Tornadoes and Hail in Wisconsin, April 28, 2025

Area Forecast Discussion
National Weather Service Green Bay WI
658 AM CDT Mon Apr 28 2025

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- A few rounds of thunderstorms are expected today and tonight. A
  significant severe weather outbreak is possible Monday evening
  into the early overnight hours. Most of the forecast area has an
  Enhanced Risk of severe thunderstorms, with parts of central WI
  in a Moderate Risk.

- Initial storms this morning through early afternoon could
  produce small to near severe hail (up to 1" in diameter),
  especially west and north of the Fox Valley.

- Thunderstorm chances are expected to be lower during the mid to
  late afternoon. However, any storms that do develop could
  quickly become severe, with tornadoes, damaging winds and large
  hail all possible. Confidence in the details is lowest for this
  round of storms.

- The third and final round of storms is forecast to impact the
  area this evening into the early overnight hours. Confidence is
  higher that a line of storms will impact the area, with
  potential for widespread damaging winds and embedded tornadoes.

&&

.DISCUSSION...
Issued at 450 AM CDT Mon Apr 28 2025

Short Term...Today...Tonight...and Tuesday

Scattered convection with small hail was poised to move into our
western counties early this morning. This activity was forming on
the leading edge of the elevated instability gradient, where WAA
was increasing on the nose of a 35 kt low-level jet.

Elevated convection will increase mainly west and north of the
Fox Valley this morning through early afternoon, with forcing
provided by a short-wave trough and increasing WAA as the low-
level jet cranks up to 50 to 55 kts. Instability will be marginal
and elevated, with MUCAPE of 300-800 j/kg. However, deep layer
shear will increase quickly, reaching 50 to 60 kts by early
afternoon. Wet-bulb zero heights will be favorable, in the 9-10K
range, so storms will have potential for small to marginally
severe hail.

Subsidence behind the departing short-wave, possible low clouds
north of the warm front, and some capping may suppress
thunderstorm activity during the mid to late afternoon. However,
the arrival of surface-based instability into central WI, combined
with the impressive shear parameters (SRH 400-600 m2/sec2) in
place, suggest that any isolated/scattered discrete storms that
develop could quickly become severe and possibly tornadic,
especially in the southwest part of the forecast area.

Storms should take on more of a linear mode (squall line/QLCS) as
a cold front approaches in the evening. This line is expected to
arrive in NC/C WI early in the evening (01z-02z/Tue), and track
ESE through the forecast area. SBCAPE of 1000-1500 j/kg and 0-3 km
CAPE of 500-100 j/kg, continued impressive shear parameters, STP
values of 3-8 and strong Updraft Helicity indices support high-end
severe potential. Damaging winds and embedded tornadoes will be
the primary threats as this line moves across the forecast area
between 8 pm and 1 am. The severe threat should diminish overnight
as surface-based CAPE gradually weakens. SPC has kept most of GRB
CWA in an Enhanced Risk of severe storms, with part of central WI
in a Moderate Risk. Nearly the entire forecast area is included
in either a 10% or 15% hatched tornado threat area, indicating
significant potential for strong tornadoes. Think that the
greatest threat for strong tornadoes will over NC/C WI, with a
lesser threat farther east.

Precipitation chances will be lower late tonight into Tuesday
morning, through a small thunder threat will likely persist until
the cold front moves through Tuesday morning. Dry conditions
return Tuesday afternoon.

South winds will increase today, with gusts reaching 25 to 35 mph
during the afternoon and evening. The winds will turn west to
northwest as the strong cold front moves through Tuesday morning,
with gusts continuing in the 25 to 35 mph range until mid
afternoon. After a mild/warm day today, strong CAA will result in
much cooler temperatures on Tuesday.

Long Term...Tuesday Night Through Sunday

High pressure will move through the western Great Lakes late
Tuesday through Wednesday, with light/variable surface winds
Tuesday night gradually becoming southeasterly by Wednesday
afternoon. The presence of mostly clear skies and light surface
winds will result in cool overnight lows Tuesday night, with most
locations in NC/far NE WI expected to fall below freezing. High
temps will rebound some on Wednesday afternoon, though onshore
flow will keep temperatures considerably cooler near Lake
Michigan.

The next disturbance approaching from the Northern Plains will
attempt to phase with low pressure tracking northeast toward Lower
MI during the Wednesday night through Thursday night period. This
will result in a good chance of showers, especially late
Wednesday night into Thursday. Relatively mild temperatures will
allow for all rain during this system's passage. A few showers
may linger into Friday as an upper trough moves through, but dry
conditions return for the weekend.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 641 AM CDT Mon Apr 28 2025

Have focused on the ongoing convection over the CWA, a line of
storms moving rapidly through MN and another line of storms
expected to move through the forecast area this evening into the
early overnight to determine timing of thunderstorms in the TAFs.
Given the messy pattern, overall confidence in the evolution of
this event is low. This morning's convection will mainly have a
hail threat, but any storms that form later today into this
evening will have a damaging wind and tornado threat.

Ceilings will drop to MVFR west and north of the Fox Valley
this morning, and could remain there for most of the day. Partial
clearing may occur in central and east central WI as the warm
front lifts through later in the day. Low ceilings will also
accompany the squall line this evening, and IFR ceilings could
arrive in NC WI as the cold front moves through late tonight.

LLWS is expected across much of the area through the TAF period,
due to a low-level jet increasing to 50 to 55 kts. Surface winds
will also be strong, with south winds gusting to 25 to 30 kts
this afternoon/evening, then turning west to northwest as the
front move through the northwest part of the forecast area late
tonight.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION.....Kieckbusch
AVIATION.......Kieckbusch

Hazardous Weather Outlook
National Weather Service Green Bay WI
502 AM CDT Mon Apr 28 2025

WIZ005-010>013-018>022-030-031-035>040-045-048>050-073-074-291015-
Vilas-Oneida-Forest-Florence-Northern Marinette County-Lincoln-
Langlade-Menominee-Northern Oconto County-Door-Marathon-Shawano-
Wood-Portage-Waupaca-Outagamie-Brown-Kewaunee-Waushara-Winnebago-
Calumet-Manitowoc-Southern Marinette County-
Southern Oconto County-
502 AM CDT Mon Apr 28 2025

This Hazardous Weather Outlook is for north-central and northeast
Wisconsin.

.DAY ONE...Today and Tonight

Multiple rounds of thunderstorms are expected over the entire
forecast area today into tonight. Storms may produce small to
marginally severe hail this morning through early afternoon,
mainly west and north of the Fox Valley. Thunderstorm coverage
will probably be lower during the mid to late afternoon, but
any isolated storms that develop will have potential to produce
tornadoes, damaging winds and large hail. There is potential for a
significant severe weather outbreak as a line of storms moves
through this evening and into the early overnight hours. Damaging
winds and embedded tornadoes will be the primary threats.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

There is a small chance of thunderstorms in northeast and east
central Wisconsin during the morning, but severe weather is not
expected.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will likely be needed today and tonight.

LMZ521-522-541>543-291015-
The Bay of Green Bay south of a line from Cedar River MI to Rock
Island Passage-Lake Michigan from Washington Island to Sheboygan-
502 AM CDT Mon Apr 28 2025

This Hazardous Weather Outlook is for portions of northwest Lake
Michigan and the waters of Green Bay.

.DAY ONE...Today and Tonight

There is a chance of thunderstorms today and tonight. Severe
thunderstorms with strong winds will be possible, especially
tonight.

Gusty south to southwest winds will cause waves to build today
into tonight. A few gusts to near gale force are possible on Lake
Michigan in the late afternoon and evening. A Small Craft
Advisory is in effect.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

A Small Craft Advisory will remain in effect on Tuesday, as
gusty winds turn west to northwest in the wake of a cold frontal
passage. There is also a small chance of thunderstorms Tuesday
morning.

Kieckbusch