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Thursday, July 15, 2021

SPC Jul 15, 2021 1730 UTC Day 2 Convective Outlook

SPC 1730Z Day 2 Outlook Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1229 PM CDT Thu Jul 15 2021 Valid 161200Z - 171200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTHEAST...OHIO VALLEY...MID MISSISSIPPI VALLEY...AND NORTHERN/CENTRAL PLAINS... ...SUMMARY... Isolated strong to severe thunderstorms may develop Friday across parts of the Mid Mississippi Valley into the Ohio Valley and the Northeast, and over portions of the northern/central Plains. ...Maine... An upper trough/low should advance eastward across Quebec on Friday, with enhanced west-southwesterly low/mid-level winds overspreading parts of ME during the day. The strongest ascent associated with the upper trough/low will likely remain in Canada. But, there will probably be enough low-level convergence along a surface cold front to support convective development by early Friday afternoon across parts of central/coastal ME. MLCAPE around 1000-1500 J/kg should be present in a narrow corridor ahead of the front, and 30-40 kt of deep-layer shear should foster some storm organization. A mix of multicells and supercells appears possible, posing a threat for isolated strong to damaging winds and perhaps some hail. The severe threat should quickly end as storms approach the coast and encounter a less favorable thermodynamic environment. ...Mid Mississippi Valley into the Ohio Valley and Southern New England... The trailing portion of an upper trough/low over Quebec should extend across much of the Northeast into the OH Valley and mid MS Valley on Friday. One or more convectively enhanced mid-level perturbations will probably be present across part of the MS Valley Friday morning. These perturbations are forecast to move generally east-northeastward across the Midwest and OH Valley through the day. A rather moist low-level airmass, with surface dewpoints mainly in the upper 60s to low 70s, should be present across these regions along and south of convectively reinforced front. Even though mid-level lapse rates are forecast to remain poor, most guidance suggests diurnal heating of this moist low-level airmass will foster around 1000-2000 J/kg of MLCAPE across the warm sector by peak afternoon heating. Mid-level west-southwesterly flow will probably remain modest, and effective bulk shear should range from 20-30 kt in general. This should be enough for some storm organization, with small clusters the main storm mode. Current expectations are for multiple clusters to develop and move east-northeastward across portions the mid MS Valley, OH Valley, and continuing into southern New England. As low-level lapse rates steepen with daytime heating, isolated strong to damaging wind gusts may occur with any of these small clusters. Have included a fairly broad Marginal risk area to account for this potential. A greater concentration of storms may occur across parts of the Midwest/OH Valley in association with a remnant MCV and related mid-level perturbation, but too much uncertainty currently exists to include greater severe wind probabilities. There also appears to be a relative minimum in storm coverage ahead of the front across southern VT/NH and far southern ME based on latest guidance, and there was not enough confidence at this time to connect the two Marginal areas in New England. ...Northern/Central Plains... Upper ridging will likely remain prominent over much of the Rockies and adjacent High Plains on Friday, with an upper high centered over AZ/NM. Even so, a belt of modestly enhanced mid-level northwesterly flow should be present over parts of the northern/central Plains Friday afternoon. Initially high-based storms that develop over the higher terrain of the northern/central Rockies should eventually move eastward across the northern/central High Plains Friday afternoon and evening. As these storms advance into a more moist and unstable airmass to the east of a surface lee trough, they should gradually strengthen. One or more weak mid-level perturbations embedded with the northwesterly flow aloft may support additional storm development across the northern/central Plains. The forecast combination of moderate to locally strong instability and sufficient deep-layer shear suggests a threat for both large hail and severe wind gusts with any storms that can form from parts of SD and vicinity southward into western/central NE and eastern CO/western KS through Friday evening. Overall storm coverage and the related severe threat should remain fairly isolated owing to the large-scale ridging over the Rockies. ..Gleason.. 07/15/2021 Read more LIVE: spc.noaa.gov
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