Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 291601

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1101 AM CDT Thu Sep 29 2016

1100 AM CDT

The going forecast for this afternoon into tonight continues
mainly on track with breezy, cloudy, and scattered shower
conditions generally along and east of the I-55 corridor gradually
creeping northward. Have gone ahead and issued a Lake Shore Flood
Advisory for northeast Illinois and Lake County, IN based on
observational trends and expected continual battering of high
waves. Not expected to be a big issue, but some minor flooding
including near or along the Chicago bike path is more probable.

Impressive broad and closed upper low for September is starting
to becoming stationary over the Ohio River Valley. Almost a
winter-like stratospheric intrusion is noted on the 12Z ILN
sounding capturing the center of this massive low, and similar to
a winter system, a warm conveyor belt is wrapping back to the west
on the northern periphery. This should continue to support a
tongue of scattered showers with lake enhancement into northeast
Illinois and parts of far northwest Indiana. Charge separation for
lightning in lake effect activity looks less likely than
yesterday, so not expecting storms but cannot totally rule it out.
The threat for waterspouts looks less as the 850mb temperatures
have warmed around 3-4C and the lower level wind fields have
increased (50 kt 850mb flow on area VWPs) with less near-surface
convergence. That said, minor streamline convergence is noted into
Cook and Lake (IN) late this morning and while narrower CAPE, it
is still supportive of cloud depths over 20,000 ft. So will
continue with a general marine statement and keep an eye on any
foci for low-level vorticity.

Along the lake front, high waves will continue to pound the shore,
with the Wilmette buoy up to 8-9 ft. The lake level at Calumet
Harbor is up about 15 inches in the 48 hours. Given that waves
will be persistent of 7-11 ft and lake levels may also inch up a
bit more, have gone ahead and issued a Lakeshore Flood Advisory.




317 AM CDT

Through tonight...

The stubborn upper level wave continues to influence the Great Lakes
region. IR imagery early this morning shows a nice swirl to the
clouds stretching from Southern Lower Michigan southwest through far
Northeast Illinois, then south through Central Illinois. A narrow
convergence zone continues to flow over the eastern half of the
forecast area early this morning, allowing enough lift to moisten
parcels and produce some rain. Surface ridge remains parked over
Ontario/Quebec and stretches southwest through the Upper Midwest and
Central Plains, which has sharpened the back edge of the cloud
shield west of a Crystal Lake to Pontiac line. Temps have remained
in the lower 60s closer to Lake Michigan, primarily from the warm
marine environment bleeding inland and moderating adjacent land.
Further inland where less cloud cover was, temps have fallen into
the low/mid 50s. With enough friction from a north wind, expect
temps not to fall much prior to daybreak but some portions of
Lee/Ogle county could touch the upper 40s.

The moist axis will continue to feed west from the Atlantic, along
the northern fringes of the shortwave over the Tenn valley, into
Southern Lower Michigan and arc southwest through Northeast Illinois
today. The pressure gradient is progged to remain relatively tight
across Northern Illinois, with a shallow convergence zone lingering
through much of the day and evening. With the lake waters still
relatively warm, and cooler air flowing over the lake, it is
possible that enough lake induced instability could develop some
thunderstorms and move inland across Northeast Illinois and
Northwest Indiana today. The other concern for today will be on
temps. With the signal of increased cloud cover arriving earlier
today, from the moist axis, have trended temps cooler or back into
the 60s. Considerable cloud cover will blanket the area again
tonight, with continued periods of showers and north/northeast
winds. This will lend to minimal temp change from this afternoon,
and likely end up in the mid/upr 50s and low 60s closer to Lake



317 AM CDT

Friday through Wednesday...

Friday through Sunday: Pattern remains blocked with the upper level
ridge locked over Ontario/Quebec, which will maintain the cutoff
shortwave across the Tenn/Ohio Valley Fri. Guidance continues to
retrograde the shortwave northwest towards Lower Michigan late in
the weekend. This will maintain periodic chances for showers for the
first half of the weekend, with the best chance for precip arriving
Fri. The moist axis will begin to lift north Fri ngt, with
decreasing chances for precip Sat and possibly drying for Sun.

Late in the weekend guidance begins to relax the mid-lvl block over
Canada and allow the shortwave to gradually become absorbed back
into the longwave pattern and depart the Great Lakes region.

Monday through Thursday: Ensembles in the later periods continue to
show minimal spread with an upstream trough pushing into the
Southwest CONUS late in the weekend into early next week. This will
allow slow amplification to the downstream ridge over the Upper
Midwest/Great Lakes, leading to several periods of dry weather and
southwest flow ushering in milder air through the first half of next



For the 12Z TAFs...

635 am...Forecast concerns remain showers...winds and cigs.

Mvfr cigs developed early this morning but have remained patchy
across the Chicago area terminals...becoming more widespread
further west. Several model guidance suggests that mvfr cigs will
become prevailing this morning and remain prevailing for the rest
of the day. Confidence is only medium but maintained mvfr cigs.
Cigs will likely lower to low mvfr this evening and possibly
become ifr overnight into Friday morning.

Low pressure over the Ohio Valley/Mid Atlantic will spread back
west tonight and Friday and this will maintain strong/gusty
northeast winds through the period. Showers will be possible
through the period...especially at mdw/gyy. Guidance still shows
the potential for more widespread and perhaps heavier showers
mid/late this afternoon into this evening. Its possible that an
isolated thunderstorm is possible during this time...but
confidence on coverage is too low to include in this forecast.



200 am...Low pressure over Ohio will wobble across the Ohio Valley
and mid Atlantic today and then move back west across Indiana
tonight and to southern Illinois Friday. The low will then slowly
drift north across the Great Lakes region this weekend. Large
strong high pressure will remain across Ontario and Quebec into
Saturday before slowly weakening. A weaker ridge will extend
southwest across the upper midwest. The gradient between these two
systems will remain strong across Lake Michgian with northeast
winds to 30kts into Friday evening. Occasional gale force gusts
will be possible but confidence is low regarding how prevailing
these may become. cms


IL...Lakeshore Flood Advisory...ILZ006-ILZ014 UNTIL NOON Friday.

IN...Lakeshore Flood Advisory...INZ001 UNTIL NOON Friday.

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...7 PM Friday TO 4 AM Saturday.




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