Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0731 AM CDT Mon Jun 12 2017 Valid 121300Z - 131200Z ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON/EVENING FOR EASTERN WY...NORTHWESTERN NE...AND SOUTHWESTERN SD... ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS SURROUNDING THE MODERATE RISK AREA...FROM NORTHEASTERN CO TO NORTHEASTERN WY AND WESTERN SD... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE NORTHERN ROCKIES TO THE MIDDLE MO VALLEY AND WI... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE NORTHERN ROCKIES TO LOWER MI... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON ACROSS PARTS OF ME... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON/EVENING FOR PARTS OF WEST TX... ...SUMMARY... Severe thunderstorms with tornadoes, very large hail, and damaging winds are likely this afternoon into tonight across the north-central High Plains. Other severe thunderstorms are expected from parts of the northern Rockies to the Upper Midwest, as well as across Maine and west Texas this afternoon into evening. ...Central/northern High Plains this afternoon into tonight... On the southeastern periphery of the closed low over NV, a 70+ kt midlevel jet streak will eject northeastward to the northern High Plains tonight. A related lee cyclone will deepen across northeast CO by this evening, before translating to central NE/SD by the end of this forecast period. Initial boundary-layer dewpoints in the low 60s will spread northwestward from northeast CO into eastern WY today as low-level upslope flow strengthens in response to lee cyclogenesis. Daytime heating within this upslope regime, beneath steep midlevel lapse rates, will drive moderate-strong buoyancy (MLCAPE of 2000-3000 J/kg) and weakening convective inhibition by mid afternoon. A special NSSL sounding from Cheyenne WY at 12z revealed a 100-mb mean mixing ratio of 12.5 g/kg and only a weak cap. Thunderstorm development appears likely by early-mid afternoon from just northeast of the lee cyclone in CO northward to the east slopes of the Laramie and Big Horn ranges. As buoyancy increases through the afternoon, both deep-layer and low-level shear (i.e., hodograph length and curvature) will also increase, resulting in a favorable environment for tornadic supercells. The initially discrete supercells will pose a threat for isolated strong tornadoes and very large hail, while eventual mergers/upscale growth will result in an increasing damaging-wind threat as storms spread from WY into SD. ...Northern Rockies this afternoon/evening... A gradual increase in low-level moisture will occur today as far west as southwestern MT. Convection is expected to form over the high terrain by early-mid afternoon and spread northward, as large-scale ascent increases in advance of the ejecting midlevel low now over NV. Weak-moderate buoyancy and substantial deep-layer southerly shear will support a threat of organized/supercell storms capable of producing isolated large hail and damaging gusts. ...Mid MO Valley to the Great Lakes through tonight... Isolated strong/severe storms have formed this morning in eastern/southeastern NE in a zone of low-level warm advection on the nose of the steepest midlevel lapse rates. Though the low-level jet will weaken some later this morning and the ongoing storms could likewise weaken, lingering outflow boundaries could also provide some focus for additional storm development this afternoon. Otherwise, scattered thunderstorm development is expected on the synoptic front farther to the north, from southeastern SD eastward near the IA/MN border. One or more thunderstorm clusters could pose a large hail/damaging wind threat along this corridor. A couple of tornadoes with more discrete storms will also be possible, given the narrow corridor of somewhat enhanced vertical shear along the boundary. Weaker buoyancy and vertical shear suggest that the severe-storm threat will diminish with eastward extent into Lower MI. ...Maine today... A series of remnant/subtle MCVs will move from the Great Lakes to Maine today and potentially help focus storm initiation. Boundary-layer dewpoints in the 60s and surface heating in cloud breaks will drive potentially moderate buoyancy this afternoon, and deep-layer unidirectional wind profiles with substantial speed shear could support some storm organization/supercell structures. The strongest storms may produce isolated large hail and/or damaging gusts. ...West TX this afternoon/evening... Strong surface heating/mixing on the west edge of the richer low-level moisture should support at least widely-scattered thunderstorm development late this afternoon from parts of the Trans-Pecos to the Caprock. Moderate buoyancy, a deep mixed boundary layer, and modest deep-layer vertical shear could support a few marginal supercells capable of producing some hail/wind. ..Thompson.. 06/12/2017Read more
from SPC Forecast Products http://ift.tt/rATNzD
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