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Saturday, June 5, 2021

SPC Jun 5, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0751 AM CDT Sat Jun 05 2021 Valid 051300Z - 061200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF NORTHERN NEW ENGLAND AND THE NORTHERN PLAINS... ...SUMMARY... Severe thunderstorms will be possible across portions of northern New England and the northern Plains. ...Synopsis... Compared to previous days, the main belt of westerlies will be displaced more northward across the CONUS, with waves that are lower in amplitude, but still convectively influential. A shortwave trough -- now apparent in moisture-channel imagery over western QC near James Bay -- will move across southeastern QC north of NY and New England. This will be followed by weaker/trailing perturbations now located over ON -- all in a regime of broadly cyclonic flow. Much farther upstream, a strong shortwave trough -- initially apparent over the coastal Pacific Northwest -- will move eastward through the period, reaching central/southern SK and central MT by 12Z tomorrow. Height falls will spread eastward over the northern Great Plains in advance of this feature, as a ridge now over southeastern SK and the western Dakotas both deamplifies and moves eastward. Meanwhile, a broad, cut-off cyclone now over central TX should drift northward and weaken further, becoming an open-wave 500-mb trough over OK and TX either very late this period or during the first half of day 2. At the surface, 11Z analysis depicted weak lows near MBG and FAR, the latter being along a wavy, quasistationary frontal zone extending across southeastern SK, northern/eastern ND, northern MN, southern Lake Superior, and northern Lake Huron. A weak cold front was drawn just to the north of that, over western QC east of James Bay and southwestward toward the Lakehead region near the northernmost part of Lake Superior. The latter front should move east-southeastward across parts of southern QC today and into northern ME tonight. A separate cold front -- initially located from southern SK southwestward across northern ID -- will become sharper as the Pacific Northwest mid/upper trough moves inland. By 12Z, this boundary should reach northwestern ND, western SD, and central/southwestern WY. ...Northern New England and vicinity... Widely scattered to scattered thunderstorms are forecast to develop over portions of northeastern ON and southern QC from midday into the afternoon. This activity should form along and ahead of a cold front in Canada, as large-scale ascent related to the aforementioned cyclonic-flow perturbations aloft moves over a diurnally destabilizing, favorably moist, warm-sector boundary layer. The main concern will be damaging to isolated severe gusts with organized multicellular clusters, mainly cold-pool-driven/bowing segments and bands of convection. However, favorable deep shear south of (and trailing behind) the leading perturbation aloft may foster some supercell potential, which would introduce at least a marginal threat for large hail and/or a tornado. The southern cutoff of both severe probabilities and convective potential should be rather abrupt in this regime. Diurnal heating, amidst lower-elevation surface dew points generally in the upper 50s to low 60s F, will offset modest midlevel lapse rates enough to support peak preconvective MLCAPE generally around 500-1500 J/kg. Forecast soundings also suggest that, despite veering of surface winds with time that will reduce both low-level hodograph size and convergence, MLCINH will become negligible, while effective-shear magnitudes remain in the 35-45-kt range. This will support a mixed-mode convective potential and maintenance of activity that develops over Canada. Isolated development over ME ahead of the Canadian convection, and in an earlier/stronger-SRH regime, also is possible. ...Northern Plains... Isolated to widely scattered thunderstorms are possible from late afternoon into evening, both moving northeastward off higher terrain of southern MT and, more conditionally, perhaps developing in association with convergence along/ahead of the main front in parts of the Dakotas. Severe gusts will be the main concern, though isolated, marginally severe hail may be noted as well. As the boundary layer diurnally destabilizes and heights fall aloft across southern MT, surface dew points will lower some in pockets due to strong mixing, but will remain in the mid 40s to mid 50s F, in support of maintaining orographically initiated/supported convection. MLCAPE should range from 300-800 J/kg over the MT part of the outlook, transitioning to 1000-1500 J/kg over the central/ eastern ND area. In contrast, effective-shear magnitudes should increase westward, with values in the 30s in kt over eastern ND and near 50 across eastern MT. A messy and ill-defined surface pattern over the Dakotas, ahead of the primary cold front, appears to include residual mesoscale boundaries left by prior convection. Those may become still more- poorly defined, yet remain influential to convective development and behavior this afternoon/evening. Activity developing over MT and perhaps western ND/northwestern SD may maintain strong-severe wind potential into much of tonight as it moves northeastward over ND into greater moisture, with gust potential eventually tempered by the deepening, diabatically stabilized layer near the surface. ..Edwards/Kerr.. 06/05/2021 Read more LIVE: spc.noaa.gov