Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 282009

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
309 PM CDT Wed Sep 28 2016

309 PM CDT

Through Thursday...

The main forecast concern continues to be with shower activity
driven by the well defined upper level low pressure pivoting
southward across the region. The current showers well inland are
likely diurnally driven, with dissipation expected by dark skies
clearing to at least partly cloudy across north central Illinois.
Closer to the lake, showers have been driven and aided by
impressive lake effect setup over the lake. After the showers over
northeast IL and far NW IN as of mid afternoon pivot further
inland and dissipate, some isolated or scattered development may
occur. Thereafter, high resolution guidance is consistent on an
organized area of showers pivoting westward across the lake and
affecting portions of NE IL and NW IN through early to mid
evening. There should then be a lull followed by more at least
scattered showers associated with upper low with some lake
enhancement possible overnight into early Thursday. This would be
most likely to occur mainly east/southeast of I-55.

More of the same can be expected on Thursday, with respect to
occasional showers and a good bet at lake effect/lake enhancement.
However with the upper low farther south, far north and northwest
parts of the area, including Rockford and vicinity, could stay
completely dry. Cannot rule out an isolated rumble of thunder
east/southeast of I-55. Warming aloft should enable warmer
temperatures in the 60s to around 70, though am concerned with
persistent clouds and strong onshore flow, the lakeshore could be
kept cooler than in current forecast. It will be a breezy day with
northeast winds gusting to 25-30 mph, and up to 35 mph near the



257 AM CDT

Thursday through Tuesday...

Thur through Fri: Cutoff upper level shortwave continues to be
progged to drift southeast towards the Tenn valley Thur aftn/eve and
deepen. Meanwhile an expansive surface ridge over Ontario will
spread southwest through the Central Plains, helping to sharpen
the backedge of the departing surface wave over the eastern
portions of the forecast area. Expect clouds to thin considerably
along the western CWA by late Thur, but this may only be short-
lived as a moist channel is poised to feed west from the Atlantic
along the northern fringes of the surface wave over Southern Ohio
late Thur/early Fri. Guidance continues to prog precip chances
returning from east to west Thur aftn, but more focused on the Fri
period. But with the surface ridge sharpening up the western edge,
it is possible the far western CWFA could remain dry Thur ngt into
much of Fri morning.

Guidance continues to indicate the 500mb vort max will be
retrograding across the Ohio Valley to the Great Lakes later in the
day Fri, which will help to push the precip shield west across
Northern Illinois. Temps will continue to be unseasonably cool, with
highs mainly in the 60s Thur to near 70, then as the upper level low
drifts northwest Fri highs could struggle to move beyond the upper

Sat through Tue: Across much of Manitoba/Ontario/Quebec is a 500mb
ridge that continues to block the northward progression of the
cutoff low over the Great Lakes; however, ensemble members continue
to indicate that over the weekend this block will weaken and will
allow the shortwave to lift northeast and depart the Great Lakes. By
the second half of the weekend a trough is poised to slide south
across the Western CONUS, which will help to increase mid-lvl
heights/ridging over the Central Plains/Great Lakes Mon/Tue of next
week. This should help to return temps to near seasonal conditions.
As for precip chances, the best chance doesn`t arrive until closer
to the middle of next week.



For the 18Z TAFs...

Concerns through Thursday:
*occasional scattered showers,particularly near the lake through
early-mid evening and then again on Thursday.

*gusty north-northeast winds

*the return of low-mid MVFR CIGs on Thursday for the eastern

Well defined cold upper level low pressure over the area is
resulting in shower activity this afternoon, particularly near the
lake where an excellent lake effect setup is in place. Have
adjusted the tempo for ORD/MDW slightly earlier and will need to
monitor radar trends for any further unscheduled adjustments. Thus
far, activity has remained light away from the lake in Illinois,
so not overly concerned with heavier showers at ORD/MDW and
associated impacts. The potential exists for additional bands of
lake effect showers rotating inland through mid evening, so
maintained a VCSH through 03z.

North-northeast winds will become increasingly strong and gusty,
with the strong winds continuing through the night at GYY. CIGs
will remain primarily VFR through the night, followed by low-mid
MVFR CIGs potentially spreading northwestward on Thursday morning
across the eastern terminals. Confidence is medium on timing and
exact cloud heights, but at this time do not expect IFR. The lake
effect setup will remain favorable on Thursday, so additional
showers are possible, but only enough confidence for a VCSH
mention for now. Strong northeast winds can be expected Thursday,
with sustained speeds of 15-18 kt and gusts of 25-30 kt.




224 am...An area of low pressure over northern Lake Michigan will
move south and merge with a second area of low pressure over
central Illinois. This combined low will slowly move southeast to
the Ohio Valley tonight into Thursday. A strong ridge of high
pressure will build across the upper midwest and Ontario tonight
and Thursday and this will tighten the gradient across the lake...
especially southern Lake Michigan tonight when gales are possible.
The low is expected to move back west Friday and Saturday...
centered over Indiana. This will maintain a strong gradient across
the lake with northeast winds to 30 kt expected for a prolonged
period...likely through Friday night. The gradient will slowly
weaken as the low drifts northeast across the eastern lakes
Saturday night into Sunday.

This low pressure combined with colder air aloft will allow the
potential for waterspouts to develop through Thursday morning...
ending from north to south this afternoon through Thursday
morning. cms


LM...Small Craft Advisory...NEARSHORE WATERS
     UNTIL 7 PM Wednesday.

     Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779...7 PM Wednesday TO 3 AM Thursday.




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