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Sunday, June 13, 2021

SPC Jun 13, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0746 AM CDT Sun Jun 13 2021 Valid 131300Z - 141200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON/EVENING ACROSS THE SOUTHERN APPALACHIANS...AND THIS EVENING INTO EARLY TONIGHT ACROSS PARTS OF THE CENTRAL HIGH PLAINS... ...SUMMARY... Damaging winds will be possible this afternoon/evneing across the central Appalachians. Damaging winds and isolated large hail will also be possible this evening into early tonight across parts of the central High Plains. ...Central Appalachians this afternoon/evening... A series of subtle midlevel shortwave troughs will progress southeastward from the Great Lakes toward PA/NY, in advance of an amplifying trough over the upper Great Lakes by tonight. Gradual strengthening of northwesterly midlevel flow will occur into the central Appalachians through this afternoon, where effective bulk shear will increase to 35-40 kt by evening. Thunderstorm initiation will be focused by a weak cold front moving slowly southeastward, with the strongest storms expected where the northeastern extent of the somewhat stronger buoyancy from the OH Valley overlaps the somewhat stronger vertical shear over the central Appalachians. Here, a few organized multicell clusters or low-end supercells will pose a primary threat of damaging winds this afternoon into this evening. ...Central High Plains this evening into early tonight... Low-level moisture is spreading northward across the central High Plains, on the western periphery of a surface high across the mid MO/MS Valleys. Boundary-layer dewpoints of 56-60 F, in conjunction with strong daytime heating and beneath steep midlevel lapse rates, will result in MLCAPE of 2500-3000 J/kg this afternoon from northeast CO/northwest KS into the NE Panhandle. Thunderstorm initiation will be possible east of the higher terrain and along a weak reinforcing cold front near the NE/SD border late this afternoon, despite small midlevel height rises associated with the high centered over western NM. Storms may subsequently coalesce through outflow interactions, with the potential for a cluster to form and develop south-southeastward into the low-level inflow and moisture/buoyancy corridor through tonight. Vertical shear will be on the lower margins for supercells initially, with some potential for splitting storms and isolated large hail possible. Thereafter, damaging winds will become the main threat given the expected cluster storm mode in an environment with strong downdraft potential. ...Interior OR/WA this evening into early tonight... Embedded speed maxima will eject north-northeastward over western OR/WA on the eastern periphery of a midlevel trough offshore. On the east edge of the more persistent rain/clouds along a baroclinic zone, weak destabilization is expected along and east of the crest of the Cascades. Though low-level moisture and buoyancy will remain modest at best (MUCAPE generally less than 500 J/kg), steep lapse rate profiles and strong deep-layer shear will favor the formation of organized clusters/line segments rooted near 700 mb. The strongest storms may produce isolated strong/damaging outflow gusts and small hail from this evening into the early overnight hours. ...MS/LA this afternoon/evening... A diffuse baroclinic zone will shift slowly westward from AL toward MS today in association with a weak midlevel trough over the Southeast. The richest low-level moisture and largest CAPE today will reside across MS/AL to the west of the diffuse boundary, amid weak deep-layer northerly shear. Scattered thunderstorm development is expected this afternoon along the boundary and in the warm sector, and storms will subsequently spread south and southwestward across MS into LA. MLCAPE in excess of 3000 J/kg and DCAPE at or above 1000 J/kg will favor occasional wet microbursts with isolated wind damage. ...WI this afternoon through late evening... A midlevel trough over southern MB this morning will dig southeastward and amplify through tonight, in response to upstream height rises over the northern High Plains. The midlevel trough will be accompanied by a weak reinforcing cold front, and deep-layer (0-6 km) bulk shear will reach 50-70 kt by this evening over WI. The primary concern this area is the lack of low-level moisture after a prior frontal intrusion. Short-term model forecasts (aside from the RAP/HRRR) appear to be over-estimating moisture advection from the west and local evapotranspiration, and are too aggressive with CAPE. With only meager buoyancy and steep low-level lapse rates expected, the most plausible threat is isolated strong outflow gusts. ..Thompson/Leitman.. 06/13/2021 Read more LIVE: spc.noaa.gov
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