Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 290535
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1235 AM CDT Thu Sep 29 2016

.UPDATE...
826 PM CDT

For evening update...

Clusters of showers and isolated thunderstorms continue to wrap
cyclonically west-southwest across southern Lake Michigan and
parts of northeast IL and northwest IN this evening, in association
with smaller scale vort lobes rotating about upper low centered
over Indiana. Cool air aloft associated with the upper low and
warm early autumn Lake Michigan waters were combining to yield
some impressive lake-enhanced instability (800 J/kg per RAP) and
a few stronger cells moving into the Chicago area earlier this
evening, along with a few waterspouts. These same RAP soundings
suggest instability will weaken somewhat and become more shallow
late this evening/overnight, which should allow thunder and spout
potential to diminish as well. Overall, going forecast generally
in good shape, though did expand higher pop coverage a little
farther north/west across most of metro area to match radar
trends. Focus for additional scattered showers overnight should
shift gradually south as the center of the upper low also drifts
slowly south tonight.

Temperatures are not expected to vary too much from current
readings in the 50s overnight, especially closer to Lake Michigan
where blustery north-northeast winds will help prevent much
radiational cooling. Mins are expected to range from the lower 50s
across north central IL, to the mid-upper 50s across the City.

Updated grids/zfp/etc available.

Ratzer

&&

.SHORT TERM...
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
lake.

RC

&&

.LONG TERM...
316 PM CDT

Thursday night through Wednesday...

The upper level low that brought fall conditions to the region today
will impact the region through most of the weekend.  The low
retrogrades north Friday afternoon through Saturday, but guidance
differs on how far west the low will be when it pushes north. There
is a chance that the low may push right back over the region which
could lead to another round of cooler than normal temps late this
week.  Have much more confidence in scattered showers and gloomy
conditions through the end of the week due to the low.  Bands of
vorticity will produce showers through the weekend.  Friday in
particular may be blustery with wind gusts up to 25 mph and
occasional showers. Guidance features some CAPE so kept a chance of
thunderstorms in the afternoon south of I-88.  High temps look to be
in the mid 60s.

The low finally shifts east early next week and an upper level ridge
builds overhead.  Temperatures become more seasonal in the low to
mid 70s and then increase to above average with mid to upper 80s
possible mid next week.  The next chance of widespread rain arrives
mid week as well along the next low`s cold front.

JEE

&&

.AVIATION...
For the 06Z TAFs...

1235 am...Forecast concerns include showers...winds and cigs.

Low pressure over Ohio will wobble around the Ohio Valley today
and then shift back west to western Indiana/eastern Illinois by
Friday morning. This will maintain gusty northeast winds through
the period with gusts into the mid/upper 20kt range later today
into this afternoon. There will also be periodic showers rotating
around this low. There does seem to be some focus for prevailing
showers from mid afternoon through early this evening and added a
tempo during this time but the chance of showers will continue for
almost the entire period and maintained vicinity mention...except
at rfd with any shower activity there likely just mid/late this
afternoon. An isolated thunderstorm is also possible this
afternoon into this evening but low confidence regarding coverage
so no mention with this forecast.

Cigs will likely remain vfr early this morning...perhaps some
patchy mvfr with any heavier showers...but mvfr cigs should
develop later this morning and possibly slowly lower this
afternoon with a rather strong signal for ifr cigs developing this
evening into Friday morning...as the surface low moves back toward
the area. cms

&&

.MARINE...
332 PM CDT

Strong north-northeast winds and high waves will be the story
through Friday evening. A period of gale force winds/gusts still
appears probable tonight on the south half of the lake and near
shore waters. Latest guidance suggests that after a brief lull to
start Thursday, there could be a brief period where winds increase
back to gale force during the day on Thursday. If confidence
increases in this scenario, the Gale Warning may need to be
extended into Thursday afternoon. This may be followed by yet
another brief period of gales or occasional gales Thursday night,
followed by solid 30 kt east-northeast winds on Friday. Winds will
diminish Friday night into Saturday, with peak speeds around 20 kt
on Saturday and then light northerly and northeast winds of 15 kt
or less Sunday and Monday.

Cold air aloft over the lake will continue to provide a favorable
setup for at least some potential for isolated waterspouts tonight
through Friday. Mariners are advised to stay alert for the
issuance of any statements or warnings pertaining to waterspout
development.

RC

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779 UNTIL 3 AM Thursday.

&&

$$

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