U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus02 kwns 211724 
Storm Prediction Center ac 211723 

Day 2 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1223 PM CDT Tue Aug 21 2018 

Valid 221200z - 231200z 

..there is a marginal risk of severe thunderstorms over portions of 
New England...the coastal Carolinas/mid-Atlantic 
states...south-Central High plains...and the intermountain west... 

Isolated strong to severe thunderstorms may occur Wednesday across 
portions of New England, the coastal Carolinas/southeast Virginia, 
south-Central High plains, and intermountain west. 

A deep upper trough will be in place over much of the east on 
Wednesday, as a series of embedded shortwaves move through portions 
of the Ohio Valley, mid-Atlantic, and northeast. A surface low will 
move northeastward across Quebec as a trailing cold front sweeps 
through much of the east. Further west, an upper trough will move 
slowly eastward across the intermountain west as a low-amplitude 
ridge remains in place over the Southern Plains. 

..southeast Idaho...Eastern Utah...WY...northwest Colorado... 
steep midlevel lapse rates and sufficient moisture are expected to 
result in MLCAPE values of 500-1500 j/kg by afternoon across 
portions of the intermountain west, depending on elevation and the 
extent of diabatic heating that can occur. As the upper trough 
begins to eject eastward, midlevel flow in the base of the trough is 
expected to increase, resulting in a corresponding increase in 
effective shear into the 35-45 kt range, sufficient for organized 
storm structures including the potential for a few supercells and/or 
bowing segments, capable of locally severe hail and wind. Severe 
probabilities have been confined to western Wyoming for now, though there 
is some potential for the threat to spread across the state through 
the evening as initial convective development spreads eastward. 

..south-Central High plains... 
Similar to Tuesday, moist upslope flow along the southern fringe of 
stronger midlevel westerlies will support a focused area of severe 
potential across the south-Central High plains. Moderate instability 
and sufficient effective shear will support a couple of supercells 
and/or bowing segments capable of hail and locally severe wind 

..Carolinas northward into New England... 
Ample moisture will be in place ahead of the cold front on Wednesday 
afternoon from the eastern Carolinas northeastward through the 
mid-Atlantic into portions of New England. However, widespread early 
day cloudiness will tend to limit the potential for significant 
destabilization, and updrafts will struggle to intensify within an 
environment characterized by only modest low-level convergence and 
weak midlevel lapse rates. The highest relative threat still appears 
to be across two areas: portions of New England where convergence 
along the cold front may be somewhat stronger in closer proximity to 
the surface low, and across eastern Virginia/NC/SC, where warmer boundary 
layer temperatures will support somewhat greater buoyancy. Effective 
shear of 30-40 kt will support some modestly organized storm 
structures with the strongest updrafts, with localized damaging 
winds being the primary threat. 

..maximum risk by hazard... 
Tornado: <2% - none 
wind: 5% - marginal 
hail: 5% - marginal 

.Dean/dial.. 08/21/2018