Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 271134

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
634 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 12Z TAFs...



- Ongoing Showers and thunderstorms
-Variable winds in the storms
-MVFR to briefly IFR cigs
-Occasional IFR visibility
-Any redevelopment this afternoon?

Showers and thunderstorms blossomed across the terminals earlier this
morning. Showers and embedded thunder continues as a warm front
and multiple shortwave disturbances lift north across northeast
Illinois. The warm front will shift north into Wisconsin late this
morning accompanied by weak low pressure that will shift winds to
southerly today. While the predominant wind will be ese with the activity
this morning, occasional highly variable winds either to north-
northeasterly remain possible and could really be out of the
southwest too. IFR visibility will occasionally accompany these
showers and storms with MVFR cigs, and possibly some brief IFR
cigs as well. IFR cigs have been more solidified across Iowa and
far NW Illinois, and so an eastward expansion under the surface
low remains possible this morning, along with some lowering across
NE Illinois as additional rain and embedded thunder fills in with
the warm front lifting northward.

A bit more uncertainty regarding the afternoon redevelopment potential,
with ensemble suite suggesting the afternoon would not be very
active across the main terminals, but as the low shifts north and
the warm sector gets into the area, expect storms to remain
possible across East central IL and Northwest IN, and possibly a
few showers across NE Illinois. At this point at least a portion
of the area may have a harder time recovering despite the muggy
airmass in place High pressure will build in late tonight and
shift winds to north and even northeast by Sunday morning near the
lake. This could also result in some lower cigs and visibilities
which will likely need additional refinement.



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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