SPC 1630Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1119 AM CDT Tue Sep 14 2021 Valid 141630Z - 151200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON INTO TONIGHT ACROSS PARTS OF THE OH VALLEY INTO LOWER MI...AND ACROSS EASTERN CO AND SOUTHWESTERN KS... ...SUMMARY... Scattered storms with damaging winds are expected this afternoon into tonight from parts of the Ohio Valley into Lower Michigan. Severe storms with large hail and damaging winds will also be possible late this afternoon into early tonight from eastern Colorado into southwestern Kansas. A tornado or two may occur through tonight across southern Louisiana with the remnants of Nicholas. ...Lower MI to the lower OH Valley this afternoon/evening... Elevated thunderstorms moved across northern Lower MI this morning in association with the primary ejecting midlevel trough over Upper MI/Lake Superior. A positive-tilt trough extends southwestward to eastern IA/northern IL, and this portion of the midlevel wave will combine with low-level ascent along a surface cold front to support additional storm development by mid afternoon from Lower MI into IN/IL. Surface temperatures warming into the 80s with dewpoints of 66-70 F will support MLCAPE greater than 2000 J/kg, which combined with straight hodographs and effective bulk shear of 25-35 kt will support line segments and clusters of storms along the front, with the potential to produce damaging outflow gusts given the strong buoyancy, steep low-level lapse rates, and 35-40+ kt midlevel flow (strongest across Lower MI). Isolated large hail will also be possible with the stronger embedded cells. ...Eastern CO/southwestern KS this afternoon into tonight... A weak upslope flow regime will establish today from western KS into eastern CO, on the cool side of a slow-moving cold front. Aloft, a series of subtle midlevel shortwave troughs will move east-southeastward atop the low-level upslope regime. Though moisture will be rather modest (boundary-layer dewpoints in the mid 40s to low 50s), midlevel lapse rates of 7.5-8 C/km and daytime heating/mixing will contribute to MLCAPE near 1000 J/kg. The most probable area for storm initiation will be from the Palmer Divide (along a reinforcing frontal surge) southward in the immediate lee of the Front Range in CO, where convective inhibition will be weaker compared to the lower plains. Relatively long, straight hodographs will favor splitting supercells capable of producing large hail initially. Storm interactions could lead to some upscale growth and an increase in the potential for strong-severe outflow winds later this evening into early tonight, as storms spread southeastward. A few storms may also form along the front into southwest KS this afternoon, in an environment with sufficient vertical shear and buoyancy for supercells with at least isolated large hail/damaging winds. ...South central/southeast LA through tonight... Tropical Storm Nicholas continues to weaken along the upper TX coast this morning, after making landfall overnight as a minimal hurricane. The convective structure of the tropical cyclone has become quite asymmetric and ragged as a result of westerly shear aloft, and entrainment of relatively cool/dry air into the cyclone core. These factors will contribute to further weakening today into tonight as the cyclone drifts eastward near the northwest Gulf coast. Well east of the cyclone core, there will still be the potential for more discrete convection in the zone of sufficient vertical shear for supercells across south central/southeast LA where buoyancy will be larger compared to father west. ...Northeast OH/northwest PA into western NY this afternoon... A surface warm front will continue to move northeastward across NY, with a warm and moderately unstable warm sector established across western NY into eastern OH. Though the surface cold front will not approach this area until overnight, localized lake breeze boundaries may provide sufficient lift to initiate isolated storms this afternoon from northeast OH into western NY, with an attendant threat for isolated damaging winds and some hail, given MLCAPE approaching 2000 J/kg and effective bulk shear near 30 kt. An isolated supercell may also occur where vertical shear will be stronger along the warm front in NY, though this threat is highly conditional. ..Thompson/Wendt.. 09/14/2021
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