Valid 221300Z – 231200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM OKLAHOMA TO
PARTS OF THE OHIO VALLEY…MID-ATLANTIC AND NEW ENGLAND…AND OVER
PORTIONS OF THE CENTRAL AND NORTHERN HIGH PLAINS…
…SUMMARY…
Damaging gusts, with a few severe, are possible this afternoon and
evening from Oklahoma to parts of the Ohio Valley, Mid-Atlantic and
New England. Isolated severe hail and wind will also be possible
over the central and northern High Plains.
…Synopsis…
An amplified and progressive northern stream will characterize the
mid/upper-level pattern trough the period, with sharply defined,
synoptic-scale ridging moving eastward across the northern Plains,
Upper Midwest and areas of Canada near western Hudson Bay. To the
east, a well-defined cyclone near the James Bay coast of QC will
progress eastward toward the northern Gulf of St. Lawrence. A
trailing, positively tilted shortwave trough will deamplify as it
moves eastward from the upper Great Lakes to New England, amidst
broadly confluent flow.
Upstream of the synoptic ridge, another substantial shortwave trough
was readily apparent in moisture-channel imagery over the interior
Pacific Northwest. This perturbation will decouple from an
initially phased but more progressive vorticity banner over western
Canada, then move slowly eastward across the northern Rockies. By
the end of the period, the 500-mb trough should extend from
north-central MT to northwestern UT. To its southeast, and south of
the synoptic ridge, a weak shortwave trough is located over the
central/southern High Plains, and should move slowly eastward across
OK and northwest TX through the period.
…Northeastern CONUS to southern Plains…
Isolated to scattered thunderstorms are expected to develop
episodically throughout the period — but maximized from afternoon
into early evening — offering sporadic damaging winds, isolated
severe gusts, and in the western parts of the outlook area, isolated
severe hail.
Mid/upper winds will be maximized over the Northeast, while moisture
is greatest across the South, and midlevel lapse rates maximized
from the southern Plains to the Mid-South. A few loosely organized
bands of convection may develop under the southern rim of somewhat
stronger flow aloft, across the north-central Appalachians into the
Mid-Atlantic coastal plain, as well as portions of southern/eastern
New England where favorable low-level moisture will remain. One of
those convective bands may involve an eastward shift of an area of
strong thunderstorms ongoing over portions of OH/WV, later impinging
upon areas of diabatically driven surface destabilization. Such
potential will depend on the structure/survival of the related cold
pool across the Appalachians. A limiting factor will be weak
deep-layer lapse rates, keeping MLCAPE below 1000 J/kg in most areas
from central VA northward/northeastward, with pockets below 500
J/kg.
Buoyancy will increase southward over the Carolinas and
southwestward across the southern Appalachians region, where heating
and low-level lapse rates will be stronger. From this region
westward to the southern Plains, prefrontal outflow/differential-
heating boundaries will be the main foci for convective development
and localized intensification. Although flow aloft and deep shear
will be weak, limiting overall organization of the severe potential,
relative maxima in convective coverage may occur over portions of OK
and the Ozarks region. Subtle large-scale lift ahead of the
southern High Plains shortwave trough will augment already steep
midlevel lapse rates, while intense surface heating occurs ahead of
the front and away from areas of persistent/antecedent cloud cover,
weakening MLCINH to negligible levels. Well-mixed subcloud layers
will support maintenance of hail and downdrafts to the surface.
Localized concentrations of severe potential probably will develop
within this lengthy outlook area, and a 15%/slight risk may be added
as mesoscale trends warrant. However, uncertainties on that and
smaller scales preclude greater than marginal unconditional severe
probabilities being assigned at this time.
…Northern Rockies/Plains to central High Plains…
Isolated to scattered thunderstorms are expected to develop across
this area during the afternoon, and into evening with eastward
extent. Isolated severe gusts/hail are the main concerns. Primary
sources for lift will include the Pacific cold front and lee
troughing over MT and northeastern WY, and upslope flow with heating
of higher terrain along the western rim of the outlook in WY/CO.
A relative concentration of convection is possible during about
00Z-06Z across portions of eastern MT and perhaps northeastern WY.
During that time frame, some of the stronger low-level mass response
and convergence (related to the approaching shortwave trough), and
perhaps the leading fringe of mid/upper synoptic forcing, will
impinge upon a narrow plume of relatively maximized buoyancy and
boundary-layer moisture. Preconvective MLCAPE of 1000-2000 J/kg is
possible this evening near an axis from RAP-GGW, amidst well-mixed
subcloud layers supporting strong-severe gust potential. However,
weak mid/upper-level winds and related minuscule values of effective
shear, along with progs of quick transition to messy convective
mode, suggest severe potential is fairly disorganized and/or
short-lived.
..Edwards.. 08/22/2019
$$