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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 202003

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
303 PM CDT Tue Aug 20 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 18Z TAFs...

Multiple forecast concerns center around:

-Departing thunderstorms
-Additional shower and some TS development
-Wind directions in the near term
-Fog and low ceilings tonight into Weds 

A mesoscale low pressure in wake of the morning storm complex will
shift through the terminals this afternoon. This will lead to
fairly messy winds today, with SE-NE winds ahead of it (including
both ORD/MDW initially) and then trending back to some westerly
component behind it, with westerly being the synoptic direction. 

In addition to variable winds, ahead of the expect additional
showers to develop. There could even be a few thunderstorms though
confidence on coverage is low. Eventually a weaker pressure
pattern will lead to lighter though generally westerly winds. With
very moist low levels, expect some fog or low ceilings, likely
even some IFR or localized LIFR, though we will have to see how
much high cloud cover from the northwest gets into the area.
Still, given the rain and very moist conditions, it seems
favorable for areas of localized fog.