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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 201953

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
253 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. 



213 AM CDT

Wednesday night through Monday...

Synoptic front expected to continue sliding south across the 
region at the start of the period, with CAA and dry advection the 
trend. However, some potential for this boundary to hang up around
the southern CWA Wednesday night. If this were to occur, 
additional upstream development to the west during this time could
ride along it and provide some chances for showers and 
thunderstorms. Main instability axis will be to the south, with 
instability lowering across the CWA through the night. So, would 
think that the bulk of any development should stay just to the 
south, but with locations along/south of the Kankakee river valley
possibly observing the northern extent of this development. Will 
be trending drier and cooler into the end of the work week, as 
high pressure builds across the region. With this high in place, 
any precip development into this weekend will stay well 
west/southwest of the area. Below normal temps are expected late 
in the work week into the weekend.



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.