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Monday, July 12, 2021

SPC Jul 12, 2021 0730 UTC Day 3 Severe Thunderstorm Outlook

SPC 0730Z Day 3 Outlook Day 3 Convective Outlook CORR 1 NWS Storm Prediction Center Norman OK 0239 AM CDT Mon Jul 12 2021 Valid 141200Z - 151200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS WEDNESDAY AFTERNOON AND EVENING ACROSS PARTS OF THE MIDDLE MISSOURI VALLEY INTO UPPER MIDWEST... CORRECTED CATEGORICAL OUTLOOK GRAPHIC ...SUMMARY... A large, organized cluster of thunderstorms may evolve across the middle Missouri Valley Wednesday and spread east-northeastward through the Upper Midwest by Wednesday night, possibly accompanied by swaths of damaging wind gusts. ...Synopsis... The more prominent mid-level ridging likely will become confined to the southern mid- and subtropical latitudes by early Wednesday, with a couple of generally zonal and progressive belts of westerlies in the northern mid-latitudes. East of the Rockies, the most significant short wave trough is forecast to migrate across southern Hudson Bay and northwestern Ontario into northwestern Quebec during this period. To its south-southeast, weaker troughing, a remnant of a more substantive trough emerging from the Mississippi Valley, is forecast to shift east of the lower Great Lakes/upper Ohio Valley through the northern Atlantic Seaboard. To its south-southwest, weaker trough progressing east of the Rockies is forecast to continue into the Upper Midwest by late Wednesday night. Of particular note, in association with the trailing perturbation, is a consistent continuing signal in the various model output of a deepening lower/mid tropospheric low emerging from the central South Dakota Missouri Valley vicinity early in the day, before migrating east-northeastward into the Upper Midwest/upper Great Lakes region by late Wednesday night. This appears likely to be the result of convective enhancement, if not a completely convectively generated perturbation, and is forecast to be accompanied by intensifying wind fields in excess of 50 kt in the 850-700 mb layer. ...Mid Missouri Valley into Upper Great Lakes region... Models suggest that the convectively generated or enhanced perturbation will emerge near the nose of a remnant plume of elevated mixed-layer air, which may be contributing to modestly steep lapse rates at least initially. It is not clear how far east-northeast this plume will advect through the day, before being cut off by increasing thunderstorm development, and daytime heating ahead of the convection is not expected to yield particularly steep low-level lapse rates due to a relatively deep moist boundary layer. This could mitigate the magnitude of the damaging wind potential. However, the seasonably moist boundary-layer air may still contribute to sizable CAPE in excess of 2000 J/kg. Coupled with a strengthening and expanding cold pool, along with the strengthening wind fields, there still appears potential for the evolution of a large and sustained, organized and forward (northeastward/eastward) propagating convective system. This could be accompanied by swaths of damaging wind gusts, and the environment may become conditionally conducive to a number of embedded mesovortices along the gust front, accompanied by the risk for tornadoes. If current trends in model output persist, and lingering uncertainties clear, severe weather probabilities will need to be increased in later outlooks for this period. ...Northeast... In association with the lead mid-level troughing, models indicate the initiation of considerable thunderstorm activity by midday Wednesday across the Allegheny Plateau/Mountains vicinity. This seems likely to intensify as modest westerly deep-layer mean flow aids propagation into developing surface troughing from the lee of the northern Blue Ridge into the lower Hudson Valley, as well as into the mountains of western New England. Daytime heating along this corridor may yield moderate CAPE on the order of 1000-2000 J/kg, which may support upscale growing clusters or line segments accompanied by potential for a few strong downbursts. ..Kerr.. 07/12/2021 Read more LIVE: spc.noaa.gov