Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1241 AM CDT Wed Aug 26 2020 Valid 261200Z - 271200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE UPPER MISSISSIPPI VALLEY AND GREAT LAKES REGION... ...SUMMARY... A threat for a few tornadoes will exist Wednesday and Wednesday night primarily across parts of Louisiana and far east Texas in association with Hurricane Laura. Otherwise, a few strong to severe storms with damaging winds and marginally severe hail will be possible over portions of the Upper Midwest, Great Lakes, Ohio Valley, and Mid-Atlantic regions. A few marginally severe storms may also occur from the northern Rockies through the northern Plains, and in Arizona. ...Upper Mississippi Valley/Great Lakes... A zonal flow pattern will be in place across the northern Plains today as a low-amplitude upper-level ridge moves from the upper Mississippi Valley into the western Great Lakes. At the surface a cold front will advance southeastward across the north-central states, and should be located from central Minnesota into eastern South Dakota by mid afternoon. Ahead of the front, surface dewpoints in the lower to mid 70s F will result in a pocket of moderate to strong instability. Thunderstorms appear likely to develop near the strongest instability in central Minnesota during the late afternoon. At that time, the wind profile in central Minnesota is forecast to be veered to the southwest at the surface but have ample speed shear in the low to mid-levels. This will result in 0-6 km shear in the 30 to 40 kt range which could support supercells with large hail. Supercells may also be associated with a wind-damage threat. Further to the east across Northern Wisconsin and northern Lower Michigan, moderate instability will likely be in place by late afternoon. In spite of the presence of a shortwave ridge, convective development is expected during the early to mid evening along the northern edge of moderate instability where low-level convergence should be maximized. This convection is forecast to track eastward across northern Wisconsin and northern Lower Michigan. Moderate instability, 0-6 km shear of 35 to 45 kt and steep lapse rates should be sufficient for a severe threat. The current thinking is that the preferred storm type will be multicells that form into short line segments. These lines will have potential for wind damage along the leading edge. A wind-damage threat may affect areas as far southeast as northwest Pennsylvania during the mid to late evening. ...Louisiana/Far East Texas/Southwest Mississippi... Hurricane Laura will move northwestward through the western Gulf of Mexico today. Rainbands to the north of the center will overspread southern Louisiana and southeast Texas today. Surface dewpoints will be in the lower to mid 70s F across the Sabine and lower Mississippi River Valleys, which will result in moderate instability by afternoon. As convection embedded in the outer band of Laura moves through this unstable airmass, the stronger storms may generate marginally severe wind gusts. As Hurricane Laura approaches southwest Louisiana this evening, low-level shear will dramatically increase from far southeast Texas eastward across southern Louisiana. As a result, a tornado threat is expected to develop as the bands of the hurricane move inland this evening and overnight. The tornado threat will continue across much of Louisiana and far east Texas as the center of Laura moves inland near Lake Charles late in the period. ...Northern Rockies/Northern Plains... An upper-level ridge will move northeastward across the northern Rockies today. At the surface, a moist and unstable airmass will be located from Idaho eastward into parts of the Dakotas. Convection is expected to develop in the higher terrain of the northern Rockies late this afternoon and then move eastward into the northern High Plains during the evening. Hail and strong gusty winds will be possible with the more intense cells. A marginal severe threat may also develop further east in parts of northeast Wyoming and western South Dakota where an axis of moderate instability is forecast late this afternoon. The marginal severe threat could continue into the evening or into the overnight period as the more persistent storm clusters move eastward across the region. ..Broyles/Moore.. 08/26/2020Read more
from SPC Forecast Products http://www.spc.noaa.gov/products/outlook/day1otlk_1200.html
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