Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 270821

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
321 AM CDT SAT AUG 27 2016

316 AM CDT

Through tonight...

Water vapor imagery very telling this morning showing a plume of
moisture flowing from the Gulf of Mexico across the Plains and
into the Upper Midwest. Broad area of low/mid level warm air
advection underneath the right entrance region to the upper jet is
resulting in numerous thunderstorms across the region early this
morning. Precipitation is expected to continue across northern
Illinois and northwest Indiana through late this morning, then the
better forcing will lift to our north and should allow coverage to
diminish from the south. Most of the hi-res CAMs are in decent
agreement showing the bulk of the precip out of the CWA by 16-18Z
with spotty activity thereafter.

The forecast details become a bit murky as we head into the
afternoon. RAP/HRRR runs don`t develop much of anything this
afternoon, but NAM/GFS continue to hone in on afternoon showers
and thunderstorms affecting portions of the CWA, particularly
along/east of a Pontiac to Chicago line where guidance indicates
there will still be some upper level divergence in the entrance
region to the jet. Unsure if any appreciable instability will be
able to redevelop today, but if we do see partial clearing then
moderate instability could result. This will allow any
thunderstorms that redevelop to become strong to severe with mid
level flow of 35 to 40 kt in place providing sufficient shear for
an organized severe weather threat. Confidence in this occurring
is low.



316 AM CDT

Sunday through Friday...

Warm, humid, and unstable conditions will be in place Sunday
through Tuesday with periodic chances for showers and
thunderstorms through this timeframe. On Sunday, potent shortwave
is expected to lift across the northern Great Lakes region with
upper ridging building into portions of the Upper Midwest.
Meanwhile, the low level ridge axis will extend from the New
England coast southwest into the lower/mid Mississippi Valley with
convection expected in the unstable air mass on the periphery of
the ridge. This pattern should generally persist through Tuesday
with afternoon highs in the mid 80s and dewpoints in the 70s
contributing to moderately strong instability across much of the
Midwest. No upper level support and little in the way of forcing
should limit thunderstorm coverage, but do anticipate some widely
scattered pop-up thunderstorms, diurnally favored, Sunday and
Monday. Lake breezes appear possible each day and could serve as
the focus for thunderstorm development. Weak mid-level flow will
limit any severe threat through this period. A cold front is
expected to push across the region on Tuesday which will bring a
better chance for showers and thunderstorms.

Stage is set for a pattern change for the latter half of the week
favoring drier and cooler weather. Through midweek, upper ridge
is expected to amplify over the midsection of the country which
will allow cooler air to spill into the Great Lakes region. The
850mb trough is expected to push across the region Tuesday night
with temps falling into the low teens Celsius in its wake, and
possibly even touching single digits briefly per the 12Z ECMWF.
Continue to see model differences in how far southwest the cold
air will spread, though the 00Z ECMWF came into a little better
agreement with the GFS keeping the coolest air just to our east.
Nudged temps down slightly for Wednesday and Thursday. Thursday
should be the coolest day, with afternoon highs only in the low to
mid 70s. Surface high will build across the Great Lakes Thursday
and Friday providing dry weather, then the upper ridge follows
suit Friday into Saturday allowing a gradual warming trend to
commence as we head into the weekend.



For the 06Z TAFs...

TAF concerns center around expansion of thunderstorms across the
airfields overnight into early Saturday. Expect rain showers to
continue across the area in the coming hours. Elevated instability
being tapped across north central Illinois will also occur farther
east as the forcing from the upper wave along with the sfc-925
warm frontal boundary lift northward across the area...and
therefore expect an increase in thunder coverage. The warm front
will lift north of the area Saturday morning which should lead to
a decrease in thunder and eventually rain coverage from southwest
to northeast. Occasional though generally brief IFR visibility
will occur in the heavier downpours, with MVFR cigs at times.
Expect the MVFR will be most prevalent across north central IL.

Most of the terminals will be in a subsident regime behind the
front. Expect modest south-southwest winds behind the front, with
shower and storm coverage lingering longest across southeast
arrival routes. Weak surface convergence and some lingering
moisture could trigger a few showers Saturday/Saturday night, but
confidence is too low to include in the terminals at this point.
Will reassess for the 12z set when convective allowing guidance
may have a better idea on some trends.



253 AM CDT

No major marine concerns through at least early next week other
than a few periods of thunderstorms, most notably today and again
possibly Tuesday. Weak high pressure currently in place across
the Great Lakes will quickly shift east of the region today as
quick moving low pressure will shift northeast across central Lake
Michigan. Low pressure across the Canadian Rockies will cross
north central Canada early next week as high pressure returns
Sunday-Tuesday. The aforementioned low will continue east across
James Bay Tuesday and to the Canadian Maritimes later in the week.
This low will send a cold front cruising south through Manitoba
and Ontario, with a portion of this front impacting Lake Michigan
beginning later Tuesday. This will shift winds to northerly before
high pressure returns for mid to late week.






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