Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1223 PM CDT Fri Apr 21 2017 Valid 221200Z - 231200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SATURDAY INTO SATURDAY NIGHT ACROSS PARTS OF THE TENNESSEE VALLEY INTO THE LOWER MISSISSIPPI VALLEY... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING AREAS OF THE TENNESSEE AND LOWER MISSISSIPPI VALLEY...EASTWARD ACROSS THE SOUTHERN APPALACHIANS AND SOUTHERN MID ATLANTIC COAST REGION... ...SUMMARY... Thunderstorms accompanied by a risk for severe wind gusts and hail are expected across parts of the Tennessee Valley into the lower Mississippi Valley Saturday through Saturday evening. Additional strong thunderstorms with some severe weather potential may develop across parts of the southern Appalachians and southern Mid Atlantic Coast region. ...Synopsis... The leading edge of strong zonal westerlies, extending across much of the mid-latitude Pacific, appears likely to nose inland across the Pacific coast during this period, accompanied by at least a couple of significant short wave perturbations. Downstream, models suggest that large-scale ridging will continue to develop/expand across the eastern Great Basin and Rockies, into the U.S. Plains, with troughing evolving further downstream forecast to gradually pivot from a positive to more neutral tilt across the southeastern Plains through the lower Mississippi Valley. The east/northeastward development of the ridging into the northern Plains may be slowed by a couple of significant short wave perturbations rotating around the southern periphery of a broad polar vortex centered near Hudson Bay. In lower levels, a reinforcing cold front associated with the latter development is forecast to advance across much of the upper Great Lakes region, Upper Midwest and northern Plains by 12Z Sunday. A preceding cold front likely will advance southeastward across the lower Mississippi Valley and northwestern Gulf coast region, and more slowly southward across the southern Mid Atlantic Coast, southern Appalachians and Tennessee Valley. Seasonably modest low-level moisture in the vicinity of the lead front probably will contribute to sufficient destabilization to support considerable thunderstorm activity Saturday through Saturday night. Thunderstorm activity also appears possible farther to the southeast, across portions of the Florida peninsula, in association with moistening ahead of a weak lower/mid-level wave within the subtropical westerlies, over the southeastern Gulf of Mexico. Additional thunderstorm activity is possible near the northern Cascades during the day Saturday, and across parts of the northern intermountain region Saturday night, aided by destabilization associated with cooling/lift accompanying the inland progressing short waves. ...Tennessee Valley/lower Mississippi Valley... Models generally indicate that a corridor of weak to moderate boundary layer destabilization is probable ahead of the southeastward advancing front, near/east of the lower Mississippi Valley, and as far north as a modest frontal wave forecast to evolve across western Tennessee into the Tennessee Valley. Near/east of the surface low, a stalled remnant outflow boundary and/or warm front may provide the focus for vigorous convective development, in the presence of strong vertical shear associated with 30-50+ kt cyclonic lower/mid tropospheric flow. Organized severe storm development appears possible, including the risk for supercells and an upscale growing/organizing convective system. Additional thunderstorm development appears likely within the pre-frontal corridor of stronger boundary layer destabilization, southward across parts of northwestern Alabama and Mississippi Saturday afternoon. Although deep layer mean wind fields and vertical shear likely will decrease with southward extent across central into southern Mississippi, the environment still may become at least marginally conducive to a risk for organized severe thunderstorm development through the afternoon into evening hours. ...Southern Mid Atlantic Coast into southern Appalachians... Severe weather potential is more unclear than across the Tennessee and lower Mississippi Valleys. However, modest destabilization along/south of the southward advancing surface front could support some thunderstorm activity capable of approaching or briefly exceeding severe limits Saturday. ..Kerr.. 04/21/2017Read more
from SPC Forecast Products http://ift.tt/ws9HZn
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