Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221935

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

235 PM CDT

Through Wednesday...

Main forecast concern will be the threat of fog again tonight
across northern IL, some possibly dense.

Skies area expected to gradually clear this evening as surface
high pressure builds into northern IL. Overall low-level moisture
continues to be elevated across the area, with dew points in the
mid 50s to low 60s across the area. With light winds under the
high tonight, expect temperatures to radiate rather efficiently
through the 50s. This should set up ample conditions for fog
again tonight as temperature fall through the cross over
temperature. I would not be surprised for a some areas of dense
fog development as well. However, with low confidence on the
extent of the dense fog, no advisory is planed at this time, but
could be needed for some of the area later.

Any fog and low clouds should dissipate across the area Wednesday
morning, with more sun likely by the afternoon. Expect warmer
temperatures, with highs in the 70s to near 80 inland, but
continued cool near the lake.



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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