SPC 1730Z Day 2 Outlook

Day 2 Outlook Image

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1226 PM CDT Wed Sep 16 2020

Valid 171200Z - 181200Z


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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