Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 281640

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1140 AM CDT Wed Sep 28 2016

1140 AM CDT

Upper level low pressure that is very well defined on water vapor
imagery will gradually pinwheel south through tonight. 850 mb
temperatures in the single digits causing steep low level lapse
rates as well as broad large scale ascent will favor isolated to
scattered brief showers inland. Near the lake, an impressive
lake effect/thermodynamic setup is in place with equilibrium
levels over 20kft AGl and at or above 500 j/kg of lake induced
CAPE. This is due to the aforementioned cold temperatures aloft
over the still mild lake waters (69F at the south buoy). As
result, periods of banded lake effect showers and even the
potential for an isolated rumble of thunder. Furthermore, the
thermodynamic setup is also very favorable for waterspouts, with
some observed relatively close to shore this morning and this
continued potential exists into the evening.



257 AM CDT

Through tonight...

Mostly cloudy skies will persist through the day and tonight, as the
upper level shortwave currently centered over Southern Lake Michigan
drifts south through tonight towards Central Indiana. The surface
feature will drift south as well, and continue to spread periodic
showers over Northern Illinois and Northwest Indiana. Some low-level
convergence areas are expected to pivot over the southern half of
the lake and much of the forecast area, which coupled with
considerable cold air aloft will lift parcels and quickly moisten to
produce periodic steadier showers. The warm lake water will help to
produce instability near the lake, and could be enough to produce
some convection through tonight. Based on latest hi-resolution
guidance, expect the coverage for any precipitation today and this
evening to remain scattered.

With respect to highs today, current indications are that the thick
cloud cover will hold temps in the upper 50s across much of the
area. The exception will be locations adjacent to Lake Michigan
where the warmer marine environment will bleed inland and modify
temps into the low 60s. As the shortwave begins to weaken and drift
southeast away from the area, the precip shield will also shift
east/southeast. Thick cloud cover is expected to linger tonight,
which will help slow temps from falling below the low/mid 50s



257 AM CDT

Thursday through Tuesday...

Thur through Fri: Cutoff upper level shortwave continues to be
progged to drift southeast towards the Tenn valley Thur aftn/eve and
deepen. Meanwhile an expanssive surface ridge over Ontario will
spread southwest through the Central Plains, helping to sharpen the
backedge of the departing surface wave over the eastern portions of
the forecast area. Expect clouds to thin considerably along the
western CWA by late Thur, but this may only be short-lived as a
moist channel is poised to feed west from the Atlantic along the
northern fringes of the surface wave over Southern Ohio late
Thur/early Fri. Guidance continues to prog precip chances returning
from east to west Thur aftn, but more focused on the Fri period. But
with the surface ridge sharpening up the western edge, it is
possible the far western CWFA could remain dry Thur ngt into much of
Fri morning.

Guidance continues to indicate the 500mb vort max will be
retrograding across the Ohio Valley to the Great Lakes later in the
day Fri, which will help to push the precip shield west across
Northern Illinois. Temps will continue to be unseasonably cool, with
highs mainly in the 60s Thur to near 70, then as the upper level low
drifts northwest Fri highs could struggle to move beyond the upper

Sat through Tue: Across much of Manitoba/Ontario/Quebec is a 500mb
ridge that continues to block the northward progression of the
cutoff low over the Great Lakes; however, ensemble members continue
to indicate that over the weekend this block will weaken and will
allow the shortwave to lift northeast and depart the Great Lakes. By
the second half of the weekend a trough is poised to slide south
across the Western CONUS, which will help to increase mid-lvl
heights/ridging over the Central Plains/Great Lakes Mon/Tue of next
week. This should help to return temps to near seasonal conditions.
As for precip chances, the best chance doesn`t arrive until closer
to the middle of next week.



For the 12Z TAFs...

643 am...Forecast concerns include mvfr cigs...chance of showers
and wind directions.

Low pressure centered over northwest Indiana will move southeast
today. After a few showers this morning...most of the precip...
and perhaps a few thunderstorms will be mainly east of the
terminals today...though an isolated shower is possible just about
any time today. But later this afternoon into this evening...model
guidance shows a band of precip developing over the lake and
spreading into the Chicago area terminals...and this appears the
best chance for more widespread activity and added a tempo for
this time period. With the cooler air over the warmer waters of
Lake Michigan...showers may fester all night over the southern end
of the lake.

Mvfr cigs are slowly spreading/developing into northern IL this
morning and its possible that mvfr cigs may become prevailing
through early afternoon but confidence is low. Eventually...mvfr
cigs should lift to at least low vfr. Cigs should continue to
lift this evening and may completely scatter out at rfd.

North/northwest winds around 10kts to start are expected to turn
more to the north/northeast later this morning and then remain
north/northeast for the rest of the period. Speeds/gusts are
expected to slowly increase this morning with gusts into the mid
20kt range this afternoon into early this evening...but confidence
regarding how strong wind gusts will become is low...and gusts
will be higher near the lake. While winds may remain gusty
tonight...northeast winds will increase again Thursday morning
with gusts into the mid/upper 20kt range possible. cms



224 am...An area of low pressure over northern Lake Michigan will
move south and merge with a second area of low pressure over
central Illinois. This combined low will slowly move southeast to
the Ohio Valley tonight into Thursday. A strong ridge of high
pressure will build across the upper midwest and Ontario tonight
and Thursday and this will tighten the gradient across the lake...
especially southern Lake Michigan tonight when gales are possible.
The low is expected to move back west Friday and Saturday...
centered over Indiana. This will maintain a strong gradient across
the lake with northeast winds to 30 kt expected for a prolonged
period...likely through Friday night. The gradient will slowly
weaken as the low drifts northeast across the eastern lakes
Saturday night into Sunday.

This low pressure combined with colder air aloft will allow the
potential for waterspouts to develop through Thursday morning...
ending from north to south this afternoon through Thursday
morning. cms


LM...Small Craft Advisory...NEARSHORE WATERS
     UNTIL 7 PM Wednesday.

     Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779...7 PM Wednesday TO 3 AM Thursday.




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