Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 282033

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
333 PM CDT Fri Oct 28 2016

250 PM CDT

Through Saturday...

Main forecast concern continues to be how strong winds get this
evening. Additional focus has been on cloud and temperature
trends. Northwest mid-level flow carrying extensive thick cirrus
into the area has slowed the temperature rise today, though the
gusty south-southwest winds have ramped up low level thermal
advection. Thus temperatures have temperatures have gradually
warmed into the 60s, though will be lower than initially thought.
After any minor fall in the early evening, temperatures will
likely rise slightly the rest of the evening and then stay nearly
steady overnight as south-southwest to southwest winds ramp up.
Lows in the 60s area wide is exceptionally mild for late October,
given normal lows in the upper 30s-low 40s.

Regarding winds this evening, intense LLJ will develop as weak low
pressure tracks across Lake Superior, with 50-60 kt winds between
1kft and 3kft AGL. Pressure fall magnitude will maximize across
northern Lake Michigan, with a glancing blow for our area. With
such high winds not far above the ground tonight, continue to have
concern for a brief period of 30-35+ kt gusts. However, it
continues to appear the limiting factor will be mixing heights,
especially since mixing will not be as deep today with slightly
cooler temperatures than originally expected. Lowest levels will
remain mixed given temperatures likely rising later this evening,
but mixing heights staying below 900 feet should prevent the
highest winds aloft from being tapped into, with peak gusts in the
30-35 mph range. With all this being said, still cannot rule out
brief isolated gusts to 40 mph since the the high winds aloft will
be really close.

On Saturday, main focus is the timing of a cold front trailing
from low pressure shifting into eastern Canada. This has been
problematic all week, and latest guidance has once again sped up,
which favors the front dropping south across northern Illinois
during the early-mid afternoon. There will also likely be some
lake enhancement to it near the lakeshore, forcing it southward
slightly quicker. Stratus will move back into the area just ahead
of and behind the cold front, if not sooner in far northern
Illinois. Prior to the frontal passage, thermal ridge will still
be over the area, featuring 850 mb temps in the mid teens and 925
mb temps in the mid to high teens Celsius. Thus anticipate highs
to reach the 70s area wide except perhaps far northeast Illinois,
where FROPA will occur earliest. Warmest temps will be south of
I-80, soaring into the mid to upper 70s, aided by west-southwest
winds gusting up to 25 mph. The tightening baroclinic zone later
in the day will start up frontogenetical processes over northern
Illinois, so cannot rule out some post frontal sprinkles.
Maintained a dry forecast for now, with better forcing moving over
the area in the evening.



222 PM CDT

Saturday night through Friday...

A chance of rain Saturday night is followed by drier conditions and
above normal temperatures through early next week.  Another round of
showers and storms is possible mid week followed by a return to
seasonal temps.

A weak cold front will sink over northern IL Saturday evening. Winds
will vary from out of the northeast north of the front to southwest
winds to the south. Guidance continues to suggest showers will form
along the front and increase in coverage and intensity through the
evening. The heaviest precip will fall north of the front along the
IL/WI state line. Precip spreads south and shifts east Saturday
night into Sunday morning as an upper level wave moves through.
Could see some embedded thunder, but have low confidence in thunder
occurrence and coverage. Forecast soundings feature a stout
inversion and little to no CAPE. However, forcing along the front
looks decent so kept isolated/slight chance of thunder in the
forecast through Sunday morning.

Cooler air and northeast winds set up Sunday, keeping high temps in
the mid 50s to the low 60s across central IL.  Cloud cover will take
awhile to clear and some areas may see a bit of sun just before the
sun sets.

High pressure shifts east Monday and winds become south. Warmer air
is brought in by the gusty south winds. Gusts look to be around 20-
25 MPH with high temps in the mid 60s to low 70s. Temps could be a
bit warmer if clouds clear quicker. Similarly, sunnier conditions
will likely lead to enhanced mixing and higher gusts, possibly
around 30+ MPH. Dry and warmer than normal conditions continue
Tuesday and into Wednesday.

The next system brings showers and storms to the region mid week.
The GFS is faster than the ECMWF, but the best chance for widespread
precip looks to be Wednesday evening and night just ahead of the
cold front.  High pressure follows the front along with more
seasonal high temps around 60.



For the 18Z TAFs...

* Strong gusty south/southwest winds this evening
* LLWS at 1500-2000 ft AGL this evening/tonight
* Patchy MVFR clouds this afternoon and possibly overnight into
  Saturday morning
* Lower MVFR CIGs Saturday afternoon
* Wind shift to northeast late Saturday afternoon

Primary focus continues to be on magnitude of surface winds/gusts
this evening along with LLWS potential. Thick high clouds have
slowed wind increase somewhat through mid day, though tightening
pressure gradient will compensate and support 15-17 kt winds and
20-25 kt gusts this afternoon. This evening into the overnight, an
intense southwest low level jet between 1000-3000 ft AGL will
shift across the area. This will feature 50-60 kt winds, so have
made earlier mention of LLWS in the TAFs. Surface temperatures
will likely not fall or even slowly rise this evening, so there
should be enough mixing to tap into some of the higher winds aloft
and generate gusts of 25-30 kt. Still cannot rule out gusts over
30 kt but confidence has diminished in prevailing 20 kt sustained
winds and 30 kt gusts at the eastern terminals. Winds will then
shift west and gradually diminish late tonight into Saturday. A
cold front will drop south on Saturday afternoon and shift winds
to northeast 10+ kt late in the afternoon.

Regarding CIGs, patchy MVFR CIGs, mainly in vicinity of RFD, but
some scattered 020 clouds occurring farther east, should
dissipate this afternoon. Forecast soundings lend uncertainty to
late tonight into tomorrow with MVFR potential, but have better
confidence in scenario being that these MVFR clouds will be just
ahead of and behind the cold front Saturday afternoon. As the cold
front pushes south late in the afternoon, cool marine air behind
it could result in low MVFR or even IFR CIGs arriving.



304 PM CDT

Headlines...Gale warning and small craft advisory remain as is.

Gusts have increased across the lake with gales being reported at
the north buoy. Max gale force gusts should be around 40 kt. Gales
become southwest this evening and then ease overnight as the
gradient weakens over the lake. The low over Minnesota shifts over
Lake Superior overnight. The low`s cold front slowly sinks south
down the lake Saturday with north winds 15-25 kt moving in behind
it. A small craft advisory may be needed Saturday night into
Sunday due to hazardous waves and possibly 20-25 kt winds.

Winds weaken and veer to southeast Sunday night as high pressure
shifts east over the lake. Southeast to south winds increase
to 30 kt ahead of the next low Monday. The low passes over or just
west of Lake Superior Monday night, and winds veer to west as the
low shifts east. High pressure arrives Wednesday leading to
lighter winds. Guidance then differs greatly on the pattern late
next week so have low confidence in the forecast past Wednesday



LM...Gale Warning...LMZ777-LMZ779 UNTIL 3 AM Saturday.

     Small Craft Advisory...NEARSHORE WATERS
     UNTIL 7 AM Saturday.




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