Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211736

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1236 PM CDT SUN AUG 21 2016

223 AM CDT

Through tonight...

A cool and breezy day is on tap for the area today. While the day
will start out on the sunny side, diurnal CU will develop over the
area by early afternoon. This will result in a partly cloudy to
partly sunny afternoon over the area. We also cannot rule out a
few isolated sprinkles or light rain showers this afternoon,
particularly over northeastern Illinois and points north. These
areas will be in better proximity to an afternoon disturbance
propogating through the northwest flow aloft. However, these
should not make much of an impact to the day, with most areas
likely staying dry. High temperatures this afternoon are only
expected to warm into the middle 70s.

A surface ridge of high pressure will set up over the region
tonight. This will result in light winds and clear skies over the
area, which should set up a chilly Monday morning over the area.
The coldest areas will be outside the warming effects of the city,
where temperatures could drop near 50. This could also result in
some patchy fog late tonight into early Monday morning.



259 AM CDT

Monday through Saturday...

Temperatures will begin to warm back to near 80 on Monday, and back
into the low to mid 80s on Tuesday as the low level flow turns
back out of the south in the wake of tonight`s surface ridge of
high pressure.

The main story during the extended period will center around mid-
week (Wednesday through Thursday). Forecast model guidance
continues in good agreement with the evolution of another stout PV
anomaly, currently located near the British Columbia coast, per
the latest water vapor imagery. This feature is expected to shift
eastward across south central Canada through Wednesday morning. As
it does so, expect surface low pressure to develop over the
Northern High Plains by Monday night, then gradually shift
eastward over towards the upper Great Lakes region by Thursday.
This systems cold front then looks to push over northern IL and NW
IN by Thursday afternoon.

Deep layer Gulf Moisture (Precipitable water values near 2") will
be likely be driven back northward over the Plains and the
Mississippi valley on Tuesday as a 35+ KT southerly low-level jet
sets up in association with the developing northern Plains low.
This gulf moisture then looks to be steered towards the southern
Great Lakes region on Wednesday into Thursday. This could be
setting up another wet period local, with possibly a couple rounds
of showers and thunderstorms during this period as an active
southwesterly flow in the upper levels sets up over the central
CONUS. So heavy rain could again become an issue for some.

Following this period of stormy weather, high pressure and cooler
weather looks to make a return to the area for Friday into
Saturday. Another disturbance could produce some more storms on
late Saturday or Sunday.



For the 18Z TAFs...

A ridge of high pressure is approaching the middle and upper MS
Vallet as strong low pressure lifts newd through nrn Quebec. Cold
advections persists over the region with gusty wnwly winds and
sct-bkn stratocu. The trend has been for cigs to improve to vfr,
so will start out all sites, except GYY at vfr. RFD/DPA have
already becm sct and ORD/MDW should scatter soon as drier air
advects over the region and the sfc ridge continues to build to
the east. GYY will remain mvfr a little longer due to nwly flow
off of Lake Michigan. Expect that sky cover should continue to
diminish through the afternoon and go skc in the evening. Winds
should remain gusty through the afternoon, but diminish as the
ridge contiues to build east. There is a very remote chance for an
isolated shower this afternoon, close to Lake Michigan, but the
general trend should be for dry conditions through the period.

Winds should becm lgt/vrbl overnight as the ridge axis moves
overhead. By tomorrow morning, the ridge should push east of the
terminals while low pressure develops over the plains. Winds
should increase and becm swly through the morning hours.


437 AM CDT

Gale warning will continue this morning for the far northern zones
of Lake Michigan, with frequency of gales diminishing quickly by
mid-morning. Winds will then becoming northwesterly between 20 to
25 kt later this afternoon. Further south the winds will be
strongest this morning to 30 kt from the west then shifting
northwest and diminishing slightly. With cooler air flowing over
the lake, waves will easily grow and should remain elevated
through this afternoon. Then with a diminishing gradient tonight,
winds and waves should steadily subside. Small craft conditions
will end this morning for the Illinois nearshore; however, with
the northwest/west winds the Northwest Indiana nearshore waters
will persist. High pressure will remain over the region late
tonight into Monday, then shift east with a gradient arriving and
producing stronger winds ahead of the next system for Monday night
through Wednesday. Then a low pressure will lift northeast of the
Lake Thur, with a frontal boundary passage expected for Thur aftn
and shifting southwest winds yet again to the northwest.



IN...Beach Hazards Statement...INZ001-INZ002 UNTIL 7 PM Sunday.





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