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Sunday, June 13, 2021

SPC Jun 13, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0258 PM CDT Sun Jun 13 2021 Valid 132000Z - 141200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE MID-ATLANTIC...CENTRAL GULF COAST...AND CENTRAL HIGH PLAINS... ...SUMMARY... Scattered damaging winds will remain possible through about dusk across the central Appalachians and the Deep South to central Gulf Coast region. Scattered severe wind and hail are possible across the central High Plains, mainly this evening. ...20Z Update... The Slight Risk across parts of the lower MS Valley/Southeast has been expanded into more of MS and western AL to account for a loosely organized cluster of storms ongoing along the MS/AL border. The very moist and strongly unstable airmass exists across this region, and severe/damaging winds will likely remain the primary threat this afternoon. See Mesoscale Discussion 950 and Severe Thunderstorm Watch 265 for more information on the near-term severe threat across this region. Scattered storms have developed this afternoon across the OH Valley and WV to the east of a weak cold front. Moderate instability and marginal deep-layer shear should continue to support mainly multicells capable of damaging wind gusts and marginally severe hail. The Slight Risk has been trimmed from parts of western MD, the eastern WV Panhandle, and south-central PA where stabilization from prior convection has occurred. See Mesoscale Discussions 951/953 and Severe Thunderstorm Watches 264/266 for more information. It still appears that robust convective development along the Laramie Range in WY and Black Hills in western SD may occur within the next few hours. Although deep-layer shear is modest owing to the influence of an upper ridge, moderate to strong instability and steep mid-level lapse rates should support an isolated hail threat with any storms that can form. See Mesoscale Discussion 952 for additional meteorological details. Considerable uncertainty remains regarding convective evolution and coverage across OK and northwest TX this afternoon and evening in association with a pronounced MCV evident on visible satellite imagery. Any storms that form and can persist will be capable of producing isolated large hail and strong/gusty winds. Minor adjustments to the Marginal Risk have been made based on the current placement of the MCV and observational trends. No changes have be made to the Marginal Risk areas across parts of the Upper Midwest and Pacific Northwest. ..Gleason.. 06/13/2021 .PREV DISCUSSION... /ISSUED 1112 AM CDT Sun Jun 13 2021/ ...Central Appalachians... Convection associated with a leading mid-level impulse is ongoing near the MD/WV/VA border region along the periphery of weak buoyancy. Additional scattered convection should develop in its wake shortly along a weak surface front from the lee of Lake Erie southwest to southern OH. Despite weak mid-level lapse rates, robust boundary-layer heating amid upper 60s to low 70s dew points will support moderate buoyancy with MLCAPE of 1500-2500 J/kg across the OH Valley. Within a belt of 25-35 kt mid-level northwesterlies, a few organized multicell clusters and low-end supercells will pose a primary threat of damaging wind, along with isolated marginally severe hail through about dusk. ...Central High Plains... Low-level moisture characterized by a plume of low 60s surface dew points is spreading north on the western periphery of a surface high. Intense boundary-layer heating beneath steep mid-level lapse rates will result in MLCAPE of 2000-3000 J/kg by late afternoon as thermodynamic profiles become deeply mixed. Thunderstorm initiation is most likely near the higher terrain in southeast WY and the NE Panhandle along a quasi-stationary front despite small mid-level height rises associated with the high centered over western NM. Storms will probably 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 at least midnight. Vertical shear will be on the lower margins for supercells initially, with some potential for splitting cells producing large hail. Thereafter, severe winds will become the main threat given the expected cluster storm mode in an environment with strong downdraft potential. ...TN Valley to central Gulf Coast... 12Z area soundings sampled a plume of probable large MLCAPE in excess of 3000 J/kg should develop as further boundary-layer heating occurs amid very rich low-level moisture characterized by 70s dew points. Scattered thunderstorms are expected from the TN Valley south-southwest towards the central Gulf Coast. Within a belt of confluent 20-25 kt mid-level northerlies, multiple loosely-organized multicells clusters are expected with occasional wet microbursts producing scattered wind damage. See MCD 0943 for additional short-term forecast information. ...WI... A shortwave trough over northwest ON this morning will dig southeast and amplify through tonight across the Great Lakes, in response to upstream height rises over the northern High Plains. The trough will be accompanied by a strong northwesterly mid-level jet which will yield a highly elongated hodograph as 0-6 km bulk shear increases to 60-70 kt this evening. The primary concern is the lack of low-level moisture return after a prior frontal intrusion. With only meager buoyancy, an inverted-v thermodynamic profile should support a few high-based/lower-topped cells capable of isolated damaging winds and marginally severe hail from late afternoon through about dusk. ...Interior OR/WA... Embedded speed maxima will eject north-northeast 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, meager buoyancy from parcels rooted near 700 mb is expected along and east of the crest of the Cascades amid modest mid-level lapse rates. Strong speed shear from the boundary-layer to mid-levels will support a few fast-moving cells with isolated severe gusts and small to marginally severe hail as the main hazards. ...Southwest OK and northwest TX... An MCV drifting across southwest OK may be the focus for intensifying thunderstorm development this afternoon. Guidance differs greatly with the degree of development, but the westward advection of rich boundary-layer moisture from the I-35 corridor should be adequate to overcome MLCIN by late afternoon. A couple of multicell clusters could develop into this evening with a risk for at least isolated severe wind and hail. Read more LIVE: spc.noaa.gov