SPC 1730Z Day 2 Outlook
Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1226 PM CDT Wed Sep 16 2020 Valid 171200Z - 181200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF GEORGIA AND THE CAROLINAS.... ...SUMMARY... A threat for tornadoes and damaging wind gusts associated with Tropical Cyclone Sally will exist from Georgia into the Carolinas. Potential for strong wind gusts also exists Thursday afternoon and evening across western Oregon and far southwest Washington. ...Georgia into the Mid-Atlantic... Predominantly weak flow aloft will persist across the southern half of the CONUS Thursday with high pressure remaining in place over the Four Corners region and modest mid-level trough moving slowly southeastward over the southern Plains. The only exception to this weak flow over the southern CONUS is in the vicinity of TC Sally. Sally is forecast to begin the Day 2 period as a tropical depression near the central AL/GA border before continuing northeastward across central GA into SC. Strong low to mid-level flow will persist throughout the eastern periphery of the system, spreading from GA across the Carolinas. This will result in a kinematic space favorable for tornadogenesis. Primary concerns, as is typical with tropical systems, is the degree of destabilization and storm mode. Faster motion of the system suggests there may be more opportunity for increased cellular convection ahead of the system while still maintaining well-formed rainbands. This will result in a greater opportunity for more persistent updrafts to realize the strong low-level vertical shear. Additionally, low to mid 70s dewpoints will likely be in place ahead of the main convective bands associated with Sally. This ample low-level moisture will support modest buoyancy, despite the poor lapse rates associated with the tropical air mass. As such, the overall parameter space appears supportive of tornadoes throughout much of Day 2, including overnight Thursday into early Friday morning across the Carolinas. ...Pacific Northwest... Farther northwest, an upper low will begin the day centered off the Pacific Northwest coast. A strong shortwave trough is forecast to progress through the base of this low, reaching the southern OR/northern CA coast during the early afternoon. Increased mid-level moisture associated with this shortwave trough will result in modest elevated instability, primarily across portions of OR west of the Cascades. The strong forcing for ascent attendant to this shortwave coupled with the modest instability is expected to result in thunderstorm development between 21Z and 00Z. While much of the environment only support elevated storms, potential for a few surface-based storms does exist during the brief window before low-level nocturnal stabilization begins. In either case, the resulting storms are expected to move quickly northward/north-northeastward during the evening. Given the strength of the vertical shear and progressive forcing for ascent, strong gusty winds could accompany these storms. ..Mosier.. 09/16/2020
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