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Thursday, May 20, 2021

SPC May 20, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0238 PM CDT Thu May 20 2021 Valid 202000Z - 211200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE NORTHERN AND CENTRAL HIGH PLAINS... ...SUMMARY... A few severe thunderstorms remain possible late this afternoon and evening across the Big Horns and Laramie Mountains vicinity into the Colorado Front Range and adjacent portions of the northern Great Plains. ...Discussion... No changes are being made to the current outlook aside from minor line tweaks, as current meteorological evolution remains in line with prior expectations/reasoning. TCU/CB development is underway over Wyoming/Colorado on western fringes of the SLGT risk area, as anticipated, and this remains the primary risk area with respect to severe storms this afternoon and evening. ..Goss.. 05/20/2021 .PREV DISCUSSION... /ISSUED 1127 AM CDT Thu May 20 2021/ ...Synopsis... A relatively stagnant, amplified mid/upper flow regime will prevail from the mid-latitude Pacific into the Atlantic through this period and beyond. An elongated blocking high appears likely to remain prominent, centered between the Mississippi Valley and the Appalachians, and flanked by large-scale troughing over the western Atlantic and across the Pacific Coast states through the Intermountain West. The deep-layer ridging may tend to build westward across the Mississippi Valley toward the Great Plains, while a deep mid-level low embedded within the western troughing digs southward, along and west of the Sierra Nevada by late tonight. Between the western troughing and the blocking ridge, remnant mid-level troughing across the Great Plains is forecast to continue to weaken. Several convectively induced or enhanced cyclonic vorticity centers are embedded within the southerly flow ahead of the weak trough axis. One currently over the central Great Plains may maintain better identity while accelerating toward the Upper Midwest today through tonight. Models suggest that another, over the southern Great Plains, may undergo more substantive shearing/deformation across parts of eastern Kansas, Oklahoma and Texas. In lower-levels, a sharp surface frontal zone is forecast to remain quasi-stationary across parts of northern Minnesota and North Dakota into the northern Rockies and Great Basin. However, surface troughing across the Great Basin may gradually fill, while deeper surface troughing to the east of the Rockies becomes more confined to the immediate lee of the Front Range into Black Hills vicinity by late this evening. ...Northern Rockies into Great Plains... The stalled surface frontal zone may provide one focus for vigorous thunderstorm development late this afternoon into tonight. However, models suggest that orographic forcing across the Big Horns, Laramies and Front Range may become the primary focus. The mid-level cold core of the Western troughing will remain well to the west and southwest of the region, and southwesterly mid/upper flow will be generally weak. However, it appears that there may be sufficient moisture to contribute to moderate CAPE, in the presence of daytime heating and steepening lapse rates. As scattered storms initiate off the higher terrain, beneath broadly difluent high level flow, stronger activity may pose a risk for marginally severe hail and locally strong surface gusts. Perhaps aided by a subtle perturbation within the flow, there is a signal in various model output that convection and associated developing cold pools may consolidate into an organizing linear convective system, before weakening across the high plains this evening. ...Parts of southeastern Great Plains into Upper Midwest... Thermodynamic profiles across the region will be marginal for vigorous deep convective development, at best. However, the boundary-layer will remain relatively moist, and supportive of weak CAPE in the lower/mid-levels. Particularly where surface dew points are at least in the mid 60s, near a belt of 30-50 kt flow in the 850-500 mb layer (along the western periphery of the ridging), the stronger, more sustained and discrete low-topped convection may occasional exhibit weak cyclonic rotation late this afternoon and evening. It might not be completely out of the question that some of this may pose a risk for a brief/weak tornado or localized potentially damaging wind gusts. Read more LIVE: spc.noaa.gov
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