Severe Thunderstorm Warning 65, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1247 PM CDT Mon Sep 3 2018

.UPDATE...
1113 AM CDT

Monitoring thunderstorm trends in the near term, with additional
thunderstorm development expected this afternoon into the evening.
Outside of areas in far northwest IL, along/west of I-39, mainly
dry conditions have been in place over the last couple of hours.
Several hours of warming in a very moist environment has allowed
instability to build and CIN to erode, with low level lapse rates
increasing. Despite upper level ridge building across the region,
will see mid level energy ride across the region this afternoon.
This increasing ascent and instability with some focus in the
lower levels should allow thunderstorm development to occur this
afternoon.

Initial coverage here in the near term will likely remain low,
however, do expect coverage to increase through mid afternoon
across much of northern IL and northwest IN. Thunderstorms are
likely, especially north of I-80, with isolated to widely
scattered thunderstorms south. Lowering bulk shear and weak mid
level lapse rates should limit any severe threat, though a few
stronger storms with gusty winds are possible. Flooding is
possible, especially near the Illinois and Wisconsin state line.

Rodriguez

&&

.SHORT TERM...
329 AM CDT

Through tonight...

Main concern for today and tonight will continue to be thunderstorms
chances and precipitation amounts.

For the remainder of the overnight hours, have been monitoring
continued development of scattered showers and thunderstorms across
northern Illinois.  The main focus for the continued development
looks to be a broad zone of isentropic lift and weak shortwave
activity rippling through a belt of fast swly flow aloft.  The
longwave pattern has varied little over the past few days, though
the main focus continues to shift slightly northwest as the center
of the Bermuda High sits centered over the Mid-Atlantic region with
gradually building ridging extending across the Middle Mississippi
Valley.  The short range guidance continues to advertise slight
height rises over the local area, which support the idea of the
focus of the most organized convective activity to be slightly to
the north and west of that observed over the past couple days.
However, there will still be enough broad instability and moisture,
with both the DVN and ILX 03/00Z soundings indicating 1.75 inch
pwats and the short range guidance indicating 2-3k J/kg CAPE, that
periods of showers and thunderstorms will be possible.  While
widespread cloud cover and weak forcing will minimize any severe
threat, localized heavy rain will be a concern.  Locations north of
the I-88 corridor will need to be monitored closely for flooding
potential since that area received anywhere from 3 to 7 inches of
rain yesterday and that area remains under a Slight Risk for
Excessive rainfall per the latest WPC Outlook.  The exact details of
convective activity today remain somewhat uncertain except that the
main area impacted should be generally north of the I-80 corridor,
but given that any residual outflow boundaries or differential
heating boundaries could provide enough sfc forcing for at least
isold-sct tsra, even the srn portions of the CWA can't be ruled out.
The temperature forecast for today will also have some bust
potential.  Currently, expect that much of the area along and south
of I-80 should peg 90 F with lower temps to the north under more
extensive cloud cover.  However, should the cloud cover become
thicker and more extensive over the srn locations, the 90 F mark
could be hard to hit.  However, the screaming is that this Labor Day
should be warm and humid with showers and thunderstorms likely over
the northern portions of the CWA and a lower chance for pcpn to the
south.

&&

.LONG TERM...
236 AM CDT

Tuesday through Sunday...

Overview...The active pattern continues through the extended with
multiple chances for showers and storms. Hot and humid conditions
also continue through Wednesday, but a cold front brings cooler
temps to the region for the latter half of the week.

Isolated afternoon storms are possible Tuesday, but I have low
confidence in coverage and timing. I have higher confidence in hot
and humid conditions resulting in heat indices in the mid to upper
90s. Afternoon storms should dissipate as the sun goes down.

The next chance of precip arrives ahead of a cold front Wednesday
morning. I have high confidence in rain across the area.  Given the
slow nature of the front, I think heat indices will once again be in
the 90s.

Luckily a cooler air mass follows the cold front with highs only in
the 70s to around 80 Thursday. Guidance continues to indicate
showers and some storms are possible mainly along and south of I-80
Thursday into Friday morning thanks to an upper level wave over
northern IL. I have medium coverage in precip and thunder
occurrence.

Precip chances increase across the forecast area Saturday as an
upper level wave and surface low move over the region. I have much
higher confidence in precip Saturday than Sunday. Sunday may be dry,
but I left low chance precip chances as some models have the low
traveling a bit slower. Zonal flow aloft would suggest the active
yet cooler pattern continues as waves pass over the region early
next week.

JEE

&&

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

An active weather period likely this afternoon through this
evening, with thunderstorms expected across all of the terminals.
Have seen thunderstorm development quickly occur early this
afternoon across much of northern IL and northwest IN, in a very
warm and moist environment. Expect the coverage of these storms to
remain the same over the next several hours, with generally higher
coverage expected for areas along/north of I-80. A few stronger
storms are possible with gusty winds possible, and with heavy
rain significantly reducing vis likely. Should see thunderstorm
coverage lower early this evening, with focus shifting further to
the north into southern WI late this evening. Ceilings should
remain VFR, however, any stronger storm could briefly lower
ceilings to MVFR. Winds will remain a challenge this afternoon for
the ORD/MDW/GYY terminals where thunderstorm outflow and lake
breeze interaction will likely cause varying winds this afternoon
into the evening.

Rodriguez

&&

.MARINE...
236 AM CDT

A cold front will push south down the lake today and stall over
the middle of the lake early this afternoon. A lake breeze will
likely push on shore this afternoon. High pressure builds over
Lake Superior this afternoon, and as it continues east, east winds
increase to 10-20 kt over the northern end of the lake.

A period of south winds of 15-25 kt begins Tuesday. Winds sharply
turn north 10-20 kt, psbly to 25 kt behind a cold front Wednesday.
A small craft advisory may be needed for hazardous waves Wednesday
night into Thursday morning. Northeast winds diminish as a high
builds over the western lakes Thursday. Northeast to east winds
are expected through at least the first half of the weekend.

JEE

&&

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

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Special Weather Statement
National Weather Service Chicago/Romeoville Il
137 PM CDT MON SEP 3 2018

ILZ013-014-022-031900-
Cook IL-DuPage IL-Will IL-
137 PM CDT MON SEP 3 2018

...SIGNIFICANT WEATHER ADVISORY FOR NORTHWESTERN WILL...SOUTHERN
DUPAGE AND CENTRAL COOK COUNTIES UNTIL 200 PM CDT...

At 136 PM CDT, Doppler radar was tracking a strong thunderstorm over
Bolingbrook, moving northeast at 20 mph.

Wind gusts of 50 to 55 mph will be possible with this storm.

Locations impacted include...
Naperville, Bolingbrook, Wheaton, Downers Grove, Elmhurst, Lombard,
Romeoville, Glen Ellyn, Westmont, Lisle, Villa Park, Hinsdale,
Lemont, Burr Ridge, Oak Brook, Willow Springs, York Center,
Woodridge, Darien and Clarendon Hills.

Including the following interstates...
 I-55 between mile markers 263 and 277.
 I-88 between mile markers 127 and 138.
 I-355 between mile markers 8 and 24.

This includes...  Argonne National Laboratory, Benedictine
University, Morton Arboretum, and North Central College.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

&&

LAT...LON 4162 8813 4178 8820 4189 8796 4165 8784
TIME...MOT...LOC 1836Z 244DEG 19KT 4171 8807

RC