Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231904
AFDLOT

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

.SHORT TERM...
203 PM CDT

Through Wednesday...

Quiet conditions are expected through this evening with increasing
chances for shra/tsra late tonight and into tomorrow.

For the remainder of the afternoon and this evening, expect
generally quiet conditions across the region.  Temperatures this
afternoon should top out in the low 80s under partly cloudy skies.
Developing sswly flow has lead to increasing low level moisture
with sfc dewpoints returning into the lower 60s across the area.
Gulf moisture should begin to return in earnest late tonight as
the moist air can finally make it`s way around the sfc ridge
extending into the lower Mississippi Valley and expect sfc
dewpoints in the middle to upper 60s by daybreak. Warm/moist
advection and modest isentropic lift should bring increasing cloud
cover with deeper layer moisture. Timing of pcpn onset is still
a bit uncertain as the models have difficulty latching onto any
particular detailed solution, but expect the general trend should
be for increasing pcpn chances durg the early morning hours with,
at least, sct shra/tsra through the morning hours. There is some
guidance suggesting that there may be a lull in the pcpn chances
durg the early afternoon hours following the passage of a warm
advection wave. The second round of convection would be more
associated with a cold front pushing across the upper Mississippi
valley and Great Lakes. The srn portion of the front should stall
out as high pressure remains parked over the sern CONUS and lower
Mississippi Valley. The progression of the front will define where
the second round of pcpn lays out, with the general consensus
being that the front should push at least into cntrl or srn IL/IN.
Regardless of whether the front lays out across cntrl IL/IN or srn
IL/IN, confidence is high enough to keep the likely PoPs across
portions of the area along and south of a Chicago to Peru line and
then tapering off to the north and perhaps some drying over nwrn
IL by late Wednesday afternoon. Warm/moist advection should
continue through the day tomorrow, with temps reaching the lower
to middle 50s and sfc dewpoints in the lower to middle 70s. Max
heat index values should reach the lower 90s across the swrn
portions of the CWA.

&&

.LONG TERM...
304 AM CDT

Wednesday through Monday...

Wed through Thur: A lobe of vorticity will be lifting northeast
across Northern Illinois in the morning Wed, which may see some
growth/organization to the convection along with some thermal
differential along leading edge with some diabatic heating possibly
enhancing the strength to convection. While the weak mid-lvl trough
and vorticity/warm-air advection wing lifts overhead in the morning,
guidance is beginning to hint that mid-lvl heights may try to
rebound midday/early afternoon ahead of the next lobe of vorticity
progged to arrive later Wed eve and create a lull in
precip/convection through the afternoon hours; however, with PWAT
values around 2" and southerly flow, it`s conceivable that an
isolated storm could pop and easily increase in coverage/intensity.
The signal is not completely clear though on timing of coverage.
Feel the greatest concern could end up being brief heavy downpours
due to the elevated PWAT and expected slow storm movement. Challenge
also arise with high temps, given the increased moisture within the
parcels and possibly thick cloud cover, temps could struggle to warm
beyond 80 to the lower 80s. Have maintained highs in the low/mid
80s, but this will hinge upon cloud cover.

Moist axis will become positioned from the Central Plains northeast
through Northern Illinois to Southern Lower Michigan late Wed ngt
into early Thur, with guidance depicting a weakly organized vort/mid-
lvl trough across Eastern Iowa/Northern Illinois late Wed eve. The
channel of warm/moist air lingering overhead will be drifting
south/southeast by midday Thur. So Thur morning the potential
remains that some stronger storms approaching severe criteria;
however, feel the greatest concern at this point will end up being
on the hydro side through Thur. The mid-lvl ridge will steadily
relax to the southeast across Alabama/Georgia by  Thur aftn, which
should allow the quasi-stationary moist boundary to drif away from
the Great Lakes region. Surface ridging will return late Thur and
bring an end to precip chances.

Fri through Mon: Ensembles continue to show the ridging relaxing
across the Southeast CONUS Fri, with weak troughing sliding south
into the Central Rockies through Sat. Then later in the weekend into
early next week the Southeast ridging retrogrades north, with the
mid-lvl flow transitioning into a quasi-zonal flow. This will likely
lead to periodic chances for precip for Sun through at least Mon of
next week. Temps will initially  be seasonal in the mid/upr 70s Fri,
then as surface ridging departs temps will warm steadily back to the
lower/middle 80s.

Beachler

&&

.AVIATION...
For the 18Z TAFs...

Main issue for the remainder of the afternoon and through the late
evening hours will focus on winds. As a large area of high
pressure lowly moves to the mid atlantic region and trough of low
pressure and associated sfc cold front move a little quicker across
the nrn plains, the swly pressure gradient across the region is
strengthening. Sustained winds have increased to 10-15 kt across
the region. With still ample sunshine, deep mixing is allowing for
gusts to 20-25kt. Expect that winds/gusts will drop off with
sunset, but a generally sly wind to 10 kt or so will persist
through the night.

Forecast concern will quickly shift to cloud cover and pcpn
chances increasing through the late night hours tonight and
morning hours tomorrow. Deeper layer moisture is advecting off of
the GLFMEX and through the ARKLATEX area and into the middle
Mississippi valley. Dewpoints across nrn IL/IN have already risen
into the lower 60s while upstream dewpoints are in the upper 60s
to lower 70s. Expect warm advection and increasing deep layer
moisture to continue to stream newd, bringing increasing chances
for tsra/shra through the late night and into the morning hours.
The models remain in relatively poor agreement on the timing of
pcpn moving into the terminals, so have maintained the general
trend of the going forecast, though did speed up timing a touch to
relflect the higher moisture return going on at the current time.
So, will maintain the general trend of increasing shra/tsra
chances as well as increasing/lower cloud cover. Expect that
confidence in timing should be increasing with subsequent runs of
the high res guidance get a better handle on convection timing and
coverage.

&&

.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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