Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200519

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1219 AM CDT Sun Aug 20 2017

156 PM CDT

Tonight and Sunday...

Dry, quiet weather expected to finish the weekend, with warmer
and slightly more humid conditions for Sunday.

High pressure was drifting east across the forecast area this
afternoon, with sunny skies and light winds making for pleasant
weather. Light winds will turn southeasterly tonight as the high
drifts off to the east, with clear skies and light flow allowing
for the development of some patchy shallow ground fog in a few
spots by sunrise Sunday morning.

Winds will turn southerly Sunday, with enough gradient flow to
limit any lake breeze cooling primarily to the immediate IL shore
area north of Chicago. Progged 925 mb temps of +23-25C should
support highs in the 85-90 degree range by afternoon, with a
gradual return of low-mid 60`s surface dew points making it feel a
bit more humid. Forecast time-height sections and soundings
suggest plenty of sunshine, though some increase in thin high
cloud is likely late in the day.



156 PM CDT

Sunday night through Saturday...

Warm and somewhat humid conditions will be in place through the
first couple days of the upcoming week, with drier and cooler
conditions arriving for the latter half of the week.

High pressure providing fair weather to the region this weekend
will shift to the mid Atlantic coast by Monday while broad and
disorganized area of low pressure sets up over the central Great
Plains. Deep corridor of southerly flow will help to transport
higher dewpoints back into the region while 850mb thermal ridge
overhead will allow surface temps to warm into the mid to upper
80s Monday afternoon. Guidance continues to suggest widespread
convection off to our west Sunday night approaching or entering
the local area Monday morning. Unstable conditions will be in
place Monday and it remains plausible for showers and
thunderstorms to develop on remnant outflow boundaries or any MCV
that is able to track across the area. Confidence in details
remains low at this distance given lack of broad scale synoptic
support Monday, but will continue to maintain chance PoPs and
partly to mostly cloudy sky cover.

Monday night into Tuesday, a strong upper wave is progged to dig
across the Upper Midwest and Great Lakes carving out a deep trough
across the eastern half of the country and is expected to persist
through the end of the week. At the surface, low pressure will
develop over the Great Lakes with a trailing cold front sagging
south across northern Illinois Tuesday morning and afternoon.
Showers and thunderstorms will be likely with fropa and the
potential for moderately strong instability and increasing deep
layer shear may allow for some degree of a severe threat across
the CWA.

Behind the front, an expansive high will build south from the
Canadian Prairies into the Upper Midwest transporting a cooler and
drier airmass over the region. Wednesday through Saturday should
see plenty of sunshine each day with highs only in the 70s and
dewpoints much more comfortable in the 50s



For the 06Z TAFs...

For the Chicago TAF sites today and tonight, concerns are south-
southwest winds approaching 10 kt during the afternoon and a
small chance for showers tonight.

Early this morning, patchy fog likely shallow in nature has been
observed at a handful of outlying locations. Could see some of
this at DPA, RFD, and GYY. Confidence in this remaining out of ORD
and MDW is high.

Light southeast winds should become south-southwest by mid-
morning, maybe as late as late morning. Speeds will increase with
8-10 kt expected at ORD and MDW by 19Z-20Z. Winds will drop in
speed near sunset and back slightly to due south or a little east
of south. Confidence in winds for this TAF is medium- high.

Convection across the Nebraska/Iowa border region early this
morning will evolve slowly east-southeast and should see a
weakening trend through the morning. An upper level disturbance is
likely to move due east though, and over the TAF sites by late
day/evening. Cannot rule out isolated showers with this, even
possibly a thunderstorm or two toward RFD. But confidence in how
much instability will be present for precipitation is low. For
overnight into Monday morning, a more noteworthy chance for
showers and storms will begin across eastern Iowa into
northwest/north central Illinois.



156 PM CDT

High pressure is centered over portions of the Upper Midwest and
across southern Lake Michigan this afternoon resulting in light
flow and allowing for lake breezes to develop this afternoon.
Modest southwest flow will persist across the northern half of
Lake Michigan peaking around 20 kt at times. South to southwest
flow will increase again Sunday and continue into Monday as low
pressure advances east across the Canadian Prairies. A trailing
cold front is expected to push south across the lake through the
day Tuesday turning winds northerly in its wake.






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