Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 291945

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

240 PM CDT

Through Tonight...

Similar to the past couple days, and upcoming ones for that
matter, tonight will continue to be dominated by the large
closed upper low centered over Kentucky this afternoon. Mid-
afternoon visible satellite imagery has indicated some breaks with
more cumuliform clouds across northern Indiana into far eastern
Illinois. Isolated showers are likely to fill in there and cannot
100% rule out a lightning strike or two. Further to the north, a
ribbon of scattered to numerous showers will continue within the
wraparound moist conveyor belt of the occluded system. This axis
will persist into this evening with likely an uptick in showers
later this evening from lift of a left entrance region of the
upper jet wrapping around the low center.

Clouds overall will continue tonight, even if some areas see a
break late in the day into early evening. This along with the
continued winds will prevent temperatures from falling much at all
with lows in the mid to upper 50s, and likely around or above 60
near the lake shore.

Speaking of the lake shore, continued battering waves of 7 to 11
ft through tonight will probably cause some minor flooding in
typical vulnerable areas immediately along the lake front. The
upstream mid-lake south buoy has been steady around 7 ft today,
and would expect to see around 115-125% of that within our
northeast Illinois nearshore given the flow pattern. The Wilmette
buoy has been near that, with 8-9 ft a good part of today and
webcams and pictures supporting that. While that is borderline
for what usually causes flooding, have seen around a 15 inch
increase in lake levels the past 48 hours from the constant
northeast flow and that could inch up the next 12-18 hours.



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 18Z TAFs...

The concerns with the aviation forecast this afternoon through
Friday are periodic showers and possible MVFR visibility, along
with continued MVFR cigs and increasing potential for IFR cigs by
daybreak Friday.

Low pressure across the Ohio River Valley continues to have a far
reaching extent with gusty northeast winds continuing over the
region through the period. Gusts exceeding 25 kt will
occasionally be seen this afternoon at Chicago area airports.
Forcing for showers will continue, some of which will be lake
enhanced. Temporary reduction in visibility is likely and had
been observed this morning as low as briefly 3SM at ORD. While
showers should undulate in coverage some through Friday morning,
the overall support for them will continue.

Steady MVFR cigs should be seen this afternoon and into early
evening. Thereafter, a slow lowering of cloud bases is likely
into the overnight. There is IFR cloud cover upstream in lower
Michigan and this should expand westward. The slightly warmer air
off the lake may result in cloud bases more at 1000 ft than below
that at ORD and MDW, but tough to say for sure and would be
surprised if not even temporary IFR during the morning rush. Not
expecting much improvement in clouds through the entire day



240 PM CDT

Strong northeast winds will continue across the lake through
Friday before easing. Waves have been steady around 7 ft at the
south buoy and 8-9 ft at the Wilmette buoy. Would expect these to
hold steady or undulate slightly, maybe even coming up some later
tonight. The occasional gale force gusts also cannot be ruled out
tonight and early Friday morning. The threat for waterspouts is
less than last night, but still present over the south given the
instability and sizable lake-induced cloud depths.

Waves will be slow to subside into Saturday and it is possible
the current Small Craft Advisory for Illinois and Indiana needs to
be extended through Saturday morning.



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

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

LM...Small Craft Advisory...NEARSHORE WATERS
     UNTIL 4 AM Saturday.




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