Mesoscale Discussion 1641
Issued: · Valid: –
Summary
Thunderstorms capable of producing severe hail and wind may develop late this afternoon across northeastern Minnesota, northwestern Wisconsin, and Upper Michigan. These storms could include supercell structures with wind gusts up to 60 mph and hail between 1 and 1.75 inches.
Product Text
000 ACUS11 KWNS 171946 SWOMCD SPC MCD 171946 MNZ000-WIZ000-172145- 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