SPC 1630Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1127 AM CDT Sat Sep 26 2020 Valid 261630Z - 271200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS NORTHERN WI TO UPPER MI... ...SUMMARY... 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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