Severe Thunderstorm Warning 13, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
323 PM CDT Fri Mar 31 2023

.MESOSCALE DISCUSSION...
Issued at 249 PM CDT Fri Mar 31 2023

The main concerns through late afternoon are the initial area of
storms from the Quad Cities through the Peoria area and any storms
that develop southeast of there. The primary threat with these is
hail with a close second being a tornado threat if they can
become anchored in the low-levels. A Tornado Watch has been issued
until 10 PM for the entire area for these storms as well as the
second line of storms which as of 245 PM extends from Des Moines,
Iowa to near Columbia, Missouri.

A low amplitude short wave and associated speed maximum continue
to move northeast from the far western Illinois area and was the
impetus for the convective cluster, which is becoming more self-
sustaining. This is overriding a subtle secondary warm front that
was diabatically enhanced earlier. Backed surface and low-level flow
has been noted at central Illinois sites and the ILX VWP, and the
supercell characteristics of the storm in the Peoria area have
continued to mature. It also has become more rooted in the
boundary layer, a sign that synoptic scale lift is starting to
quickly wear away at a pronounced cap sampled on the ILX 18Z
sounding. Surface temperatures and dew points are similar ahead of
the storm into the southwestern and southern CWA. There likely
still is some cap per visible satellite imagery an hour ago that
showed transverse waves in the low clouds before the anvil shield
spread over. But again that is likely fading quickly, so will need
to watch this southern part of the cluster in particular for a
tornado threat. The large hail environment is noteworthy with
steep elevated lapse rates within the CAPE growth layer with much
of buoyancy above the-10C level and an increasing overall mass to
that supercell.

So through 4 PM, southern LaSalle and Livingston Counties will be
the most at risk, and potentially Ford County too. Parameter
space should favor these storms to continue toward the south
Chicago metro toward Kankakee in that 4-5 PM area. Persons in
those counties show remain extra vigilant. To the north for
northern LaSalle County toward Dixon and Rockford, the Quad
Cities area storm cells will likely become closer to boundary
layer rooted but still uncertain if conditions will destabilize in
time. There definitely will be a hail threat with these.

The second line of storms to the west from central Iowa to central
Missouri looks to arrive into the western forecast area after 6-7
PM. Oblique shear orientation to the frontal boundary (which will
almost be a pre-frontal dryline) looks to support a broken line of
storms that would have an all hazards threat, but more so wind and
a tornado risk with a low-level warm sector ahead of it remaining
unstable after dark. There may be some "imprint" of these
afternoon storms, but synoptic scale forcing will probably be
strong enough to overcome that.

MTF

&&

.SHORT TERM...
Issued at 322 PM CDT Fri Mar 31 2023

Through Saturday night...

For details on thunderstorm trends through the remainder of this
afternoon and evening, see the latest mesoscale forecast
discussion above.

990 mb surface low over central IA at mid-afternoon is expected
to continue to deepen slightly as it moves east-northeast across
south central WI and central LM to northern lower MI through
Saturday morning. In the process, a sharp cold front will push
quickly across the forecast area this evening. as will an initial
strong mid-level vort. Strong subsidence and drying is expected
behind these features during the late evening and early overnight
hours, with a quick end to thunderstorms and showers. The main
concern during this period, is for strong west-southwest winds
behind the front, with a combination of strong surface pressure
rises and a deepening boundary layer within a low-level cold
advection regime characterized by steep low level lapse rates
supporting wind gusts in the 40-45 kt (wind advisory) range. While
areas along/south of the I-80 corridor would appear to have the
better potential for prolonged advisory criteria winds overnight
into Saturday morning, latest guidance soundings suggest at least
some potential to tap into 40+ kt gusts for a time overnight as
well. Based on this have expanded our wind advisory north to the
IL/WI state line (though with a later 10 pm start time behind the
cold front). All wind headlines will continue through 1 pm
Saturday, with northwest winds gradually diminishing through the
afternoon and evening.

A secondary stout mid-level vort is progged to wrap eastward
across the forecast area early Saturday, coincident with a
secondary surface trough and renewed advection of low-level
moisture from the northwest. Along with the steep low-level lapse
rates, this would support rain/snow showers. Surface temps are
forecast to settle into the mid-upper 30s during this time, which
may support brief all-snow showers (especially near the IL/WI
border) and some spotty minor accums. Warm ground temps and air
temps trending back up toward 40 degree should melt anything which
might accumulate by afternoon. Surface high pressure then builds
east into the area Saturday evening, with diminishing winds and
clearing skies.

Ratzer

&&

.LONG TERM...
Issued at 322 PM CDT Fri Mar 31 2023

Sunday through Friday...

No significant changes to previous extended forecast thinking.

Primary forecast concern is the potential for another period of
severe thunderstorms Tuesday into Wednesday along with another
period of possible wind advisory level winds.

A frontal boundary will become stationary across the southern cwa,
or just south of the area Monday night with chance of showers and
thunderstorms returning Monday night. This frontal boundary is
expected to lift back north as a warm front on Tuesday as low
pressure deepens moving from the central Plains to the upper
midwest. A strong cold front is expected to move across the area
Tuesday night and this time period would be most favored for the
potential for severe weather as shown in the days 4-8 outlook from
SPC. Strong westerly winds are then possible Wednesday, possibly
into advisory level.

Weather does look to quiet down late Thursday into Friday, with
global deterministic and ensemble runs depicting broad surface
high pressure building across the Midwest.

cms/Ratzer

&&

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

Aviation Forecast Concerns:

* Two periods of strong to possibly severe thunderstorms likely
  from mid-afternoon through mid-evening. Some severe storms
  possible.

* Southwest winds may turn south-southeasterly ahead of storms
  later this afternoon, with some convective disruption to winds
  possible through early evening.

* West-southwest winds increase with gusts 35-40 kts overnight
  into early Saturday, then becoming northwest and gradually
  decreasing Saturday afternoon.

* Period of rain/snow showers Saturday morning/midday.

Deep surface low pressure over western IA at midday will continue
to deepen as it moves east-northeast across southern WI through
this evening. As this occurs, a warm front will lift across the
terminals late this afternoon, then a strong cold front will push
east across the forecast area this evening. Until that time, the
terminals will reside in a moist and unstable warm sector, with
temperatures approaching 70 degrees. Two periods of thunderstorms
appear poised to affect the region, the first after about 3 pm
associated with a mid-level disturbance approaching from the
southwest, and the second roughly 7-9 pm or so ahead of the
aforementioned cold front. Coverage in the first round is a little
less certain than the second round, which looks to be more of a
solid pre-frontal convective line. Both of these periods may
feature strong to severe storms. MVFR to brief IFR conditions are
possible.

In the wake of the cold frontal passage mid-evening, winds will
shift west-southwest and strengthen, with gusts 35-40 kts
possible. Strongest winds may actually occur during the pre-dawn
and early Saturday morning hours, before shifting northwest mid-
morning and eventually gradually diminishing in the afternoon.

VFR conditions are expected behind the cold front tonight, before
the primary upper trough axis shifts across the area late tonight
and Saturday. Scattered rain/snow showers are likely with MVFR
ceilings, and occasional MVFR visibilities possible. Precip threat
should quickly decrease early Saturday afternoon.

Ratzer

&&

.MARINE...
Issued at 322 PM CDT Fri Mar 31 2023

No significant changes to previous marine discussion.

A strong area of low pressure will track north of the region this
afternoon and tonight. While some occasional gales are possible
this morning, more frequent gales are expected to develop this
evening, and especially late tonight through Saturday afternoon.
The strongest winds are currently expected across the Indiana
nearshore waters where gales to 45 kt are possible. Winds will
ease quickly Saturday evening, but lingering large wave action
will necessitate a follow-up Small Craft Advisory into early
Sunday morning.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Wind Advisory...ILZ019-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ107-
     ILZ108 until 1 PM Saturday.

     Wind Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-
     ILZ012-ILZ013-ILZ020-ILZ103-ILZ104-ILZ105-ILZ106...10 PM
     Friday to 1 PM Saturday.

IN...Wind Advisory...INZ001-INZ002...10 PM Friday to 1 PM Saturday.

     Wind Advisory...INZ010-INZ011-INZ019 until 1 PM Saturday.

LM...Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...10 PM
     Friday to 7 PM Saturday.

     Small Craft Advisory...nearshore waters until 10 PM Friday.

&&

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Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
1215 PM CDT Fri Mar 31 2023

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-011715-
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-
1215 PM CDT Fri Mar 31 2023 /115 PM EDT Fri Mar 31 2023/

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

.DAY ONE...This Afternoon and Tonight.

Weather hazards expected...

  Significant Thunderstorm Risk...with an associated:
     Elevated Tornado Risk.
     Elevated Hail Risk...up to golf ball size.
     Significant Damaging Wind Risk...up to 80 mph.
  Limited Flooding Risk.
  Elevated Non Thunderstorm Wind Risk.

DISCUSSION...

A robust early spring storm system will track across the
Midwest and into the Great Lakes region today. A significant
severe threat exists across the area this afternoon through mid
evening. All severe weather hazards are possible, including
significant damaging winds and strong tornadoes. Stay up to date
with the latest forecast information.

A period of strong non-thunderstorm winds to around 50 mph is
expected late this afternoon and into Saturday morning. At this
time, the strongest winds are favored to be south of I-80 this
afternoon and near the Wisconsin line late tonight.

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

Saturday...
  Elevated Non Thunderstorm Wind Risk in the morning south of I-80.
  Elevated Lakeshore Flood Risk for northwest Indiana.
Monday...
  Limited Thunderstorm Risk.
Tuesday...
  Limited Severe Thunderstorm Risk.
  Limited Non Thunderstorm Wind Risk.
Wednesday...
  Limited Non Thunderstorm Wind Risk.

.SPOTTER INFORMATION STATEMENT...

  Spotters will likely be needed this afternoon and evening.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the northeast at 50 mph this afternoon, and to
  the east at 65 mph this evening.

LMZ740>745-011715-
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-
1215 PM CDT Fri Mar 31 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...This Afternoon and Tonight.

Weather hazards expected...

  Gale force winds and or very hazardous waves.
  Significant Thunderstorm Risk...with an associated:
     Limited waterspout risk.
     Limited hail risk...up to half dollar size.
     Elevated thunderstorm wind gust risk...up to 70 knots.

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

Saturday...
  Gale force winds and very hazardous waves are likely.
Sunday...
  Hazardous conditions for small craft are likely.
Tuesday...
  Hazardous conditions for small craft are likely.
  Limited Severe Thunderstorm Risk.
Wednesday...
  Gale force winds and very hazardous waves are likely.
Thursday...
  Hazardous conditions for small craft are likely.

Yack

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Special Weather Statement
National Weather Service Chicago/Romeoville
353 PM CDT Fri Mar 31 2023

ILZ033-039-312115-
Iroquois IL-Ford IL-
353 PM CDT Fri Mar 31 2023

...A strong thunderstorm will impact portions of north central Ford
and northwestern Iroquois Counties through 415 PM CDT...

At 353 PM CDT, Doppler radar was tracking a strong thunderstorm near
Cullom, or 11 miles northeast of Chatsworth, moving northeast at 55
mph.

HAZARD...Nickel size hail.

SOURCE...Radar indicated.

IMPACT...Minor damage to outdoor objects is possible.

Locations impacted include...
Cabery, Kempton and Stelle.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

A Tornado Watch remains in effect until 1000 PM CDT for central,
northeastern and east central Illinois.

&&

LAT...LON 4077 8824 4099 8824 4100 8822 4100 8810
      4084 8799
TIME...MOT...LOC 2053Z 244DEG 49KT 4088 8816

MAX HAIL SIZE...0.88 IN
MAX WIND GUST...<30 MPH

Izzi

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE CHICAGO IL
414 PM CDT FRI MAR 31 2023

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

0403 PM     HAIL             MAZON                   41.24N 88.42W
03/31/2023  M0.88 INCH       GRUNDY             IL   TRAINED SPOTTER

            NICKEL SIZE HAIL.

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE CHICAGO IL
425 PM CDT FRI MAR 31 2023

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

0423 PM     HAIL             KANKAKEE                41.12N 87.86W
03/31/2023  M0.50 INCH       KANKAKEE           IL   PUBLIC

            RELAYED VIA SOCIAL MEDIA.

&&

ZY