Flash Flood Warning 3, NWS Duluth, MN

Area Forecast Discussion
National Weather Service Duluth MN
635 PM CDT Fri Jun 20 2025

.KEY MESSAGES...

- Moderate to high confidence in severe storms impacting the
  Northland tonight with all threats on the table.

- The weekend will see hot and humid conditions across the
  Northland with heat indices in the 90s and low 100s. Extreme
  Heat Warnings and Heat Advisories are in effect.

- Additional strong to severe storm potential late Sunday into
  Sunday night.

&&

.DISCUSSION...
Issued at 1254 PM CDT Fri Jun 20 2025

Tonight, a significant severe thunderstorm event is expected to
occur, driven by an approaching potent warm front. Convective
Allowing Models (CAMS) guidance indicates that initial storm
development is anticipated to commence around 03Z, with a more
widespread and organized Mesoscale Convective System (MCS)
expected to traverse the region closer to 08Z. The atmospheric
environment preceding this front is exceptionally primed for
severe convection. Key thermodynamic and kinematic parameters
point to a high-impact event, including remarkably high MUCAPE
values near 3000 J/kg, which signifies a substantial amount of
instability. Deep layer shear near 50 kt will support organized,
long-lived storm structures, while a robust 60 kt low-level jet
(LLJ) will provide critical low-level shear and ample moisture
transport into the storm environment. Precipitable Water (PWATS)
values are elevated at 2.0, indicating a very moist airmass
capable of producing heavy rainfall. Furthermore, 0−3km Storm
Relative Helicity (SRH) values exceeding 600 m2/s2 are highly
favorable for the development of strong, rotating updrafts.
Given these impressive parameters, all severe weather threats
are considered possible, encompassing large hail, damaging
winds, and tornadoes. While hodographs are notably long and
curvy, suggesting a favorable supercell environment, current
assessments indicate that large hail and damaging winds are the
primary threats. A unique local factor will be the influence of
the Lake Superior lake breeze, which is forecast to extend
westward into Itasca County. Within this localized boundary, the
lake breeze could introduce a stabilizing effect, potentially
inhibiting tornado potential; however, it may also locally
enhance the tornado threats in areas immediately adjacent to its
boundary. Due to the very high moisture content and the
potential for storms to train over the same areas, coupled with
a favorable pivot axis for the MCS, localized flash flooding is
a significant concern, necessitating the likely issuance of a
Flash Flood Watch for affected portions of the forecast area.

Following tonights severe weather, a substantial and robust heat
dome is forecast to build over the region, ushering in a period
of record to near-record heat and oppressive humidity throughout
Saturday and Sunday. This prolonged period of extreme heat will
lead to dangerous heat indices, prompting the issuance of
widespread Extreme Heat Warnings and Heat Advisories across the
forecast area. Residents are strongly advised to take all
necessary precautions to mitigate heat-related risks. As Sunday
progresses, the core of the ridge responsible for the intense
heat is expected to shift eastward, which will allow a low-
pressure system to track through the forecast area. With
residual atmospheric instability from the preceding heat, this
transition highlights another potential period of severe weather
development. Forecasters will be closely monitoring this systems
exact timing and trajectory to assess the precise nature and
extent of the severe threat. Finally, as the low-pressure system
departs, cold air advection will usher in a significant and
welcome change in weather patterns for the latter half of next
week. Dew points are anticipated to plummet back into the 40s,
bringing much cooler and more comfortable temperatures. No
notable large-scale forcing mechanisms are currently apparent
for specific thunderstorm timing during this cooler period,
suggesting that any isolated convection would likely be
diurnally driven.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 633 PM CDT Fri Jun 20 2025

Mostly VFR conditions to start the forecast period with a few
pockets of MVFR ceilings near BRD. Overnight the Northland will be
inundated with showers and storms, some of which will be strong to
severe. The primary threats will be large hail and damaging winds
with heavy rainfall at times. This system moves through at a fairly
quick pace with most terminals clearing the hazardous weather by
tomorrow morning. On the backside of the system we may have some
lingering MVFR/IFR ceilings through the latter morning hours before
returning to VFR conditions.

&&

.MARINE /FOR NEAR SHORE WATERS OF WESTERN LAKE SUPERIOR/...
Issued at 1254 PM CDT Fri Jun 20 2025

A line of storms is expected to cross the forecast area tonight
and will impact the waters of western Lake Superior with sever
storms overnight. Thinking that large hail will be the main
threat there though can't rule a line of strong wind gusts
either. Storms should move east of our nearshore waters after
7 am.

For the open water discussion, refer to the NWS Marquette Area
Forecast Discussion at weather.gov/mqt.

&&

.DLH WATCHES/WARNINGS/ADVISORIES...
MN...Heat Advisory from noon Saturday to 9 PM CDT Sunday for
     MNZ010>012-018-019-025-026-035-037.
     Flood Watch from 10 PM CDT this evening through Saturday
     morning for MNZ011-012-018>021-025-026-035-037.
     Extreme Heat Warning from noon Saturday to 9 PM CDT Sunday for
     MNZ033-034-036-038.
     Beach Hazards Statement until 10 PM CDT this evening for
     MNZ037.
WI...Flood Watch from 10 PM CDT this evening through Saturday
     morning for WIZ001>004.
     Extreme Heat Warning from noon Saturday to 9 PM CDT Sunday for
     WIZ001>004-006>009.
     Beach Hazards Statement until 10 PM CDT this evening for
     WIZ001.
MARINE...Small Craft Advisory until 9 AM CDT Saturday for LSZ144-145.

&&

DISCUSSION...Wolfe
AVIATION...Britt
MARINE...Wolfe

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