SPC 1630Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1121 AM CDT Fri Sep 25 2020 Valid 251630Z - 261200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FAR NORTHERN WI TO WESTERN UPPER MI... ...SUMMARY... A couple tornadoes, golf to tennis ball size hail, and damaging winds will be possible between 5 to 10 PM CDT across far northern Wisconsin and western Upper Michigan. ...Lake Superior vicinity... A series of low-amplitude shortwave troughs will progress eastward from the northern Rockies/Great Plains to the upper Great Lakes through tonight. At the surface, a cyclone in west-central MN will track east-northeast into northwest WI by about 00Z before shifting across Lake Superior tonight. To the east of the cyclone, a warm front will develop northward to near the southern shore of Lake Superior by late afternoon. Surface temperatures warming into the 70s and boundary-layer dewpoints increasing into in the low 60s beneath steep mid-level lapse rates will contribute to a narrow corridor of MLCAPE near 1500 J/kg immediately ahead of the surface cyclone. The stronger forcing for ascent will likely be confined to the warm front, with areas farther south in the warm sector remaining capped given the modest moisture return. Thus, the more probable corridor for surface-based storms will be across extreme northwest WI into western Upper MI during the early evening (22-00Z). The conditional storm environment will be rather favorable for a supercell or two given the moderate buoyancy and large, curved hodographs with effective bulk shear of 50-60 kt and effective SRH in excess of 300 m2/s2. There will be a window of opportunity for a couple tornadoes, significant severe hail, and damaging winds within a spatially confined corridor. The overall threat is expected to diminish after about 03Z. ...SC vicinity... The remnants of post-tropical cyclone Beta have phased with a shortwave trough over the southern Appalachians. The surface reflection has weakened during the past 24 hours, but there will still be a baroclinic zone to focus isolated to scattered thunderstorms across the Carolinas. Mid-level lapse rates will remain poor, and low-level wind fields should continue to slowly subside. However, deep-layer vertical shear will be sufficient for a few transient, low-end supercells capable of isolated damaging wind gusts and a brief tornado through about sunset. ..Grams/Smith.. 09/25/2020
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