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Thursday, June 3, 2021

SPC Jun 3, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0747 AM CDT Thu Jun 03 2021 Valid 031300Z - 041200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE MID-ATLANTIC AND NORTHEAST REGIONS THIS AFTERNOON AND EVENING... ...SUMMARY... The main threat area today is over parts of the Mid-Atlantic and Northeast regions this afternoon and evening, with damaging to severe wind the primary concern. ...Synopsis... The general synoptic pattern aloft will remain characterized by progressive western ridging and central/eastern troughing through the period, but with some northern-stream deamplification and southern-stream breakdown noted. Moisture-channel imagery indicates nearly phased shortwave troughs offshore from northern CA and the Pacific Northwest Coast. These will move inland through the period, with the northern portion reaching the BC Rockies by 12Z tomorrow. To its south, an anticyclone will drift eastward over the Great Basin, and a cutoff low over northern/central Baja essentially will weaken in place and lose definition. To the east, a synoptic-scale trough is apparent from the upper Great Lakes through a pronounced shortwave trough and vorticity max over southeastern MO, southwestward to a closely spaced series of lesser vorticity lobes over west TX. The latter will evolve into a weak, closed, elongated, cutoff cyclone by tomorrow morning, from northern Coahuila to north-central/northwest TX. Meanwhile, the MO perturbation will move eastward and elongate with positive tilt, reaching middle TN, WV and western PA by 12Z. At the surface, 11Z analysis showed weak lows near STL, GEZ and ERI, connected by a quasistationary frontal zone. A weak cold front arched from the western low across southern AR and north-central through southwest TX. A wavy warm front was drawn over western NY, central PA, and the Delmarva Peninsula, and this boundary should shift northeastward over more of the Mid-Atlantic region through the day. ...Eastern CONUS including Mid-Atlantic/Northeast... A vast area of at least marginal severe-weather potential exists today ahead of the mid/upper trough, from portions of the Tennessee and Ohio Valleys to Georgia, the Carolinas and western new England. Isolated to scattered thunderstorms will offer sporadic damaging gusts and isolated, marginally severe hail over those regions, primarily during daylight hours, but with some persistence possible into mid evening. A tornado also may occur somewhere within a long corridor from eastern NC to eastern NY, where any sustained supercell(s) can develop and mature. That area will have weak midlevel lapse rates, but also a moist, low-LCL environment with marginal environmental shear. Any such threat should be conditional on local low-level hodograph/vorticity enhancement along outflow, differential-heating and bay-breeze boundaries. The best-focused regime of severe potential still appears to be for damaging wind in and near the 15%/"slight" probability area, east from VA to southern NY, including much of the I-95 megalopolis corridor. Favorable low-level moisture is anticipated, with moist advection commonly increasing surface dew points into the 60s F over the lower Delaware Valley to NJ/southeastern NY, and maintaining 60s over the eastern VA/MD/Delmarva region, even amidst some offset due to boundary-layer heating/mixing by mid/late afternoon. Steepening low-level lapse rates related to the latter process will foster increasing wind potential with scattered thunderstorms whose inflow layers will support 500-1000 J/kg MLCAPE in the north, and 1000-1500 J/kg roughly southwest of the Mason-Dixon line. 30-45-kt effective- shear magnitudes will support a blend of organized multicells, quasi-linear segments with possible bowing configurations, and a few embedded or discrete supercells. ...Interior Northwest... Isolated to widely scattered thunderstorms should develop this afternoon in a narrow corridor from central/eastern OR to the northern Rockies, with initiation favoring frontal lift over higher terrain in each area. One or two clusters of high-based convection may evolve from this activity and move swiftly northeastward, amidst strengthening deep-layer flow that will precede the shortwave troughs. Moving atop a well-mixed and deep subcloud layer, this activity will offer sporadic strong to severe gusts, despite the lack of more-robust buoyancy. Modified RAOBs and forecast soundings suggest 100-500 J/kg MLCAPE west of the ID Rockies, with greater height-adjusted low-level moisture contributing to 400-800 J/kg values from northern ID into portions of western MT. Some damaging thunderstorm-wind potential may continue across the High Plains of northwestern/north-central MT this evening into tonight, before near-surface cooling more-deeply stabilizes the boundary layer. ..Edwards/Gleason.. 06/03/2021 Read more LIVE: spc.noaa.gov