Valid 201630Z – 211200Z
…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
FOR PARTS OF IL/IN…
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
ENHANCED RISK ACROSS THE MIDDLE MS AND OH VALLEYS THIS
AFTERNOON…NORTHERN VA AND VICINITY THIS AFTERNOON…AND PARTS OF
THE CENTRAL PLAINS THIS EVENING INTO TONIGHT…
…SUMMARY…
An ongoing thunderstorm cluster across Illinois will continue to
pose a threat for damaging winds into this afternoon into Indiana.
Damaging winds will be possible with a few storms this afternoon in
the vicinity of northern Virginia. Large hail and damaging winds
will be possible this evening into tonight across parts of the
central Plains.
…Mid MS/OH Valley region through this afternoon…
A well-developed MCS and associated MCV will continue to move
east-southeastward across IL/IN through the afternoon. Convection
will likely be maintained into the afternoon along the gust front as
the cold pool encounters a warming boundary layer with dewpoints in
the mid-upper 70s, resulting in MLCAPE in excess of 3000 J/kg with
weakening convective inhibition. Vertical shear will weaken with
southeastward extent aside from the MCV itself, which suggests that
convective organization will be driven more by lift along the gust
front by mid-late afternoon. Still, the established cold pool and
potential for strong downdrafts will maintain a threat for damaging
winds through the afternoon at least into IN/KY, and perhaps into OH
late this afternoon.
…Northern VA and vicinity this afternoon…
A small thunderstorm cluster and diffuse MCV will move eastward from
the eastern WV Panhandle across northern VA/MD this afternoon.
Surface heating in advance of this minor wave will contribute to
moderate buoyancy (MLCAPE near 2000 J/kg), and some loose clustering
of storms is expected this afternoon. Steep low-level lapse rates
and DCAPE near 1000 J/kg will favor occasional downbursts with some
potential for wind damage.
…Central Plains/High Plains this evening into tonight…
The western extent of the outflow is effectively reinforcing a front
across NE. Dry abiabatic midlevel lapse rates atop boundary-layer
dewpoints in the 70s, with surface heating, will contribute to
MLCAPE of 3000-4000 J/kg. Regional 12z soundings suggest that
surface temperatures will need to warm to near 100-105 F to
eliminate convective inhibition, thus storm formation this afternoon
along the effective front is in question. The more probable
scenarios are for storms to form this evening in the upslope regime
near the WY/CO/NE border, and overnight across NE in the
strengthening warm advection immediately north of the front. The
evening upslope storms could be supercells initially as deep-layer
vertical shear is augmented within the easterly low-level flow along
and north of the front, and large hail would be the main concern.
Some upscale growth into a cluster with damaging winds will be
possible early tonight as a southerly low-level jet and warm
advection strengthen. The warm advection could also support
additional elevated storm development farther east across NE
overnight, with an attendant threat for isolated large hail/damaging
winds.
..Thompson/Coniglio.. 08/20/2019
$$