Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 291756

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1256 PM 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 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




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...NEARSHORE WATERS
     UNTIL 4 AM Saturday.




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