Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 142339

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
639 PM CDT Thu Sep 14 2017

145 PM CDT

Through Friday...

The only weather of impacting note in the short term is the
possibility for fog redevelopment tonight. Overall the pattern is
a little less favorable with the surface high pressure ridge
having inched southeast. Surface dew points have remained above
mid- September normals though, with upper 50s to mid 60s. Visible
satellite imagery also indicates a decent cumulus field in the
east, a sign that condensate remains. Believe this eastern half of
the forecast area, primarily outlying locations, is most
susceptible to fog development tonight being under the ridge.
Western areas cannot be ruled out though as low temperatures may
dip below guidance values with the dry ground and clear sky. There
will be potential for at least patchy dense fog some of which
could linger into the Friday a.m. commute hours.

Assuming no issues with lingering fog or stratus, Friday should
see afternoon highs achieve mid 80s given forecast mixing depths.



149 PM CDT

Friday night through Thursday...

After a period of relatively quiet weather, the southern stream will
become more unsettled as the upper level pattern trends toward more
zonal flow.  By Friday night, the upper trough over the west will
begin to progress over the plains.  The parent upper low over the
Canadian Rockies will progress east, helping to break down the upper
ridge over the east.  The above normal temperatures will continue
into the weekend while the upper ridge and swly flow prevails, with
highs in the middle 80s, but into early next week, temperatures
should trend down closer to more seasonable levels of upper 70s to
lower 80s.  As the upper level pattern trends to more zonal, a more
active southern stream will set up with a series of weak shortwaves
rippling through the fast wswly-wly flow aloft brings periodic
chances for showers and thunderstorms for much of next week.  Given
the low amplitude pattern, timing and intensity of individual waves
will become increasingly uncertain through the period, so, will
generally keep PoPs in the slight chance to low chance range.


For the 00Z TAFs...

Main concern tonight is with potential for fog redevelopment,
though is not expected to be as favorable for dense fog as last
night/this morning.

VFR conditions and light south winds in place across the terminals
early this evening. Lake breeze has been slow to push inland, and
remains 5+ miles east of ORD/MDW. Classic lake breeze passage is
not expected at this point, with the loss of mixing at sunset
allowing a general backing to light southeast winds across the

Surface dew point temps remain in the upper 50s/low 60s across the
area, while AMDAR aircraft ascent soundings indicate drier air
aloft. Generally clear skies and light winds should allow fairly
strong radiational cooling and development of some areas of at
least MVFR fog by sunrise. High-res model guidance focuses on
northwest IN and areas south of Chicago for greater potential for
lower vsbys, closer to surface ridge axis and lighter/calm winds
overnight. Have maintained MVFR vis fog for DPA and GYY, with RFD
and more urban ORD/MDW having lower potential for fog.

South winds around 10 kt and clear/VFR conditions expected Friday.



200 PM CDT

Persistent high pressure parked over the East Coast will prevent a
developing low over the central plains from making much eastward
progress over the next few days. The high will weaken and set up
more over the southeastern states and allow the low to finally
progress to the northeast, pushing across the western Great Lakes
and into Ontario while dragging a cold front across Lake Michigan
on Sunday. Until then, generally southerly flow and quiet weather
will persist across the lake.





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