Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230735

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
235 AM CDT Wed May 23 2018

234 AM CDT

Through tonight...

Main forecast concerns/challenges are with fog trends this
morning across much of the area, with an area of dense fog
situated across far northwest Indiana.

High pressure overhead early this morning supporting another
night/morning of fog development across the CWA. An area of dense
fog has developed across the northern portions of Lake and Porter
counties in IN, and at this time, it has remained fairly
stationary. Given how dense this fog was and that it wasn`t likely
going to dissipate, went ahead and issued a Dense Fog Advisory.
Guidance does continue to indicate this fog will expand into
remaining areas of northwest IN while other areas of dense fog
develop across portions of northern IL. Have already seen fog
development this morning, with any other areas of dense fog
remaining isolated. Overall pattern along with low dewpoint
depressions would support continued fog development, some of which
could be dense this morning. Have continued the forecast of
areas/widespread fog over much of northern IL and northwest IN
through this morning, and will continue to monitor for more
widespread dense fog and the possibility of including other areas
in the Advisory. The fog/any low clouds should scatter out through
mid morning, with partly cloudy to sunny skies expected today.
Warmer temps around 80 expected today, while locations near the
lake observe onshore flow and cooler temps in the 60s.



234 AM CDT

Thursday through Tuesday...

Overall trend into the end of the work week will be upper level
ridging with a continued warming trend. Any guidance that
indicated the possibility for some weak mid level energy to ride
along this ridge into Thursday morning is now consistent with
keeping this energy and any precip chances to the north/northwest
of the area. Dry conditions for the entire area expected through
early Friday. By late Friday into the start of the weekend, the
ridge breaks down as upstream energy dips down across the region.
This will coincide with increasing moisture, and while instability
shifts overhead. While higher focus for thunderstorm development
should stay to the north, guidance indicating the possibility for
sagging development into at least the northern half of the CWA
late Friday/Friday night. Although confidence is not overly high,
its enough to keep slight chance to low chance mention of thunder
Friday night.

Pattern going into the weekend will support a continued warming
trend with temps well above normal. Temp forecasts in the upper
80s still seem reasonable including near the lake, with some
locations possibly climbing more to around 90 this weekend. Have
not trended this way quite yet given the continued possibility for
periodic and scattered shower and thunderstorm development that
could limit some heating. Given the summer like pattern, this
thunderstorm development still seems reasonable at this time.



For the 06Z TAFs...

High pressure has moved over northern Illinois and northwest
Indiana and will provide light winds through most of the forecast
period. Mostly clear skies will continue overnight with only some
passing high cirrus. Patchy fog has already developed and should
continue to expand overnight. Some locations may experience dense
fog with visibility a quarter mile or less. Fog should clear
around mid morning Wednesday with VFR conditions the remainder of
the day. Light east to southeast winds are expected to develop
later in the day Wednesday behind the ridge axis. There is an
additional, albeit low, chance for fog Wednesday night into
Thursday morning.



330 PM CDT

There are no significant/impactful winds forecast through the
holiday weekend, with the strongest winds up to 25 kt over the
north half Friday into Friday evening. The main marine concern in
the near term is dense fog over the south half of the lake, with
the Dense Fog Advisory now in effect through Wednesday morning.
It`s again possible the advisory may need to be further extended
depending on trends on Wednesday.

Weak high pressure over the western Great Lakes will gradually
shift east to the eastern Great Lakes by Thursday. Weak flow will
be in place across Lake Michigan through this time frame. 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 south-southwest flow will develop, especially
over the north end of the lake, where again speeds could be up to
25 kt Friday PM. 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 Lakes. There are periodic low
end chances for thunderstorms. Will also need to watch for
possibility of dense fog at times into and through the weekend as
very and increasingly moist air mass moves over the still chilly
lake waters.

Over the IL/IN nearshore, winds will be driven by lake and land
breeze influences much of the time through the weekend, with
possible exception of Friday and Saturday.



IN...Dense Fog Advisory...INZ001-INZ002 until 9 AM Wednesday.

LM...Dense Fog Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-
     LMZ777-LMZ779 until 10 AM Wednesday.




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