Tornado Warning 33, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1021 PM CDT Fri Jul 28 2023

.MESOSCALE DISCUSSION...
Issued at 1021 PM CDT Fri Jul 28 2023

A few HP supercells have been festering about from Winnebago
County east into the northern Chicago suburbs. Recent radar trends
suggest these cells are becoming increasingly outflow dominant
with consolidating cold pools. Despite thunderstorms in and around
the Chicago metro area and earlier passage of a lake breeze, sfc
obs suggest that the air mass over the Chicago metro has recovered
and is strongly unstable. The cluster of storms over the northwest
suburbs appear likely to bow east-southeastward across the heart
of the Chicago metro with damaging winds.

Farther west into Ogle and Lee Counties, very impressive rear
inflow jet evident on our radar with 70-80 kt inbounds noted at
5000 ft agl on KLOT 88D. Recent radar trends show the convection
associated with this rear inflow jet seemingly catching up with
the outflow ahead of it. If this trend continues, it would suggest
a increasing damaging wind threat as these storms track eastward
across the I-80 corridor. Air mass ahead of this line is extremely
unstable, with sfc dewpoints still around 80F, lending credence
the >3500 j/kg shown on latest SPC mesoanalysis.

If the western convection continues to catch up with the outflow
and becomes more shear balanced, then we would need to watch for
potential QLCS tornadogenesis with this western line. Particularly
where the outflow from the NW burbs bow intersects this second
western bow nearly perpendicularly (near DeKalb/LaSalle line as of
1020 PM).

- Izzi

&&

.SHORT TERM...
Issued at 325 PM CDT Fri Jul 28 2023

Through Saturday...

Key Forecast Message:

* Thunderstorms expected to develop near a cold front late this
  afternoon this evening with an attendant severe and flash flood
  threat

Heat and oppressive humidity this afternoon with dew points of
75-80F are expected to provide the fuel for potentially explosive
thunderstorm development as a back-door lake breeze enhanced
front sags south through this evening. While confidence as of this
writing remains on the lower side regarding exact trends, no big
changes to the thinking regarding severe and flash flooding
threats.

The main severe threats are expected to be scattered damaging to
locally destructive winds and isolated to scattered instances of
large hail. In addition, a brief tornado or two may occur near
frontal/outflow boundary intersections, particularly with any
bowing structures. A Flood Watch may be issued for portions of the
area over the next few hours as convective evolution and focus
area becomes more clear. Preliminary thinking regarding watch
issuance is generally the I-80 and north counties, with the flash
flood vulnerable Chicago and Rockford metros potentially in the
cross-hairs.

...General Expected Convective Development Trends into Early Evening:

Exceptionally high dew points near to locally just above 80F are
pooling ahead of the back-door front, making its way into far
northeast Illinois this hour. Recent AMDAR soundings and the
visual look to the Cu field suggest a capping inversion remains in
place. The "clumpiest" Cu resides from inland of the lake breeze
across northern Illinois west-northwestward into Wisconsin and
north central IA, where SPC/RAP mesoanalysis is indicating that
the cap may be on its way to eroding. In addition to the capping,
we're lacking strong large scale forcing as of the 3pm hour, with
the most notable short- wave over the upper MS Valley and forecast
neutral mid- level height tendencies into the early evening.

May be signs of first CI over north central IA closer to the
aforementioned short-wave and near the cold front. Expectation
locally is for the back-door cold front to have CI once the cap
breaks given enhanced low-level convergence, plus upstream
convection to the west/northwest to fill in and gradually sag
south with the front this evening. Lake enhanced frontal
convection is forecast to develop in the 4-6pm timeframe, with
outflow gradually expanding the footprint southward, plus
additional convection northwest of the area likely to move in
toward and beyond 7pm for areas far inland of Lake Michigan.

...Severe Weather Threat:

Steep mid-level lapse rates and the swamp-like air mass are
contributing to very large MLCAPE up to 4000-4500 J/kg per recent
mesoanalysis. This is a "powder keg" sort of scenario in which
updrafts can quickly become severe thunderstorms in short order.
Effective deep layer bulk shear up to 30-40 kt with extreme
instability is sufficient for a severe threat, with storm mode
modulating residence time of variance threats. Downdraft CAPE
(DCAPE) pushing 1500 J/kg with steepening low-level lapse rates
suggests clusters becoming outflow dominant and potentially
congealing into bowing segments that would transition to more of a
damaging wind threat. Prior to that, the extreme instability and
steep mid-level lapse rates paired with supportive wind shear will
support both downburst damaging winds and large to potentially
very large damaging hail.

While damaging winds and hail are expected to be the primary
severe threats, we'd be remiss to discount any tornado threat
through a portion of this evening. 0-3km MLCAPE is forecast to
grow to up to 150 J/kg inland of the lake breeze front, so pooled
vorticity near the front introduce a concern for boundary
interaction related tornado-genesis. While low-level shear and
effective SRH are currently sub-marginal, effective SRH may
briefly increase to up to 200 m2/s2 early this evening. With the
sagging front, outflow boundaries, and increased low-level SRF in
play, we'll need to monitor for quick spin-ups, especially on
bowing segments interacting with aforementioned boundaries.

We're continuing to message the main window for the severe threat
to be from shortly after CI late this afternoon through about
midnight CDT tonight.

...Flash Flooding Threat:

Moisture pooling and advection streaming toward the sagging
frontal boundary of an already exceptionally moist air mass at the
low levels is expected to contribute to PWATs of up to 2-2.25" by
early this evening. With some localized areas in northwest Illinois
seeing over 2" of rain last night, and the ever-present vulnerability
of the Chicago and Rockford metros to flash flooding in these
areas, concern is certainly there for corridors of more than
localized flash flooding. Conceptually, this set-up checks some of
the key boxes for a frontal flash flood archetype, with potential
for back-building and training convection north of the sagging
cold front. The 12z HREF LPMM notably features some pockets of
3-5" of rain, with ensemble max hinting at upside potential being
5-7" amounts where repeated rounds of intense convection occur.
See WPC's recent MPD for additional thoughts on this. We're
looking at a decent chance of issuing a Flood Watch once
convective trends have shown their hand more conclusively.

Later Tonight-Saturday:

It's unclear how long more intense convection will persist
tonight, but generally showing higher PoPs stair-stepping south
along with thunder chances with time overnight. We'll have a large
reservoir of instability to work over, but sometimes reinforcing
outflows and force the effective surface front and front aloft
more quickly southward, so PoP trends will undoubtedly need
refining this evening.

On Saturday, most of if not the entire area is expected to be dry
with increasing sun from north to south. Minor exception to this
may be south of the Kankakee River where lingering instability
axis will take the longest to be shunted south. Model guidance has
trended downward with the magnitude of northerly winds over Lake
Michigan on Saturday and with it the potential for dangerous
swimming conditions at the beaches has also come down some. Can't
rule out the northerly winds being strong enough to necessitate a
Beach Hazards Statement for portions of the lakeshore, but at this
time, a moderate swim risk and possibly hazardous conditions will
be messaged. High temperatures on Saturday will be in the lower to
mid 80s for most of the area away from the lake, with isolated
upper 80s in far interior sections, while upper 70s to around 80F
are expected near the lake. Humidity levels will start out
uncomfortable but will moderate with time (especially north of
I-80) as drier air works in from the north during the afternoon.

Castro

&&

.LONG TERM...
Issued at 203 PM CDT Fri Jul 28 2023

Saturday night through Friday...

Key Messages:

* Quiet weather through the weekend and majority of the week

* Temperatures return to seasonal average

* Chances for storms return towards the end of the week

Discussion:

A reprieve from the hot and stormy weather pattern will come this
weekend into next week, with temperatures returning to seasonal
average. The ridge will slowly weaken and push west. Expect highs to
be in the low-to-mid 80s through midweek next week, with cooler
temperatures along the lakeshore (potentially in the mid-to-upper
70s). Temperatures could push into the upper 80s towards the end of
the week. Dew points will drop through the weekend, retreating into
the 50s, bringing an end to the oppressive humidity. Dry air in the
mid-levels will limit the potential for rain and storms through
midweek. Chances for storms will return to the forecast late next
week.

Carothers

&&

.AVIATION...
For the 00Z TAFs...

Aviation Forecast Concerns:

* Period of scattered strong to severe thunderstorms this evening.

* Generally north-northeast winds through the period, though
  variable wind directions in thunderstorms this evening.

* Low-confidence potential for a period MVFR ceilings late
  tonight/early Saturday.

Early evening surface analysis depicts low pressure over northeast
IA, with with a nearly stationary frontal boundary extending into
southwest WI then southeast across northern IL where the effective
boundary appears to combine with the lake breeze off of Lake
Michigan. AMDAR aircraft soundings through the mid-late afternoon
have depicted a slowly weakening capping inversion near the 850 mb
level across he Chicago metro area which has resulted in fairly
slow growth of convective clouds along the lake breeze boundary,
though most recent soundings indicate this is eroding more quickly
now and suspect taller updrafts from west of KDPA southeast to
near KMDW and KGYY will soon be capable of lightning production.
Weaker capping upstream across parts of IA/southwest WI were being
overcome by large scale ascent associated with an eastward moving
mid-level disturbance, and it appears that convection will develop
more rapidly into our forecast area over the next couple of hours
as this forcing works its way east. Very warm/humid air mass
across the area combined with increasing mid-level flow aloft will
likely support the potential for strong to severe storms across
the terminals this evening, with damaging wind gusts and large
hail the primary severe threats. Storms likely to be in the
vicinity of several TAF sites at 00Z, with better coverage
expected to develop by/after 01Z. High-res CAM guidance shows some
differences in details of evolution and ending later tonight,
though it appears that storms may linger in the vicinity to/a
little past midnight.

Weather quiets down late tonight, with north-northeast winds
persisting through the remainder of the period. Winds may gust
near 20 kts at times by Saturday afternoon, especially within a
few miles of Lake Michigan. Some potential exists for a period of
MVFR ceilings into Saturday morning, with conditions otherwise
VFR.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-
     ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ103-ILZ104-
     ILZ105-ILZ106-ILZ107-ILZ108 until 3 AM Saturday.

IN...Flood Watch...INZ001-INZ002-INZ010-INZ011 until 3 AM Saturday.

LM...None.
&&

Visit us at weather.gov/chicago

Follow us on Facebook, Twitter, and YouTube at:
www.facebook.com/NWSChicago
www.twitter.com/NWSChicago
www.youtube.com/NWSChicago

Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
921 PM CDT Fri Jul 28 2023

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-300230-
Winnebago-Boone-McHenry-Lake IL-Ogle-Lee-De Kalb-Kane-DuPage-
La Salle-Kendall-Grundy-Kankakee-Livingston-Iroquois-Ford-
Northern Cook-Central Cook-Southern Cook-Northern Will-
Southern Will-Eastern Will-Lake IN-Porter-Newton-Jasper-Benton-
921 PM CDT Fri Jul 28 2023 /1021 PM EDT Fri Jul 28 2023/

This Hazardous Weather Outlook is for portions of North Central
Illinois...Northeast Illinois and Northwest Indiana.

.DAY ONE...Tonight.

Weather hazards expected...

  Significant Thunderstorm Risk...with an associated:
      Elevated Damaging Wind Risk...up to 70 mph.
      Limited Hail Risk...up to half dollar size.
      Limited Tornado Risk.
  Elevated Flooding Risk.

DISCUSSION...

Strong to severe thunderstorms will continue to overspread the
area late this evening into the early overnight hours, ahead of a
cold front which will move through the area late. Some of these
storms will be severe, capable of producing damaging to locally
destructive winds and large hail. A brief tornado or two can not
be ruled out. In addition, some of the storms will produce
torrential rainfall which could lead to flash flooding, especially
where several storms train across the same areas.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

Saturday...
  Limited Thunderstorm Risk south of the Kankakee River.
  Moderate Swim Risk Lake Michigan Shore.
Thursday...
  Limited Thunderstorm Risk.

DISCUSSION...

Potentially hazardous swimming conditions may occur on Saturday
at Lake Michigan beaches.

.SPOTTER INFORMATION STATEMENT...

  Spotters may be needed through this evening.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the east-southeast around 30 mph.

LMZ740>745-300230-
Winthrop Harbor to Wilmette Harbor IL-
Wilmette Harbor to Northerly Island IL-
Northerly Island to Calumet Harbor IL-
Calumet Harbor IL to Gary IN-Gary to Burns Harbor IN-
Burns Harbor to Michigan City IN-
921 PM CDT Fri Jul 28 2023

This Hazardous Weather Outlook is for portions of the Illinois
nearshore waters of Lake Michigan and the Indiana nearshore
waters of Lake Michigan.

.DAY ONE...Tonight.

Weather hazards expected...

  Significant Thunderstorm Risk...with an associated:
     Limited Hail Risk...up to half dollar size.
     Elevated Thunderstorm Wind Gust Risk...up to 60 knots.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

Thursday...
  Limited Thunderstorm Risk.

Ratzer

Visit us at www.weather.gov/chicago

Follow us on Facebook, Twitter, and Youtube at:
www.facebook.com/nwschicago
www.twitter.com/nwschicago
www.youtube.com/nwschicago

Preliminary Local Storm Report
National Weather Service Chicago IL
1057 PM CDT Fri Jul 28 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1024 PM     Non-Tstm Wnd Gst 2 NNW Marseilles        41.36N 88.72W
07/28/2023  M54 MPH          La Salle           IL   Public

            Private weather station KILMARSE11 recorded
            54mph gust.

&&

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Service Notice</title>
  <link rel="stylesheet" href="index.css">
</head>
<body>
  <main class="wrap">
    <section class="card" role="status" aria-live="polite">
      <div class="status" id="status">Temporary Notice</div>
      <h1 id="headline">This service is currently unavailable.</h1>
      <p id="message1">
        The site you requested is temporarily down for maintenance or an unexpected outage.
      </p>
      <p id="message2">
        Please try again in a few minutes.
      </p>
      <p class="small" id="detail">
        If you are an administrator, restore upstream service and remove the redirect/fallback rule when normal operations resume.
      </p>
    </section>
  </main>
  <script src="index.js"></script>
</body>
</html>