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Saturday, August 28, 2021

SPC Aug 28, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0251 PM CDT Sat Aug 28 2021 Valid 282000Z - 291200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF EASTERN SOUTH DAKOTA INTO SOUTHERN MINNESOTA AND FAR WEST-CENTRAL WISCONSIN... ...SUMMARY... Severe thunderstorms are expected through this evening from southeast South Dakota across southern Minnesota. The most intense storms will be capable of significant damaging wind gusts in excess of 75 mph, a few tornadoes, and large hail. ...20z Update -- Upper Midwest... The Enhanced risk has been removed from the western Upper Peninsula of Michigan and across much of WI, as well as parts of central MN. Morning convection and cloud cover has limited destabilization across these areas, with a surface warm front remaining further south across southern MN. The warm front could lift a bit northward still as the upper trough progresses eastward, but the most favorable environment for severe storms during the evening into the early overnight period will likely remain focused from eastern South Dakota into southern MN. Near-term guidance continues to indicate vigorous thunderstorm development this evening across eastern South Dakota. This activity will likely develop into clusters/bands shifting eastward in the vicinity of the warm front/outflow across southern MN. An increasing low level jet, steep midlevel lapse rates and moderate instability should aid in organized convection capable of a few intense/damaging gusts, large hail and a few tornadoes. The Marginal and Slight risk areas also have been trimmed on the northern edges from ND/MN into NE/southeast CO based on the current location of the surface boundary. Otherwise, no changes have been made to the Marginal risk areas across PA/VA or in relation to the tropical cyclone tornado risk associated with approaching Hurricane Ida across the central Gulf Coast. ..Leitman.. 08/28/2021 .PREV DISCUSSION... /ISSUED 1129 AM CDT Sat Aug 28 2021/ ...Upper Midwest to NE... Ongoing cluster across central MN is expected to track east along the primary baroclinic zone through east-central MN through northwest WI. The airmass ahead of this MCS will continue to destabilize amid ample MUCAPE (around 3000 J/kg) sampled by the 12Z MPX sounding. With a leading mid-level jet and further cold pool development, this cluster should persist with compact bowing structures yielding severe wind gusts as the primary hazard (a few which should be significant). Damaging winds should remain spatially confined in latitudinal extent, but an elongated west-east swath appears probable. A couple QLCS tornadoes will also be possible, especially as mature bow stage in the northwest WI vicinity this afternoon. A separate round of severe thunderstorms is expected towards late afternoon, initially centered on southeast SD. This activity will likely expand east along the trailing outflow from the leading MCS across southern MN and southwest along the surface cold front into central NE. The stronger vertical shear will be confined to along and north of the baroclinic zone, with a mix of supercells and bowing segments probable over southeast SD and southern MN. All severe hazards will be possible but a transition to primarily damaging winds will occur during the evening as convection grows upscale into multiple bands/clusters and a broader MCS evolves east-southeast. Overall intensity will wane towards late evening as instability diminishes and outflows become farther separated from the stronger vertical shear. ...North-central Gulf Coast... Hurricane Ida is expected to strengthen today over a deep/warm Loop Current eddy and continue northwestward between the southeast Atlantic coast midlevel ridge and a weak midlevel low over the northwest Gulf. Ida is forecast to reach the southeast LA coast as a major hurricane Sunday afternoon/evening (see latest NHC advisories for details). The outer northeast quadrant of the hurricane will approach a portion of the north-central Gulf Coast late in the period, when strengthening wind profiles will begin to support the threat of supercells and a brief tornado or two early Sunday morning. ...South central PA to northwest VA... Thunderstorm coverage should be lower today compared to yesterday, in the wake of an MCV now moving off the DE coast. However, boundary-layer dewpoints remain in the 68-72 F range to the southwest of a weak backdoor front, and surface heating over the higher terrain of the Alleghenies and Blue Ridge should support thunderstorm development by mid afternoon. Thermodynamic profiles are fairly similar to previous days with MLCAPE of 1500-2500 J/kg and DCAPE of 750-1000 J/kg, which could be sufficient to support isolated downbursts capable of producing localized tree damage. Read more LIVE: spc.noaa.gov
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