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Wednesday, June 9, 2021

SPC Jun 9, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0254 PM CDT Wed Jun 09 2021 Valid 092000Z - 101200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF MONTANA... ...SUMMARY... Severe thunderstorms are expected particularly across western/northern Montana, but other isolated strong to severe thunderstorms will also be possible across the Lower Mississippi Valley, Eastern States, and possibly the southern High Plains and Upper Midwest. ...20Z Update... ...Montana... Forecast reasoning across MT remains unchanged from the previous outlook. Widely scattered thunderstorms should develop late this afternoon and evening, initially over higher terrain of southwestern Montana and nearby Idaho, then in areas northeastward across northwestern/north-central Montana through the evening. Supercells capable of all severe hazards are possible, including very large hail, significant severe wind gusts, and a tornado or two. ...Northeast/Mid-Atlantic States... Several outflow-dominant thunderstorm clusters have developed within the very moist and buoyant air mass covering much of the eastern CONUS. Individual updrafts within each of these clusters have been relatively short-lived and disorganize, owing predominantly to the weak vertical shear across the region. Even so, ample low-level moisture has resulted numerous precipitation loaded downbursts. A 51 kt gust was recently measured at DOV with the cluster over the Delmarva Peninsula. A 40 kt gust was also recently measured at ELZ (in Allegany County NY). MCD #898 was recent issued regarding short-term strong to severe thunderstorm potential for southeastern NY and portions of southern New England. In this area, isolated thunderstorms along a weak surface boundary will bring potential for damaging downdraft winds this afternoon. ...Elsewhere... Isolated thunderstorms will continue to develop over the Lower MS Valley, along the southern periphery of the broadly cyclonic flow centered over the Mid MS Valley. Weakly sheared but very moist environment will support isolated updrafts strong enough for water-loading and resultant damaging wind gusts. As strong storm or two also remains possible across north-central MN ahead of an approaching shortwave trough and weak cold front. Conditional severe threat also exists through this evening along the dryline from southwest OK into Far West TX. ..Mosier.. 06/09/2021 .PREV DISCUSSION... /ISSUED 1134 AM CDT Wed Jun 09 2021/ ...Montana... Widely scattered thunderstorms should develop late this afternoon and evening, initially over higher terrain of southwestern Montana and nearby Idaho, then in areas northeastward across northwestern/north-central Montana through the evening. Some of this activity should evolve into supercells, offering damaging gusts and large hail (potentially in excess of 2 inches in diameter) along with the possibility of a tornado. Bowing clusters are possible, which would enhanced the damaging wind risk, potentially including some significant-caliber wind gusts. A substantial easterly component of low-level flow, north and east of the cyclogenesis area, will advect higher theta-e into the region in support of convective potential. Though the source region over the western Dakotas and eastern Montana includes a convectively processed boundary layer, surface dew points in the 50s F should become common over northern Montana. Diurnal heating and a steepening of low-level lapse rates, beneath steep mid-level lapse rates of around 8 deg C/km, will support MLCAPE around 1000-1500 J/kg. Meanwhile, gradually strengthening mid/upper flow and relatively backed near-surface winds will support effective-shear magnitudes of 45-55 kt, particularly across west-central/northern Montana. ...West Texas/western Oklahoma... While the likelihood and coverage are far from certain, it still seems that isolated to widely scattered, slow-moving, high-based thunderstorms are possible late this afternoon into this evening. This would be in a narrow corridor near the dryline from the Big Bend/Davis Mountains north-northeastward to the eastern Panhandle/far western Oklahoma. That said, the most probable area for development late this afternoon/early evening may be across the Texas South Plains/Low Rolling Plains vicinity where low-level convergence should maximize. If/where storms do occur across the region, isolated instances of severe hail/wind will be a distinct possibility given the robust buoyancy and steep lapse rate environment. ...Northern Minnesota/northeast North Dakota... While the peak/most probable severe risk will likely be north of the International border, at least isolated to widely scattered thunderstorms may redevelop across the region later this afternoon into tonight on the southern periphery of advancing shortwave trough and in vicinity of the cold front or nearby triple point. Where storms do form, 25-35 kt effective shear in conjunction with moderate buoyancy could support some severe storms capable of hail/wind. ...ArkLaMiss/Lower Mississippi Valley... The boundary layer will moderately to strongly destabilize (2000-4000 J/kg MLCAPE) this afternoon particularly across the ArkLaMiss vicinity, on the south/southwest periphery of persistent heavy rainfall/residual cloud cover from yesterday. Cyclonically curved westerlies along with possible weak MCV influences/differential heating will aid thunderstorm development within the moist/unstable air mass. Thermodynamic conditions will support the potential, if not likelihood, of wet microbursts capable of localized wind damage across the region this afternoon through around sunset. ...Northeast/Mid-Atlantic States... A moist/moderately unstable air mass will across a broad north-south region in vicinity of a surface trough as well as to the south of a southward-advancing cold front across New York/Pennsylvania into southern New England. While low/mid-troposphere winds will be weak (southern New England an exception), a diurnal steepening of low-level lapse rates and moderate buoyancy may support downbursts capable of localized damaging winds. Somewhat more sustained/organized storms in terms of southeastward-moving clusters could materialize across southeast New England and/or southeast Virginia and northeast North Carolina during the mid/late afternoon through early evening. Read more LIVE: spc.noaa.gov
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