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Friday, June 4, 2021

SPC Jun 4, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0748 AM CDT Fri Jun 04 2021 Valid 041300Z - 051200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF CENTRAL MONTANA TO EXTREME NORTHWESTERN MINNESOTA... ...SUMMARY... Severe thunderstorms will be possible from parts of central Montana to extreme northwestern Minnesota this afternoon and evening. Damaging wind gusts are expected to be the primary threat. ...Synopsis... In mid/upper levels, the northern-stream pattern generally will deamplify through the period, from the northern Rockies across the Great Lakes to the Northeast. A synoptic-scale trough -- evident in moisture-channel imagery from southern ON across eastern KY to AR -- is expected to weaken and move off the Atlantic Coast tonight. This will occur in step with an embedded shortwave trough/vorticity max, initially located over WV and forecast to cross southern New England during the 00-06Z time frame. Upstream ridging will flatten over the northern Plains as the decaying remains of a positively tilted shortwave perturbation -- now over the Pacific Northwest -- traverse the area. Heights also will fall this evening and tonight, from MT westward to the Northwest Coast, in advance of a stronger shortwave trough moving inland. Meanwhile, a weak, cut-off cyclone will drift eastward over portions of TX and OK. Surface analysis at 11Z showed an occluded front over southern QC to northeastern PA, where a warm front branched off to the east- southeast across the northern NJ/NYC area. A slow-moving cold front extended across eastern PA to western NC and northern GA, becoming quasistationary southwestward across central portions of MS/LA to a low near BRO. The warm front will drift northward ahead of the occluded/cold front today, while the cold front proceeds slowly eastward over the Mid-Atlantic and central Piedmont regions. Farther west, a low initially over southern MB will move east- southeastward to the southern part of northwestern ON, while a trailing weak cold front crosses the international border and moves over northern/eastern MT and parts of western/northern ND. ...Northern Plains... Isolated to widely scattered thunderstorm clusters are expected to develop this afternoon into evening near the front over ND, and over higher terrain in southwestern MT. Activity should move generally eastward to east-northeastward over the outlook area, offering sporadic severe gusts. Isolated large hail also may occur, especially over western parts of the outlook area where strong midlevel lapse rates will overlie at least marginal inflow-layer moisture content, in a regime of favorable deep/speed shear. Across the entire corridor, diurnal heating will foster very deep, well-mixed subcloud/boundary layers with almost dry-adiabatic lapse rates. Dewpoints currently in the upper 40s to low 50s should lessen slightly amidst the very strong heating/mixing, but still support the development of MLCAPE values generally in the 500-1000 J/kg range over MT, increasing eastward across lower elevations in ND (but with somewhat greater CINH, per midlevel stable layers evident in 12Z Dakotas RAOBs). Resulting high-based convection will pose a threat for severe gusts that may persist to near the Red River of the North overnight, before diabatic stabilization of the boundary layer finally becomes too strong to support the threat for damaging gusts to penetrate to the surface. ...Coastal Carolinas/Mid-Atlantic... Episodic, widely scattered to scattered thunderstorms are expected through this evening, developing along an assortment of features such as a prefrontal trough, a near-coastal Carolinas convergence zone ahead of the ejecting mid/upper trough, as well as outflow, sea-breeze and differential-heating zones ahead of the front. Damaging to marginally severe gusts will be the main concern. Rich low-level moisture is in place across this corridor, with surface dew points commonly in the mid/upper 60s F, with 70s over the Carolinas. This, along with diurnal heating behind bands of clouds/precip now over the eastern Carolinas to Delmarva region, will offset modest midlevel lapse rates enough to support around 1000-2000 J/kg MLCAPE from southeastern VA southward and roughly 500-1500 J/kg farther north. MLCINH should be minimal enough to support convective forcing along subtle boundaries this afternoon. Strong mid/upper winds will aid in storm organization, though veered low-level winds should help to keep effective-shear magnitudes below 40 kt over most of the area. Multicell clusters/bands should be the main modes, with occasional damaging gusts in the most intense downdrafts. ..Edwards/Kerr.. 06/04/2021 Read more LIVE: spc.noaa.gov