- 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