Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 230641
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
141 AM CDT TUE AUG 23 2016

.SHORT TERM...
117 PM CDT

Through Tuesday...

Quiet conditions are expected through the short term forecast
period.

For the remainder of this afternoon and tonight, mostly clear to
clear skies with light sswly-sly winds will prevail as high
pressure begins to shift east, becoming centered over the ern
Great Lakes, while low pressure develops over the nrn plains. The
return flow of Gulf moisture will be very gradual as ridging
extends through the lower Mississippi Valley. Little change to the
general air mass is expected through tonight, with temps topping
out today in the middle to upper 70s. A few spots may yet hit arnd
80 F. Dry air, dewpoints in the lower 50s, and mostly clear skies,
should allow temps to drop off quickly after sunset. With winds
remaining arnd 5 to 10 mph overnight, radiative cooling should be
limited and lows should only be the upper 50s away from the
Chicago urban heat island, where lows tonight should be in the
lower to middle 60s.

The return flow of moisture should begin tomorrow as high pressure
moves off the mid atlantic coast and low pressure moves toward
the upper Mississippi Valley with a trailing cold front extending
through the cntrl plains. While conditions should remain dry
through the day tomorrow, dewpoints are expected to rebound into
the lower to middle 60s. Increasing low level warm advection and
weak height rises as an upper ridge builds across the region
should allow for temps to be a few degrees higher tomorrow, with
highs in the lower 80s.

&&

.LONG TERM...
230 PM CDT

Tuesday night through Monday...

The upper air pattern to start the long term will be characterized
by high pressure across the southeastern U.S. and troughing across
the west.  A closed low embedded in the mean longwave trough will be
centered across the Canadian prairies to start the period, and then
move eastward across the Canadian border Wednesday/Wednesday night
before weakening into an open wave across Ontario Thursday.

A shortwave on the leading edge of the trough will push from the
Central Plains into Wisconsin on Wednesday. This will steer a warm
front across Northeast Illinois late Tuesday night into Wednesday.
Warm/moist advection across the boundary on the nose of a strong low
level jet will lead to a fairly organized round of showers and
thunderstorms, especially late Tuesday night into early Wednesday
with a weakening trend toward midday. Elevated instability is not
super high with this first wave. Following the morning activity,
some recoveries are expected with a warm, moist and muggy airmass
filling in behind the warm front. The airmass will therefore
become quite unstable, but forcing mechanisms and their respective
locations are still a bit muddled at this point to suggest how
widespread any afternoon/evening activity will be. There will be a
surface trough/boundary to our west and continued disturbances in
the sw flow between the western trough and southeastern ridge and
warm moist advection continued so will hold onto some chance pops
Wednesday afternoon, but at this point the upper and lower forcing
would suggest the more organized activity will be in Iowa.

The surface trough axis moves closer Wednesday night as an upper
level jet strengthens to our north and west which would suggest
another uptick in activity during this period. The cold front moves
through Thursday morning which should greatly reduce coverage, with
afternoon activity focused into Northwest Indiana.

Friday looks to be a repeat of the fabulous weather we are having
today. High pressure in place will result in sunny skies, dewpoints
in the 50s to lower 60s, and highs in the upper 70s. The high
departs east, and we return our focus to troughing in the west and
the ridge holding in the southeast. High upper heights and the
departing surface low look to keep most of Saturday dry, but the
shortwave in the west will become more progressive and shift into
the Central Plains Saturday and reach Lake Superior Sunday. Current
guidance keeps the influence of the ridge strong enough that most of
the energy and forcing for precipitation will head north. Still we
look to at least get clipped by this wave such that chances of
showers and storms are warranted with the focus on late Saturday
night into early Sunday.

KMD

&&

.AVIATION...
For the 06Z TAFs...

Another quiet weather day is expected across the area today. Winds
will remain southerly and increase up around 10 KT by mid to late
morning, with a few occasional gusts up to 16 KT also possible
this afternoon. Otherwise, no significant weather is expected
until Wednesday, when a disturbed weather pattern will likely
result in a period or two of thunderstorms through Thursday. Given
that the chances for these thunderstorms could enter the picture
as early as 10 to 12 UTC on Wednesday I added a PROB30 group to
the end of the new 30 hour ORD TAF.

KJB

&&

.MARINE...
141 AM CDT

The combination of a surface ridge of high pressure over the
Atlantic coast and a developing surface low over the Dakotas will
continue to support stout south-southwesterly winds (up to 25 to
30 KT at times) over the lake through Wednesday. The Dakota low is
expected to gradually shift east-northeastward towards the upper
Great Lakes and southern Ontario by late Wednesday. Ultimately
this will allow an associated cold front to shift across the lake
by early Thursday, essentially ending the southerly winds and
resulting in a west-northwesterly wind into Friday. Surface high
pressure will then build over the western Lakes region by Friday
night.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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