Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 010808

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
308 AM CDT Sat Oct 1 2016

307 AM CDT

Through Tonight...

The main focus for today is the timing of showers and
thunderstorms with the next disturbance expected to impact
portions of the area.

Water vapor imagery this morning shows the center circulation of
the upper low now centered over far southeastern Indiana. This
continues to allow small scale perturbations to rotate westward
along its northern periphery, namely over the lower Great Lakes.
One such disturbance is currently driving a complex of showers,
with some embedded thunderstorms over portions of northeastern
Indiana. Believe it or not, these storms will be headed for
portions of northeastern Illinois and northwestern Indiana this
morning as they shift westward within the deep broad cyclonic flow
over the region. Current timing of these showers and storms puts
them in and around the Chicago area by around 14Z (9am) this
morning. As a result, I have boosted POPs into the 70 to 80
percent range across the eastern CWA mid to late this morning.
Although it is not a good diurnal time of day for thunderstorms
this time of year, the northwestward influx of cold mid-level
temperatures (500MB temps -18 to -20C) associated with the
northward moving upper low, will support the presence of steep
enough lapse rates for charge separation over the area. Therefore,
I have also mentioned isolated thunderstorms in the forecast with
these showers this morning.

Additional showers and storms will remain a good bet (at least on
a scattered basis) over the eastern half of the area into the
early to mid afternoon hours as the main PV anomaly retrogrades
northwestward towards the area. Forecast guidance continues to
indicate that this will act to drive an inverted surface trough
northwestward towards southern portions of the Chicago area by
early afternoon. As a result, this will continue to act as a focus
for showers and some storms through mid afternoon, especially over
my southern and eastern CWA.

Any lingering showers or storms should come to an end over the
area by early this evening. However, there will remain a small
chance for a few showers later Saturday night as another area of
showers tries to drop southward over portions of northeastern



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 06Z TAFs...

One batch of drizzle is passing through our western terminals as
of 6z, while a batch of showers and thunderstorms continues to
cycle northeastward ahead of inverted surface trough axis.
Rap/Hrrr guidance rotate this trough axis through the area this
morning, with shower coverage expected to increase around 10-11z
in NW Indiana, then spread westward. This axis does not completely
spread west through the terminals, so these showers may have their
best kick along and east of the Chicago TAF sites. Have not
introduced thunder, but isolated thunder looks to at least get
into NW Indiana and adjacent Lake Michigan. Will monitor overnight
progression for ORD/MDW but at this point feel that better threat
would be east and south.

Cigs are a bit of a smorgasbord with a mix of IFR and MVFR (most
dominant) and some VFR. Winds remain elevated such that widespread
IFR is not expected, but some downward trend is possible later
tonight into early Saturday when the next batch of precip arrives.
At least scattered showers continue in the afternoon with some
increase in Cigs, and flow off the lake could contribute to lake
effect showers as well.

Once again guidance is trying to bring conditions back to IFR
later tonight. This has been somewhat overdone in nights past, but
winds will lighten tonight and shift to NNW which may allow some
IFR/LIFR cigs to sneak in. Still a ways off on this so continued
the downward trend of the past TAF cycle.



306 AM CDT

An inverted low pressure trough will expand over the lake this
morning as the center of low pressure moves to the southern tip of
Lake Michigan. This will maintain an east-northeast wind today.
Waves at the Wilmette buoy have settled in the 3.5- 4 ft range or
so, and therefore will carry 3-5 occ 7 ft in the NSH and let the
small craft Advisory go with the wind field weakening. Expect
waves will hold in this range at least through the morning for the
IL nearshore. The low will eject to the northeast toward Lake
Huron tonight into Sunday which will finally allow high pressure
to begin moving east, and will also let winds shift to more NNW.
This will aid in decreasing the waves tonight into Sunday. The
high will build across the northern and western Great Lakes Sunday
night, and then it will shift to New England through the middle of
the week. The next low will head north of the lakes mid to late



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




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