Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0759 AM CDT Sat Apr 22 2017 Valid 221300Z - 231200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF EASTERN LOUISIANA TO PARTS OF SOUTHEASTERN TENNESSEE AND EXTREME NORTHWESTERN GEORGIA.... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM LOUISIANA TO THE VIRGINIA/NORTH CAROLINA BORDER REGION... ...SUMMARY... Thunderstorms may produce at least isolated severe weather from Louisiana to the southeastern Virginia/North Carolina Tidewater region. The best-organized potential appears to be this afternoon into evening between the lower Delta region and eastern Tennessee, with damaging gusts and large hail possible. ...Synopsis... A complex upper-air pattern resides over the contiguous U.S., downstream from progressive ridging that will move eastward over the Rocky Mountain States tonight. A vorticity max is evident over central/southern MO, with a trough westward across KS to an initially distinct perturbation over northwestern CO and south-central WY. Through 00Z, the latter will move southeastward across CO and parts of the OK/TX Panhandles, into confluent cyclonic flow related to the Ozarks perturbation, which should form a nearly closed 500-mb low over western TN around the end of the period. By then, a unified, positively tilted shortwave trough WILL trail southwestward from there across parts of northern LA and east TX, with increasingly difluent mid/upper-level flow over much of the Southeast downstream. Farther west, a strong shortwave trough is approaching the Pacific Northwest. This perturbation will pivot northeastward across parts of OR and WA through this evening, with associated destabilization aloft contributing to isolated general-thunder potential. Another perturbation -- now evident in moisture-channel imagery west of CA near 140W -- will move ashore near the end of the period. At the surface, the 11Z analysis showed a wavy frontal zone with several attached, weak lows from the southern Delmarva Peninsula across southern VA, TN, southern AR, northeast through southwest TX, and northern Chihuahua. The western segment of this boundary, from roughly the lower Mississippi Valley southwestward, should proceed southward as a cold front to central MS, the upper TX coast, and deep south TX by 00Z, while the eastern segment over VA drifts southward. In between, a frontal-wave low should form this evening over the middle TN/northern AL area in response to the approaching mid/upper-level trough/low. By 12Z, the frontal-wave low should be located near the ATL-CHA corridor, with the front extending southeastward over northern SC and southwestward across southern AL. ...MS Delta to TN Valley... Scattered thunderstorms -- some offering severe hail and damaging gusts -- are forecast to develop along and ahead of the surface front this afternoon and persist into this evening before weakening. An ongoing swath of precip and embedded thunderstorms from parts of northern MS into middle TN and southern KY should shift eastward and weaken through the remainder of the morning, as the supportive low-level jet continues to veer and weaken. Large-scale ascent related to the approaching and gradually deepening mid/upper-level trough -- both as DCVA and low-level warm advection -- will combine with areas of diabatic surface heating this afternoon to destabilize the region in the wake of morning clouds/precip. The prospect for destabilization behind the morning convection becomes more uncertain with northward extent from northern MS across western/middle TN, given the longer low-level return-flow trajectories from the higher-theta-e Gulf source above and through a previously stabilized boundary layer. Accordingly, some trimming has been done to the northwest part of the probabilities in this area, and more may be needed as mesoscale trends determine through the daylight hours. Otherwise, the warm sector over LA/MS should remain characterized by surface dew points in the 60s F, while moisture advects northeastward across parts of AL. Meanwhile, forecast flow trajectories indicate some cooling is expected of initially stable, convectively inhibiting midlevel layers evident in 12Z JAN, BMX and LIX soundings, as the surface heats. Resultant steepening of lapse rates and weakening of MLCINH should support thunderstorm development this afternoon and gradual intensification, despite weak and somewhat veered surface flow in most areas. MLCAPE 1000-1500 J/kg is possible amidst 35-45 kt effective-shear magnitudes, supporting organized multicells and transient supercells. ...Southern Appalachians to Atlantic... Widely scattered to scattered thunderstorms are expected to develop near the front around midday and move eastward across this region through the afternoon. Isolated cells may become severe, before the onset of unfavorable stabilization resulting from a combination of diabatic cooling and CAA related to the southward frontal movement. Along and just south of the front, surface dew points in the 60s F and diabatic heating will offset modest midlevel lapse rates to yield a narrow band of 1000-1500 J/kg MLCAPE, with effective-shear magnitudes around 30-40 kt. Surface flow also should be weak and veered over the warm sector in this region, though a narrow near-frontal ribbon of backed winds may yield favorable hodographs for supercells and overlap with the northern rim of surface-based effective-inflow parcels. ..Edwards/Peters.. 04/22/2017Read more
from SPC Forecast Products http://ift.tt/rATNzD
No comments:
Post a Comment