Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231916
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
216 PM CDT SAT JUL 23 2016

.UPDATE...
1052 AM CDT

No big changes made to the forecast for this afternoon. Still
feel as if isolated thunderstorms will spring early this
afternoon, and if so they would have slow storm motion and pose
localized heavy rain and brief gusty wind hazards. Also no changes
made to the heat headlines. The consistent forecast message
remains that it is very humid today, with the combined effects of
higher temperatures and sustained heat primarily in the
southwest/south half of the forecast area.

A quick rise today to both temperatures...already in the mid to
upper 80s...and dew points...already 75 to 80. These already high
dew points should climb a degree or two. Outside of the southern
CWA, temperatures will begin to be stunted by incoming cirrus and
already thickened cumulus. Webcams over Chicago have indicated
this cumulus is already somewhat stratified, which with light
onshore flow should allow a temperature rise of only a couple more
degrees in the city. Because of the influx of cloud cover in
northern locations and the forecast still remaining well below
criteria, have not made any additions to the heat headline. Again
very high dew points will make for humid conditions area-wide
which could have impacts to anyone doing prolonged strenuous
activities.

The warm humid air mass is plenty unstable across the area with
a modified 12Z DVN sounding and local aircraft soundings
indicating only a very small cap left. So the quick cumulus is
not surprising. Given the very light low-level wind flow and no
real mid-level impulse, there still appears to be only two areas
of focus...1) 750-900mb warm advection across north central
Illinois mainly north of I-80 and 2.) the lake breeze across the
Chicago area and far northwest Indiana. The first area has seen
some isolated radar echoes develop recently and expect that to
continue, though the cirrus shield will likely activity from
becoming too numerous or intense. A slow east-northeast motion is
expected with this. As for the lake breeze, radar and mesonet
sites indicate this boundary near I-94 in Lake County IL down to
just east of Midway Airport. convergence along this feature will
remain minimal as it inches westward but enough with such weak of
a cap to probably spring isolated showers/storms during the early
afternoon. This is all well covered in the going forecast.

As noted in earlier AFD, the high instability/low shear profiles
will make for locally heavy rain and brief gusty wind hazard. If
storms develop, they will have a propensity to produce brief
outflows that might trigger other isolated storms.

Overall not seeing anything at this time screaming for higher
storm chances this evening, as low-level jet and organized storm
activity upstream should be still to our northwest with higher
potential appearing to arrive overnight.

MTF

&&

.SHORT TERM...
335 AM CDT

Through tonight...

Decided to keep heat headlines as is today with another hot and
muggy day expected.  A few isolated thunderstorms popped up
overnight and have very low slight chance PoPs through this morning
for any additional isolated storms that may form.  Confidence is low
in thunderstorm coverage this morning, but increases as forcing
develops this afternoon.  An upper level wave shifts east and may
kick off storms mainly west of a Rockford to Fowler IN line early
this afternoon. A lake breeze is expected and confidence is
increasing in storms forming along it early this afternoon. While
CAPE is abundant, shear is not. The steering flow is also weak, but
PWATs remain high at 1.5-2 inches. Therefore expecting storms to
move slowly and produce locally heavy rainfall. Could see an
isolated storm produce damaging winds as well but widespread severe
weather is not expected.

Hi-res guidance then suggests a convective complex will pass over
Wisconsin tonight. Guidance differs on the complex`s exact path, but
a CAPE gradient could direct it southeast through the warning area.
The complex should be deteriorating, so once again not expecting
widespread high winds, but heavy rain is possible.

Cloud cover and precip will have a direct impact on temps today so
opted to not make any changes to the going heat headlines.  Many
locations are beginning the day in the mid 70s to around 80 with
dewpoints in the mid 70s. So regardless of how hot we actually get,
it will feel muggy and icky outside.

JEE

&&

.LONG TERM...
213 PM CDT

Sunday night through Saturday...

Sunday evening through Monday...The upper trough currently moving
east across southern Saskatchewan will swing across
western/central Ontario Sunday night and Monday. The associated
cool front will be dropping southward across the southern half or
so of the local area Sunday evening. Will continue with chance
pops Sunday evening/night across the south to southeast half of
the area as the front will be slow to progress and there is a
decent chance of an embedded shortwave moving through the main
upper flow. Some activity may linger Monday morning across the far
southern counties but surface high pressure will be quick to build
in bringing drier air with it. 850 mb temps cool to 16-17C Monday
with abundant sunshine expected once any lingering clouds clear
south. This supports highs in the mid to upper 80s but onshore
flow will develop near Lake Michigan keeping areas within a few
miles of the lake in the lower 80s. Lows Monday night will fall
into the mid 60s with upper 60s/around 70 in the Chicago Metro.
High pressure will be overhead Tuesday and shift to the east
Tuesday night. High temps will be similar to Monday with
continued cooling near Lake Michigan.

Wednesday through Saturday...The generally zonal upper flow
of Monday and Tuesday will buckle a bit mid week with medium
range guidance suggesting a more pronounced larger scale trough
developing into and over the weekend. Until then...the surface
pattern is fairly ill-defined through Wednesday but the upper
flow patter supports the passage of embedded shortwaves. This will
probably lead to transient shower/storm chances Wednesday and
Thursday. A more coherent surface low may move across Thursday
with a stronger upper wave noted in guidance but this has not
been the most consistent feature. Guidance is in decent agreement
on a well defined upper trough crossing the area sometime Friday
or Saturday which will bring a more organized chance for storms
but the timing is not clear at this range. Low/mid level temps do
not fluctuate much through late week. Cloud cover may be rather
persistent from Wednesday onward with any convection also working
to hold temps down some as well. Will continue with highs in the
lower to mid 80s...but may end up being warmer if convection is
kept to a minimum.

MDB

&&

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

The concerns with the Chicago area airports are isolated to
scattered storms this afternoon, the chance of storms this
evening, and then likely thunderstorms overnight. In addition, a
nearby slow-moving lake breeze will lead to winds shifting
easterly early this afternoon.

A lake breeze boundary as of 1745Z is just east of MDW and on the
eastern part of the airfield at ORD. This should creep its way
westward, bringing a change to easterly winds of 5 to 9 kt after
its passage. There have been signs of towering cumulus and even
brief light echoes on the lake breeze. Believe there will be
isolated showers or storms in the vicinity this afternoon, but the
coverage may be even less than 20 percent. Still, any storms will
produce brief outflow that could change the wind direction or
undulate the lake breeze.

Scattered storms across southwest Wisconsin may become greater in
coverage and could track a little south of east and clip at least
ORD 21Z-23Z, but this is a low confidence scenario.

As for tonight, there is the likelihood of scattered storms
developing across the eastern half of Iowa late this afternoon and
some of this may work its way into northern Illinois. Confidence
on this is also low so continue the TAF dry for the evening. More
likely is overnight storms, at least for a 2-3 hour period, as
part of the southern edge of an organized complex moving mainly
through Wisconsin.

A cold front will move east on Sunday with southwest winds in
advance of this. There could be some MVFR cigs early, especially
if rain/storms pan out late tonight. Along the cold front,
scattered storms are expected to develop and could impact the
airports during the mid-afternoon through early evening on Sunday.

MTF

&&

.MARINE...
213 PM CDT

Light and variable winds will transition to a steadier but still
fairly light south to southeast direction this evening and
tonight. Surface low pressure over southern Saskatchewan this
afternoon will track into western Ontario by Sunday afternoon.
This will bring a steady south to southwest breeze across the lake
by daybreak or so Sunday and persist through the afternoon. Gusts
near 20 kt are likely across the IL/IN nearshore but not expecting
small craft advisory thresholds being met at this time. A cool
frontal boundary will move east- southeast across the lake late
Sunday afternoon and Sunday evening shifting winds to the west and
eventually northwest by late Sunday evening. Thunderstorms will
become likely across much of the lake by late evening ahead of the
advancing front...then diminish in coverage from northwest to
southeast Sunday.

Wind speeds will diminish into Monday afternoon. High pressure
will spread in from the west so winds may end up coming onshore
around the lake later Monday afternoon. Light onshore flow is more
likely Tuesday with high pressure pretty much overhead. The
surface pattern becomes more ill-defined Wednesday into the
weekend keeping wind speeds generally light with erratic direction
at times...especially Wednesday and Thursday. A more persistent
north to northeast direction is favored into the weekend.
Thunderstorm chances increase again Wednesday through Saturday and
may substantially modify the wind fields depending on how
organized they are.

MDB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Excessive Heat Warning...ILZ008-ILZ010-ILZ019-ILZ021-ILZ023-
     ILZ032-ILZ033-ILZ039 UNTIL 7 PM Saturday.

     Heat Advisory...ILZ020-ILZ022 UNTIL 7 PM Saturday.

IN...Heat Advisory...INZ010-INZ011-INZ019 UNTIL 7 PM Saturday.

LM...NONE.
&&

$$

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