SPC 1630Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1127 AM CDT Sat Sep 26 2020

Valid 261630Z - 271200Z


Scattered large hail from quarter to tennis ball size is the most
likely hazard, with a conditional risk for a couple tornadoes and
damaging winds between 5 PM and 1 AM CDT across northern Wisconsin
into Upper Michigan.

...Northern WI to Upper MI...
A shortwave trough along the MT/ND/Saskatchewan border will amplify
towards the Lake Superior/northwest Ontario region through early
morning. 500-mb southwesterlies will intensify late in the period,
supporting deepening of a surface cyclone over east-central SD as it
tracks from southeast MN into Lake Superior. The potential for
severe thunderstorms will be focused this evening into tonight along
the baroclinic zone to the east of the cyclone.

Low-level moisture will slowly increase today from eastern KS to WI
with mid 60s boundary-layer dewpoints expected along and south of
the warm front by evening. This moistening will occur beneath the
northern extent of a steep mid-level lapse rate plume from the
southern High Plains to the Great Lakes. Pervasive mid to
upper-level cloud coverage will limit boundary-layer heating, likely
holding MLCAPE to between 1000-1500 J/kg. 

Low-level shear/hodograph curvature and deep-layer
shear/hodograph length will increase with the approach of the
shortwave trough, resulting in an environment conditionally
favorable for supercells. Storm development in the open warm sector
is unlikely given the lingering cloud cover and MLCIN.
Stronger/deeper forcing for ascent should be confined to
along and north of the warm front, with the severe threat beginning 
between 22-00Z. Surface-based storms will be possible near and north
of the warm front, but most of the convection should tend to become
increasingly elevated farther north away from the boundary. Thus,
large hail is the most likely hazard type with potential for a
supercell or two to produce significant severe. A couple of
tornadoes and damaging wind gusts will also be possible with any
sustained supercells closer to the warm front, prior to the overall
severe threat waning overnight.

..Grams/Elliott.. 09/26/2020

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