Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0800 PM CDT Thu Aug 27 2020 Valid 280100Z - 281200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTHERN PLAINS... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE LOWER TO MID MISSISSIPPI VALLEY AND FROM PARTS OF THE GREAT LAKES TO THE NORTHEAST... ...SUMMARY... Significant severe wind, large hail and a couple tornadoes will be possible this evening in parts of the northern Plains. A severe threat will also be possible in parts of the southern Great Lakes, central Appalachians and mid-Atlantic. A few brief tornadoes may also occur in association with Tropical Storm Laura across the Lower Mississippi Valley. ...Northern Plains/Upper Mississippi Valley... The latest water vapor imagery shows a zonal flow pattern in place across the north-central states. A well-defined shortwave trough and associated vorticity max is evident in the flow across eastern Montana. At the surface, a most airmass is located in much of the northern Plains where dewpoints are mostly in the 60s and lower 70s F. Surface winds backed to the east across much of South Dakota and a surface low is present across far northwest Nebraska. The RAP is analyzing a corridor of moderate to strong instability to the northeast of the surface low, extending from the Black hills east-southeastward across southern South Dakota. MLCAPE along this corridor is estimated from 2500 to 3500 J/kg. As large-scale ascent associated with the approaching shortwave trough increases, thunderstorms will gradually expand in coverage across the northern High Plains. The strong instability combined with 0-6 km shear in the 45 to 55 kt and steep low to mid-level lapse rates, will be favorable for supercells with large hail. Hailstones of greater than 2 inches in diameter may occur with the more dominant supercells. A couple tornadoes will be possible in areas where low-level shear is strongest. As convective coverage expands this evening, an MCS is expected to organize in western South Dakota. This MCS will move eastward across central and eastern South Dakota during the mid to late evening. Wind damage will be likely with the faster moving line segments embedded in the MCS. Damaging wind gusts greater than 65 kt will be possible with the more intense part of the MCS. ...Central Appalachians/Mid Atlantic... The latest water vapor imagery shows west-northwest mid-level flow is located across the Northeast. At the surface, a boundary is analyzed across Pennsylvania extending eastward into northern New Jersey. Thunderstorms are ongoing along parts of this boundary. To the south, surface dewpoints are in the lower 70s F which is contributing to MLCAPE in the 1500 to 2500 J/kg range. This combined with steep low-level lapse rates will be favorable for damaging wind gusts for a few more hours as cells move east-southeastward across the region. The severe threat could eventually reach eastern Maryland and Delaware later this evening. ...Lower to Mid Mississippi Valley/Central Gulf Coast States... The center of Tropical Storm Laura is currently located in central Arkansas. Bands of heavy rainfall extend from north of the center in northern Arkansas south-southeastward into northern and central Mississippi. The environment ahead of this band is characterized by surface dewpoints in the mid 70s F, moderate instability (MLCAPE near 1500 J/kg) and strong low-level shear. For this reason, rotating cells embedded in this rain band will have a tornado threat. The tornado threat is expected to persist trough the evening into the early overnight period as Laura tracks to the northeast into the mid Mississippi Valley. ..Broyles.. 08/28/2020Read more
from SPC Forecast Products http://www.spc.noaa.gov/products/outlook/day1otlk_0100.html
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