SPC 1630Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1121 AM CDT Fri Sep 25 2020

Valid 251630Z - 261200Z


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

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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