Area Forecast Discussion
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FXUS63 KLOT 301958

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
258 PM CDT Fri Sep 30 2016

255 PM CDT

Through Tonight...

The inability to shake this low pressure system will be the story
into the first half of the weekend, with basically six straight
days of effects of this system by tomorrow.

The broad but still very well-defined upper low is presently
centered over far southern Indiana and inching back northward as a
100+ kt jet maximum rotates around its eastern side. While the
moist conveyor belt has become slightly more disjointed with this
system, there continues to be pockets of deeper moisture rotating
around the northern side supporting scattered showers over the
CWA. In the low-levels, there is an inverted surface trough
across northern Indiana into lower Michigan that looks to serve as
a focus for periods of showers as it pivots back to the west-
southwest tonight. With support of a blend of convective-
allowing models, do have chances for showers (and likely drizzle
given low-level profiles) on the increase from east-to-west
through the evening into overnight. Believe the thunder threat
continues to be confined to just these next several hours over
east central Illinois into northwest Indiana but cannot totally
rule some out through the night as the low center re approaches.

While a few gaps in cloud cover may open up late this afternoon,
once again clouds should fill in this evening. This with the
northeast flow will keep temperatures from dropping much again.
Last night, the entire CWA had minimums basically between 58 and
61 degrees  and would expect basically the same tonight.



255 PM CDT

Saturday through Friday...

Saturday is the final day of showers and gloomy conditions.  The
upper level low pushes north into Indiana, and then shifts northeast
Saturday night and Sunday.  Similar to Friday, better coverage of
showers and possibly a couple of embedded thunderstorms will be
along and east of a McHenry to Pontiac line, closer to the low. Think
the NAM is over forecasting CAPE values as usual.  Shower coverage
diminishes quickly in the evening, but a few isolated
showers/drizzle may linger overnight as upper level vorticity lobes
rotate overhead.

The drying trend continues Sunday. Guidance suggests lake effect
showers may occur over northwest Indiana Sunday morning.  Have
medium confidence in showers impacting NW IN because more northwest
winds will likely push showers east of Porter County.  High pressure
builds overhead Sunday evening and night so odds are we will not see
ample sunshine until Monday.

High pressure and the upper level ridge bring warmer temps to the
region early next week.  A weak upper level shortwave/vort streamer
passes over the region Monday night, but thinking the drier
conditions will inhibit rain from forming.

Latest guidance slows the progression of the next system even
further. The GFS remains the slowest while the ECMWF barely has
precip reaching central IL by Wednesday afternoon. Decided to cap
pop chances at slight chance through Wed aftn, and if the GFS is
correct, Wed night may be dry as well. The other major difference
between the solutions is how robust the line of showers and storms
will be when it marches through.  The ECMWF features a stout swath
of precip ahead of the cold front and strong vorticity streamer. The
GFS has a weaker vort streamer and a smaller line. However, since it
is moving through at a favorable time for thunderstorms, the line
grows as thunderstorms develop. CAPE values are minimal, but it will
be interesting to see how the models handle the line.  The line of
showers and storms shifts east out of the area Friday morning with
cooler, more seasonal air moving in.



For the 18Z TAFs...

The concerns with the aviation forecasts continue to be low
clouds, periods of showers and/or drizzle at times tonight into
Saturday, and gusty northeast winds.

A persistent upper low across southern Indiana will inch its way
northward with moisture and lift continuing to support potential
for showers and lowering clouds. This afternoon though, shower
coverage is likely to be scattered. There are IFR ceilings near
the IL/WI state line and into WI, but these are expected to remain
away from ORD and MDW through the afternoon. Greater coverage of
showers are likely to rotate back over the area during the
nighttime hours and into Saturday. With that, and the time of
day, clouds are likely to lower into the IFR category with even
LIFR clouds possible. Confidence in IFR clouds is high, although
in terms of start time is low. It could be as early as 00Z,
depending on when SHRA/DZ remains. Confidence in sub 800 ft
ceilings during early Saturday morning at ORD and MDW is high.
Clouds are expected to lift very gradually late in the morning on
Saturday, with continued chances for showers and drizzle.

Northeast winds will be gusty through the TAF period and later
Saturday afternoon are likely to back to closer to due north.



255 PM CDT

Northeast winds have ever so slowly eased today and waves have
followed suit, though even at a slower rate. Small Craft Advisory
waves may persist in the Illinois nearshore through Saturday
morning, but at this time confidence is somewhat low. As the low
pressure across the Ohio River Valley slowly works its way back
northward, the possibility of waterspouts will again be on the
increase over the southern part of the lake, mainly on Saturday.
The setup will not be anywhere near as favorable as Wednesday
though. Winds turn toward due north on the lake by Sunday with
waves possibly come back up a bit along the Indiana nearshore.



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




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