Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0738 AM CDT Wed Sep 20 2017 Valid 201300Z - 211200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS UPPER MI TO NORTHERN MO... ...SUMMARY... Isolated severe hail and damaging winds are possible from the Upper Peninsula of Michigan to northern Missouri between about 4 to 11 pm CDT. ...Upper MI to northern MO... A potent shortwave trough over the southern Canadian Prairies to Red River Valley will move northeast towards Hudson Bay. Attendant cold front will push east into central WI with trailing portion about to stall over the Lower MO Valley. Middle 60s to lower 70s surface dew points will be maintained ahead of this boundary with mean mixing ratios of 12-16 g/kg common in area 12Z observed soundings. The 06Z NAM appears overdone with depictions of mean mixing ratios reaching 17-19 g/kg and extreme MLCAPE of 3500-5000 J/kg. A plume of 2000-3500 J/kg appears more probable from central WI southwestward at peak heating. Capping will inhibit surface-based storm development until after 21Z amid weak mid-level height rises. Convergence near the front, aided by modest low-level warm advection, may eventually yield widely scattered storms during the evening. Hodographs will remain small west of the MS River. The WI vicinity will lie on the fringe of moderate mid-level southwesterlies, marginally favorable for a couple transient supercell structures. The predominant mode should be multicells offering a risk of isolated severe hail and damaging winds. Storms should weaken by late evening owing to the modest shear and forcing for ascent. ..Grams/Kerr.. 09/20/2017Read more
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