Area Forecast Discussion
Issued by NWS Chicago, IL

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247
FXUS63 KLOT 231754
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1254 PM CDT TUE AUG 23 2016

.SHORT TERM...
304 AM CDT

Through tonight...

Broad surface ridging remains an influence on the Western Great
Lakes, coupled with mid-lvl height rises this morning. But with the
surface ridge becoming centered downstream of the area, flow has
turned southerly and holding temps early this morning in the lower
60s and may only fall prior to daybreak into the upper 50s in a few
locations. Dewpoints will remain in the 50s, but with moisture
beginning to stream north this afternoon, expect a steady increase
in humidity across Northern Illinois by late this afternoon/evening.
Mid-lvl height rises will linger through much of the aftn/eve,
delaying the chance of precip until possibly just prior to daybreak
Wed. Hi-res guidance is pretty consistent with minimal spread in
heights aloft maintaining some diffluence until around 6-12z Wed,
and more focused in the 9-12z timeframe, so have delayed precip
chances until that time. Temps this afternoon will easily warm into
the lower 80s, with the abundance of sunshine and a gradual increase
in cirrus clouds late this afternoon. Thick cloud cover coupled with
warm-air advection and steadily increasing low-level moisture will
keep temps elevated tonight in the mid/upr 60s. Could see a few
sites not dip below 70 in the far southwest portions of
LaSalle/Livingston Counties.

Beachler

&&

.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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