Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231552 AAA
AFDLOT

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
1052 AM 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...
335 AM CDT

Sunday through Friday...

Debated issuing a heat advisory for Sunday as many areas may have
heat indices in excess of 105 degrees. However, a cold front shifts
through and storms are possible ahead of it mainly south of I-88.
The storms and associated clouds could lead to cooler temps and heat
indices, so opted to hold off on the advisory for now.  One could
also argue that a heat advisory may be needed due to the cumulative
effects of multiple days of heat and humidity. Will pass these
concerns on to the day shift.

Widespread severe weather is not expected Sunday, but do have
concerns about locally heavy rainfall.

Outside of morning storms well south of I-80, Monday looks dry and
cooler.  Highs will be in the mid 80s, but only the low 80s along
the lake. Tuesday looks very similar to Monday with continued dry
conditions.

Mid to late next week looks to have near normal temps, but multiple
chances of showers and storms.  Another cold front moves through
Wednesday followed by a surface low Thursday and then an upper level
wave Saturday. There will be dry hours, but the end of the week
looks wet and busy.

JEE

&&

.AVIATION...
For the 12Z TAFs...

Concerns
-MVFR cigs and vsbys
-Thunder chances today and again overnight

SPC analysis depicts we are unstable and only slightly capped if
not at all. Meanwhile AMDAR soundings early this morning do depict
a warm nose at 700 mb that may be tough to overcome for signficant
coverage storms. We do not have a trigger quite yet but there are
some weaker upstream shortwaves along with a wind shift coming
near the lake. Hi res guidance including the rap which was way too
low on dewpoints this morning and now depicts more in the way of
isolated scattered thunderstorms later this morning near the lake
breeze. These could produce heavy rain and gusty winds. They
could be slow moving and send outflows given the slow storm motion
as well creating erratic winds. Confidence on coverage is still
not super high, but high enough to continue the VCTS for now. The
focus is for the Chicago terminals on an axis along and south of
ORD/MDW. Expect lower VFR/MVFR cigs to ease this morning as we
warm in the coming hours.

Forecast soundings from the rap/hrrr depict stabilization
following any of these storms and with the lake breeze and
possibly some upper level subsidence late in the afternoon.

Renewed chances for thunderstorms are then tied to the slow
moving compact shortwave across the central plains that will
approach the Mississippi River late this afternoon/early evening.
With best forcing headed north initially, it may just graze our
area (KRFD) this afternoon and evening and may be more of an issue
on late tonight into Sunday, but coverage across NE Illinois is
still uncertain at this point given the main forcing in Wisconsin
and a warm advective wing across Chicago overnight into early
Sunday.

KMD

&&

.MARINE...
309 AM CDT

High pressure will build over the Western Great Lakes today
resulting in generally light winds. A weak cold front will shift
winds to northerly before continuing to veer east-southeasterly.
The pressure gradient will increase Saturday night as a trough of
a low pressure system moving across Ontario approaches. This will
result in south to southwest winds/gusts up to 25 kt on Sunday. A
cold front trailing from the low pressure system will move across
the lake Sunday night, with fairly light west and northwest winds
on Monday behind the front. High pressure spreading to the Eastern
Great Lakes by mid week will bring light winds Monday night
through Wednesday.

Thunderstorms are possible across the lake over the weekend,
especially tonight into Sunday, with locally higher winds and
waves possible with any stronger thunderstorms.

KMD/RC

&&

.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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