Valid 131200Z – 141200Z
…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF THE MIDDLE ATLANTIC…
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE TN
VALLEY AND OVER THE CENTRAL HIGH PLAINS…
…SUMMARY…
Scattered severe thunderstorms are expected across the southern
Middle Atlantic into the Tennessee Valley. Severe thunderstorms
should also develop across the central High Plains.
…Middle Atlantic/TN Valley…
Early-morning water-vapor imagery depicts a mid-level vort max over
central IL which appears to be partly responsible for organized deep
convection over the lower OH Valley. This feature is forecast to
progress east-southeast across the OH Valley and leading edge of
affiliated MCS is expected to spread into WV shortly after sunrise.
Considerable amount of clouds/precipitation should limit buoyancy
across the central Appalachians while models favor stronger
boundary-layer heating downstream across the Piedmont. For these
reasons will lower severe probs across WV/western VA and orient ENH
Risk east of the mountains where renewed thunderstorm development is
expected by early afternoon. Damaging winds are the primary severe
threat with scattered severe storms as they organize across the
Piedmont. While initial convection may be cellular in nature, line
segments capable of producing wind should mature across the southern
Delmarva into southern VA.
Scattered severe thunderstorms are also expected to develop along
weak frontal zone as it sags southeast across KY/TN during the
afternoon. This activity will evolve along southern fringe of modest
northwesterly flow which should contribute to some organization.
Strong storms may spread as far south as northern AL by late
evening.
…Central High Plains…
Weak mid-level height rises are forecast across the Great Basin
Tuesday which should maintain modest 500mb flow across the central
High Plains along the front side of southwestern U.S. anticyclone.
While it’s not evident that any meaningful disturbance will approach
southern WY/NE, strong boundary-layer heating is forecast across
eastern WY/CO. Forecast soundings suggest surface parcels will
reach their convective temperatures by 20-21z. Early-evening CAMs
aggressively develop convection across this region with supercell
structures forecast early in the convective cycle. Local hail
algorithm generates hail in excess of 2″ and this seems reasonable
given the steep lapse rates and seasonally high buoyancy. There is
some concern that scattered supercells will merge into an MCS that
should propagate southeast across western KS toward northwestern OK
during the overnight hours. Wind threat should increase if an
organized squall line emerges across this region.
…Upper MS Valley…
Notable short-wave trough is beginning to dig southeast across the
northern Plains. Large-scale forcing ahead of this feature is
forecast to spread across southeast MN/western WI by early
afternoon. In response to the short wave, weak surface low will
develop over southeast MN which should enhance low-level convergence
for isolated thunderstorm development. 00z NAM suggests strong
boundary-layer heating will be noted into this feature which will
result in convective temperatures being breached by 20-21z. Will
maintain 5% severe probs across this region due to the expected
isolated nature of convection.
..Darrow/Cook.. 08/13/2019
$$