Valid 202000Z – 211200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
AFTERNOON ACROSS PARTS OF THE OHIO VALLEY AND MID ATLANTIC…AND
TONIGHT ACROSS PARTS OF THE CENTRAL PLAINS INTO MIDDLE/LOWER
MISSOURI VALLEY…
…SUMMARY…
Strong thunderstorms will continue to pose a risk for damaging wind
gusts late this afternoon across the Ohio Valley and parts of the
Mid Atlantic Coast region. Storms developing mainly this evening
into the overnight hours may also pose a risk for severe hail and
wind across the north central Plains into lower Missouri Valley.
…20Z Outlook Update…
Adjustments to categorical and probabilistic lines near/east of the
Mississippi Valley have been made, mostly to account for the
progression of a convectively generated cold pool, which continues
to advance southeastward into the lower Ohio Valley. In general,
the convective system has weakened, but the leading edge of the cold
pool may remain a focus for continuing vigorous thunderstorm
development and potentially damaging surface gusts into the 23-01Z
time frame.
To the west, a plume of very warm elevated mixed-layer air persists
across much of the Plains into the mid and lower Missouri Valley,
contributing to substantial mid-level convective inhibition. Late
afternoon into early evening thunderstorm development may remain
fairly sparse and generally focused near the Rockies and adjacent
high Plains. More substantive convective development probably will
await forcing for ascent associated with lower/mid tropospheric warm
advection, near/north of a strengthening central Plains nocturnal
southerly low-level jet during the 03-06Z time frame. Strongest
storms probably will be focused just north of the axis of warmest
mid-level temperatures (around 700 mb), which currently generally
extends across northeastern Colorado through southern Nebraska into
southwestern Iowa. The eastern edge of the Plains “slight” risk
area has been adjusted somewhat to account for this.
..Kerr.. 08/20/2019
.PREV DISCUSSION… /ISSUED 1123 AM CDT Tue Aug 20 2019/
…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.
$$