Day 3 Convective Outlook NWS Storm Prediction Center Norman OK 0215 AM CDT Fri Aug 18 2017 Valid 201200Z - 211200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE SUNDAY AFTERNOON INTO SUNDAY NIGHT ACROSS PARTS OF CENTRAL AND EASTERN SOUTH DAKOTA... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING AREAS OF THE NORTHERN PLAINS INTO UPPER MISSISSIPPI VALLEY... ...SUMMARY... A few severe thunderstorms are possible across parts of central and eastern South Dakota late Sunday afternoon into Sunday night. This may include the eventual formation of a large thunderstorm cluster by late Sunday evening, accompanied by a risk for potentially damaging wind gusts. ...Synopsis... Models suggest that the stronger mid-latitude westerlies may continue a northward retreat into areas mostly north of the Canadian/U.S. border during this period. At lower latitudes, western Atlantic subtropical ridging is forecast to continue building westward, and expanding northward, across the Southeast, into the southern Plains, middle Mississippi and Ohio Valleys. Farther west, broad weak troughing with numerous embedded perturbations may continue to evolve west of the California coast into the southern Great Basin and Rockies. On the southern fringe of the mid-latitude westerlies, a plume of warm elevated mixed layer air may nose east of the Wyoming/Colorado Rockies into the mid Missouri Valley. At the same time, modest southerly low-level flow will contribute to northward moisture transport across the Plains, into the vicinity of a cold front advancing southward into the northern Plains and Upper Midwest, before stalling. This latter feature is expected to accompany a short wave trough crossing the Canadian prairies, before turning northeastward across northwestern Ontario by early Sunday. Another significant short wave impulse may dig east southeast of the Canadian Rockies through the Canadian prairies by late Sunday night. ...Parts of northern Plains into upper Mississippi Valley... There remains considerable spread among the various models concerning pertinent features which could impact convective development Sunday afternoon and evening. This includes the positioning of the zone of potentially strong differential surface heating and plume of elevated mixed layer air, and possible subtle perturbations near the southern-most fringe of the mid-latitude westerlies. In general, though, the development of large CAPE, with continued moistening beneath steepening mid-level lapse rates, in the presence of favorable shear for organized storm development (largely due to pronounced veering with height), appears likely to contribute to an environment potentially supportive of severe storm development. At the present time, this seems most probable across parts of central/eastern South Dakota Sunday evening, where/when forcing for ascent may become enhanced near the nose of a modest strengthening southerly 850 mb jet. A transition from isolated supercell development to a consolidating/upscale growing mesoscale convective system appears possible. ..Kerr.. 08/18/2017Read more
from SPC Forecast Products http://ift.tt/GAMjPN
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