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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 210537

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1237 AM CDT Wed Aug 21 2019

253 PM CDT

Through Wednesday night...

Mesolow continues to progress eastward through the area this
afternoon. It is very moist, with some instability aloft still
present such that we expect showers and isolated thunderstorms to
continue through late afternoon before ending. 

High pressure will nose in from the west this afternoon and
evening, and thus expect continued clearing ahead of a cold front
across the upper Midwest. With the moist conditions in place and 
weak flow, expect that there will be fog tonight. It could be
dense in some areas.

The forcing near the front to our north passes well north such
that most of the overnight precip should stay out of the area,
though some lighter precip could occur ahead of it in the
morning/early afternoon, with the better chance for some isolated
TS across northwest Indiana. Meanwhile, additional convection 
will fire to our west along the southern extension of the boundary
across the plains overnight. As additional energy spreads in from
the west, some of this may sneak into our south and west counties
in the late morning or the afternoon also. We do not think
coverage will be too widespread tomorrow, but with these multiple
though weaker forcing mechanisms will require us to carry some
lower chance precip chances. 



259 PM CDT

Wednesday Night through Tuesday...

A pattern change is on the horizon for the latter part of the
week, one featuring drier air and more settled sensible weather. 
First for Wednesday night as a trough digs in eastern Canada, a 
cold front will be working southward through the region. A short
wave will be translating the strengthening mid- to upper-level 
flow around the trough. This looks to serve as some focus for 
mid-level ascent along the baroclinic zone at least across the 
southern forecast area and possibly further north depending on 
the impulse track. The precipitation with this should be mainly 
showers with a lower lightning threat, especially any lingering 
activity into early Thursday. As the pressure gradient increases 
Thursday afternoon south of an incoming Canadian surface high to 
the north, the northeast winds will increase especially over the 
lake. There is potential for a heightened rip current risk 
because of this into early Friday. Otherwise these winds will 
steer in lower dew points on Thursday night resulting in a dry, 
cooler feel for Friday. 

The surface high that builds into the eastern Great Lakes will be
sprawling and may keep the area dry all the way through Monday.
The GFS does indicate a small chance of some showers Sunday, but
the next decent synoptic signature in long range guidance is not
until Monday night into Tuesday, and the GFS ensemble generally
supports this too. Highs in the warm advection regime ahead of
this front could reach into the upper 80s on Monday.



For the 06Z TAFs...

Dry/VFR conditions in place early this morning, however, am
monitoring fog and low ceiling development this morning. Only
noting patchy fog at this time, but expect further development
over the next several hours. At this time, it does appear that DPA
and RFD will have the higher chance for observing the lowest
ceilings and vis in fog this morning. Will continue to monitor the
other sites for possible lower conditions this morning. Should see
any fog and low ceilings erode through mid/late morning, with VFR
conditions then expected. Have continued a dry forecast for this 
afternoon into this evening, but this could change with later 
forecasts. Most guidance developing precip just to the south today
and think this is still the more likely scenario. It is possible 
for this to develop further north, and will monitor trends for 
possible changes to the forecast this afternoon. Lighter westerly 
winds will gradually turn more northwest today, and then continue 
to the north later today. For locations near the lake, a sooner 
wind shift to the northeast this afternoon is appearing probable,
with lake breeze.