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Thursday, August 5, 2021

SPC Aug 5, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0750 AM CDT Thu Aug 05 2021 Valid 051300Z - 061200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ROM CENTRAL OREGON AND NORTHERN GREAT BASIN INTO THE NORTHERN ROCKIES AND ACROSS THE UPPER MISSISSIPPI VALLEY REGION... ...SUMMARY... Isolated damaging gusts and hail are possible with thunderstorms from central Oregon and northern Great Basin into the northern Rockies and across the upper Mississippi Valley region. ...Synopsis... Substantial mid/upper-level pattern transition is expected this period, away from the longstanding, high-amplitude western-ridge/ eastern-trough configuration. Heights will rise over much of the CONUS east of the Mississippi River as the trough weakens and ejects northeastward. The western ridging will break down substantially as a strong shortwave trough and associated vorticity max -- evident in moisture-channel imagery offshore from Cape Mendocino CA -- move inland. This feature should come ashore over northwestern CA and southwestern OR around 18Z, then reach the ID/OR border region and western/northwestern NV by 12Z tomorrow. A lengthy but weak shortwave trough extends from the Red River of the North across east-central NE to the TX South Plains and southeastern NM. This trough should move across the upper and mid Mississippi Valley through the period, also reaching the Ozarks and central TX. In between, a small, weak perturbation now over northeastern MT and southern SK should move southeastward across the Dakotas today and this evening, reaching southwestern MN by 12Z. At the surface, 11Z analysis showed a low south of the southern coast of New England, with warm front northeastward near ACK, and cold front southwestward to northern FL, becoming slow-moving to quasistationary over the northern Gulf and south-central TX. The front will move over the Atlantic behind the low, once the low clears Cape Cod, with the bulk of convection and favorable instability remaining offshore in the warm sector. A low over southeastern MB will move eastward to northwestern ON, while a weak cold front moves southeastward/southward across northern MN and parts of SD before the SD part stalls tonight. ...Interior Northwest to northern Great Basin/Rockies.... Isolated to scattered, high-based thunderstorms are expected to form this afternoon over a broad area of the interior Northwest to northern Great Basin, offering locally damaging/isolated severe gusts as they move northeastward toward the northern Rockies. Activity should develop as strong boundary layer heating removed MLCINH, amidst moisture profiles that will become drier each preconvective hour via vertical mixing, but still sufficient for CAPE. MLCAPE values will reach 200-700 J/kg in much of the area, likely maximized from central/eastern OR to central ID, all atop a very deep subcloud mixed layer supporting gusty downdrafts. A low-level cold front -- preceding the mid/upper trough across western to middle parts of the outlook area -- will provide additional lift, as well as a rough western bound for strong-severe thunderstorm potential today. Strengthening mid/upper winds also will spread across the area ahead of the shortwave trough, leading to faster cell motions than seen in previous days, and somewhat more widespread potential for isolated damaging gusts. This potential should shift into the higher terrain of western MT and eastern ID this evening before weakening with enough boundary-layer cooling. ...Upper Mississippi Valley... Isolated to scattered thunderstorms should develop late this afternoon into evening over parts of northeastern, eastern and southern MN, with development less certain into northwestern IA. The most intense among this activity may produce marginally severe hail/gusts before weakening later this evening across western Lake Superior, northern/western WI and southern MN. This convection should form near or even slightly behind the first shortwave trough approaching from the Dakotas, ahead of the cold front near a surface trough, and ahead of the trailing mid/upper perturbation. The regime will be characterized by weakening MLCINH and favorable boundary-layer heating, especially once the clouds from the early trough thin and exit the area. Sufficient low-level moistening is expected to occur from a combination of moist advection and evapotranspirative processes, with surface dew points commonly reaching the mid/upper 60s F. This will support peak/ preconvective MLCAPE in the 1000-1500 J/kg range, locally near 2000 J/kg over southern MN. With weak mid/upper winds and somewhat veered boundary-layer flow, deep shear will be weak, limiting overall organization to mostly discrete multicells and messy clusters. ..Edwards/Leitman.. 08/05/2021 Read more LIVE: spc.noaa.gov