Mesoscale Discussion 1641
NWS Storm Prediction Center Norman OK
0246 PM CDT Fri Jul 17 2026
Areas affected...the western Lake Superior vicinity...including
parts of northeastern Minnesota...northwestern Wisconsin and
adjacent portions of Upper Michigan
Concerning...Severe potential...Watch unlikely
Valid 171946Z - 172145Z
Probability of Watch Issuance...20 percent
SUMMARY...Initiating thunderstorm activity is likely to intensify
late this afternoon across the western Lake Superior vicinity.
Coverage of stronger storm development remains unclear due to the
stabilizing lake boundary layer, but a few supercell structures
posing a risk for severe hail and wind are possible. It is not
clear that this will require a severe weather watch, but trends will
be monitored.
DISCUSSION...A short wave trough within the westerlies is beginning
to dig southeast of the the Canadian Prairies, with the southern
fringe of modest mid-level height falls overspreading the Minnesota
international border vicinity. An associated developing frontal
wave is slowly beginning to deepen across Ontario, to the northwest
of Lake Superior, and is forecast to gradually migrate
east-southeastward into early evening. A trailing surface front is
already advancing through the northern U.S. Great Plains and
becoming better defined with differential surface heating, now near
or northwest of the Hibbing, Brainerd and Alexandria MN vicinities.
Perhaps preceded by a pocket of mid/lower level subsidence drying,
the deeper moist boundary layer appears largely focused along the
cold frontal zone, based on latest Rapid Refresh output and
supported by surface analysis. However, this is advancing
southeastward beneath a plume of warm elevated mixed-layer air (as
roughly depicted by 10-12C temperatures around 700 mb) overspreading
the Red River Valley toward upper Great Lakes late this afternoon.
Deepening convective development is underway within the moist
environment beneath this regime, which may be characterized by mixed
layer CAPE on the order of 2000-3000 J/jg. As this spreads toward
the western Lake Superior vicinity through 22-00Z, guidance suggests
that some combination of weaker inhibition and strengthening
large-scale forcing for ascent may allow for continued thunderstorm
initiation and intensification. Beneath moderate northwesterly
mid-level shear this may include a few evolving supercell structures
posing a risk for severe hail and wind.
..Kerr/Guyer.. 07/17/2026
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...DLH...
LAT...LON 47919211 48218994 47408907 46399081 46189173 46429262
47249315 47919211
MOST PROBABLE PEAK TORNADO INTENSITY...UP TO 90 MPH
MOST PROBABLE PEAK WIND GUST...UP TO 60 MPH
MOST PROBABLE PEAK HAIL SIZE...1.00-1.75 IN
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