into Tuesday morning as 
favorable radiational cooling conditions develop. If skies clear 
earlier than expected today, this may help limit fog potential 
overnight, otherwise saturated ground conditions and cloud cover 
lingering later into the day limiting heating would favor denser 
fog tonight. 



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 



IL...Dense Fog Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ023-ILZ032 until 10 AM Tuesday.

IN...Dense Fog Advisory...INZ001-INZ002-INZ010-INZ011 until 10 AM 

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




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