SPC Sep 13, 2017 0730 UTC Day 3 Severe Thunderstorm Outlook

By Newsroom America Feeds at 13 Sep 2017

SPC 0730Z Day 3 Outlook
Day 3 Convective Outlook NWS Storm Prediction Center Norman OK 0139 AM CDT Wed Sep 13 2017 Valid 151200Z - 161200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FRIDAY AFTERNOON AND EVENING ACROSS PARTS OF THE MID MISSOURI INTO UPPER MISSISSIPPI VALLEYS... ...SUMMARY... Strong thunderstorms may impact portions of the mid Missouri Valley into upper Mississippi Valley Friday late Friday afternoon and evening, accompanied by at least some risk for severe weather. ...Synopsis... Models suggest some consolidation of the mid-latitude and subtropical westerlies is possible by late this week, with digging short wave perturbations within the mid-latitude westerlies contributing to the evolution of amplified large-scale mid/upper troughing across much of the Intermountain West and Rockies, into the Plains, by late Friday night. Downstream, it appears that subtropical ridging will become increasingly prominent across the Gulf and southern Atlantic Coast states, while ridging within the mid-latitude westerlies builds across the Great Lakes/Ohio Valley region, but a residual weak trough/cyclonic shear axis may linger from the vicinity of the north Atlantic Seaboard southwestward toward the northwestern Gulf Coast. ...Northern Plains...mid Missouri/upper Mississippi Valleys... A closed low within the subtropical westerlies (currently beginning to migrate inland across the California coast) is expected to continue inland and become absorbed within the developing larger-scale cyclonic flow regime. By Friday, models indicate that it will accelerate northeast of the Colorado Rockies, accompanied by modest strengthening of southwesterly lower/mid tropospheric flow (to 30-40+ kt) within a belt near/just ahead of it. It may also provide support for modest cyclogenesis by late Friday night, along a frontal zone across the eastern Dakotas and Minnesota. Beneath a plume of warm elevated mixed layer air advecting into the Plains ahead of the large-scale troughing, a corridor of strong heating/deep boundary layer mixing appears possible ahead of the front and pre-frontal surface trough. While lapse rates are expected to become at least modestly steep, the mid-level cold core of the approaching upper trough is expected to lag well to the west of the axis of stronger surface heating. At the same time, a substantive low-level moisture return from the Gulf of Mexico appears unlikely through this period, and the extent to which thermodynamic profiles become supportive of appreciable severe weather potential remains unclear. Moisture now present across the northern Plains (lower to mid 60s F), perhaps augmented by some evapotranspiration, may be sufficient to support mixed layer CAPE on the order of 1000-1500 J/kg. Coupled with shear/momentum associated with the 30-40 kt lower/mid tropospheric flow, it appears that the environment may become favorable for an evolving line of vigorous storms by late Friday afternoon, which may become capable of producing marginally severe hail and localized potentially damaging wind gusts. This threat probably will diminish fairly early Friday evening (by around 02-03Z), as the boundary layer stabilizes with the loss of daytime heating. ..Kerr.. 09/13/2017 Read more



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