Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 281751

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1251 PM CDT Fri Oct 28 2016


1115 AM CDT

Main forecast concern continues to be how strong winds get this
evening. Additional focus is on cloud and temperature trends.
Northwest mid- level flow is carrying extensive thick cirrus into
the area. There also has been some low cloud development in north
central Illinois. Tweaked up sky cover through mid afternoon and
may need to make further adjustments. Increasing south- southwest
surface flow and warm advection aloft will try to counteract the
fairly thick clouds to warm temperatures through the afternoon,
though highs appear they may fall short of previous forecast based
on where readings are as of late morning. The gusty south-
southwest winds will ramp up low level thermal advection, so it`s
possible that highs occur later than usual with such a low sun
angle and stay nearly steady into the evening hours. Overall,
lowered highs a few degrees to mid 60s to around 70, but they
could end up more in the mid-upper 60s range.

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+ kt gusts, though it does appear
the limiting factor will be mixing heights. An already sharp
inversion on the morning RAOBs will further steepen as core of
thermal ridge gets advected in. Mixing heights as a result will
likely not get much above 1500 ft this afternoon, and though
lowest levels will remain mixed given little/no temperature drop
this evening, mixing heights will diminish some and prevent us
from tapping into the highest winds aloft. Suspect peak gusts will
be up to 35 mph or so, and less likely to be in the 40-45 mph
range for the above reasons. Looking back at the evening high wind
event on October 17th, DVN 00z sounding was still mixed above 850
mb and surface temperatures were well into the 70s, supporting
tapping into 40+ kt winds aloft, along with the much deeper
surface low in that event.



354 AM CDT

Today through Sunday night...

Main short term forecast concerns are breezy and unseasonably mild
conditions today into Saturday, before shifting to a cold frontal
passage Saturday afternoon signaling a shift to more typical
seasonable temperatures. Rain is expected to develop post-
frontally Saturday evening, with the highest potential overnight
Saturday night into early Sunday.

Early morning surface analysis depicts high pressure centered
across the Midwest, just off to the east of the forecast area. Low
pressure was developing to our northwest across the Dakotas in
response to a 115 kt upper jet streak and associated mid-level
short wave propagating eastward from the northern Rockies. The
surface low is progged to develop quickly eastward across the
northern Great Lakes through tonight, with surface pressure falls
and a tightening surface pressure gradient inducing strengthening
south winds across the area. Given the relative lack of low
cloudiness upstream this morning, suspect that model soundings mix
the boundary layer a little too shallowly this afternoon this
evening/overnight. Thus have continued to lean 2-3 degrees warmer
than mos guidance for temps today, closer to what 925 mb
climatology would support, with afternoon highs from the low 70`s
west/south to the upper 60`s in the northeast. With models likely
decoupling boundary layer too strongly tonight, expect gusty
conditions to continue with strongest winds likely occurring this
evening in the 35 mph range. Temps will likely dip a little with
sunset, but should then remain fairly steady in the low 60`s
overnight in the persistent south-southwest winds.

As the surface low continues to track away from the northern Great
Lakes Saturday, a trailing cold front will sag across the region.
Guidance has wavered a little with the timing of the front into
the cwa over the past couple of days, though there is now pretty
good agreement that the boundary will settle into northern IL and
to the south end of Lake Michigan during the afternoon. Southwest
winds and dry thermodynamic profiles in the warm sector ahead of
the front will once again support unseasonably mild temperatures
in the mid-upper 70`s in many area, though cold fropa and a shift
to northeast winds will temper things a bit across far
northern/northeast IL. By evening, another short-wave is progged
to be approaching from the west, with increasing forcing for deep
ascent developing and inducing frontogenesis within the baroclinic
zone north of the surface cold front, leading to saturation and
precipitation development across the region. Most of this appears
to be initially off to the north or across the far northern
portion of the cwa near the IL/WI border area into early evening,
eventually spreading south across the area overnight. Passage of
the short wave trough axis Sunday morning, and development of
subsidence should bring an end to precip by midday, with high
pressure spreading in Sunday afternoon and night. While cooler air
spreads in behind the front Saturday night and Sunday,
temperatures are expected to be near normal for late October, in
the mid-upper 50`s north and low-mid 60`s south.



405 AM CDT

Monday through Friday...

Above normal warmth returns early next week as upper pattern
remains a progressive and largely zonal configuration. Broad
ridging across the Midwest Monday, ahead of an upper trough
exiting from the Rockies, will set the stage for another
unseasonably warm day for Halloween as surface low pressure
deepening across the northern plains again produces gusty south
winds across the region. A weak cold front trailing this low is
progged across the area late Monday night, though a dry fropa is
expected with the return of moisture blocked by high pressure
across the Gulf. Guidance is in generally good agreement with
another upper trough and surface low pressure combination
approaching mid-week, with a little better potential for precip.
Rebuilding of upper ridge is depicted by the end of the week, with
no real intrusions of cold air noted. Thus after temperatures in
the 70`s with Monday`s gusty south winds, the remainder of the
week appears to remain slightly above average in temperature with
highs in the upper 50`s and 60`s.



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.



409 AM CDT

Active and changing conditions will be the rule across the lake
the next few days with a few periods of gales possible.

South winds will ramp up considerably today as low pressure will
quickly move from the upper Midwest to north of Lake Superior
tonight. A modestly strong pressure gradient will exist ahead
of the low as high pressure remains anchored over the
Appalachians. The strongest winds will be from mid afternoon
through evening as winds shift more southwesterly. Gale force
winds to 40 kt, possibly higher across the north, are expected.

The low will continue east tonight into Saturday with a weaker
pressure gradient behind it. High pressure will move into the
Canadian prairies and the upper Midwest Saturday, and this will
help steer a cold front across the lake from north to south
Saturday and Saturday evening. A weak low will pass just to the
south of the lake Saturday night into Sunday which will maintain a
modestly breezy northeast wind, which if a bit stronger could
result in small craft advisory winds along the nearshore for a
brief time early Sunday. The high will transit the lake later
Sunday and Sunday night.

This will be quickly followed by another quick moving low
pressure system that will move from the upper Midwest to across
Lake Superior Monday night. This will likely result in another
period of southwest gales ahead of the low. High pressure will
bring quieter conditions Tuesday.



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

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

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




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