SPC AC 141410
Day 1 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
0910 AM CDT Tue Aug 14 2018
Valid 141300Z - 151200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE CENTRAL HIGH PLAINS...SOUTH-CENTRAL PLAINS...AND EASTERN NORTH
CORRECTED FOR NDFD THUNDERSTORM GRID IN NEW ENGLAND
Marginally severe thunderstorms are possible this afternoon into
evening over parts of the north-central Rockies/High Plains region,
eastern North Carolina, as well as portions of Oklahoma and southern
The mid/upper-level pattern over the contiguous U.S. will be rather
disheveled through the period, with the primary belt of northern-
stream westerlies remaining over Canada. A broad cyclone now
covering the central/northern Appalachians and much of the Northeast
will move slowly northeastward while gradually weakening to an open
wave late this period or early day 2. Another cyclone -- currently
covering much of the central/southern plains with a primary
circulation center over central KS -- will move slowly east-
northeastward through the period, also becoming an open-wave trough.
By 12Z the trough should be near the KS/MO line and over eastern OK.
An embedded vorticity lobe -- now apparent in moisture-channel
imagery over the HYS/HLC area -- is forecast to dig southeastward to
northern OK this evening then pivot eastward toward the western
Ozarks. A basal shortwave trough over the northern Rockies occupies
the southwest end of a persistent/positively tilted synoptic trough
that extends to Hudson Bay. The shortwave trough will amplify
steadily as it moves east-southeastward across MT, reaching the
western Dakotas and northeastern WY by 12Z.
At the surface, a weak, wavy, quasistationary frontal zone was drawn
from the coastal Mid-Atlantic across eastern/central NC, central SC,
northern portions of GA/AL, then northwestward arching across the
STL and OMA areas as a warm front, then southwestward to a surface
low near SPD. A cold front associated with the northern-stream
synoptic trough was drawn from the James Bay region to south-central
MN, becoming quasistationary to the central High Plains.
...North-central Rockies/High Plains...
Convection evolving from an ongoing cloud/precip shield over
southeastern MT/northeastern WY may produce isolated severe hail
through early afternoon as it moves east-southeastward. Additional
thunderstorms are expected to develop this afternoon over higher
terrain of eastern WY and move east-southeastward to southeastward,
offering hail as well as isolated severe gusts.
Convective potential will be aided by a combination of favorable
residual moisture (surface dew points commonly 50s F), diurnal
heating south of the morning cloud/precip area, and post-frontal
upslope flow. The latter will provide enhanced storm-relative
boundary-layer winds, as well as contribute to enough veering of
flow with height to yield 40-45-kt effective-shear magnitudes.
Buoyancy will be on the marginal side, with MLCAPE reaching 800-1200
J/kg, but still sufficient to support multicells and transient
supercells moving into a favorably well-mixed boundary layer for
sporadic hail and strong-severe gusts. Activity should weaken this
evening as it moves into lower-theta-e air.
Scattered thunderstorms are expected to develop this afternoon over
the outlook area, offering the potential for isolated severe
hail/gusts and a conditional, lower-end tornado risk.
Satellite imagery shows a broad clear slot across the region, over
the southern semicircle of the mid/upper cyclone. This feature is
related to a corridor of midlevel drying analyzed on last night's
00Z upper-air charts, and evident in available soundings this
morning. It should persist today, north of the southwest-northeast
plume of profuse low/middle-level moisture, cloud cover and embedded
convective precip that covers parts of north TX, southern/eastern OK
and into the Ozarks. Forecast soundings suggest that sustained
heating of a favorably moist boundary layer in that slot will offset
modest lapse rates aloft over central/northeastern OK, and act in
concert with somewhat greater lapse rates over western/northern
areas nearer the cyclone core and in a zone of DCVA aloft. The
result should be around 2000-3000 J/kg MLCAPE.
Favorable buoyancy and low-level theta-e will be juxtaposed with a
belt of somewhat enhanced middle-level flow to aid in storm
organization. Anvil-level flow, low-level winds and hodographs each
are progged to remain modest on the meso-alpha scale. However,
localized vorticity enhancement along outflow/differential-heating
boundaries may promote short-lived development/tightening of
low-level rotation on the storm scale. As such, a marginal tornado
risk exists, as well as locally strong-severe gusts and sporadic
Isolated to widely scattered thunderstorms may develop this
afternoon near a surface lee trough and residual frontal zone,
across the eastern Piedmont/western coastal plain. The most intense
cores may yield marginally severe hail/gusts.
The frontal zone, while losing baroclinicity, still demarcates very
moisture-rich Atlantic marine air to the south from lower theta-e in
VA, beneath the southern rim of somewhat enhanced mid/upper cyclonic
flow. Low-level flow/lift along these boundaries should be weak,
but in concert with strong surface diabatic heating, still
sufficient for at least a few sustained cells to develop. By
early/mid-afternoon, weak MLCINH and near-70 F surface dew points
will be in place, supporting 2000-2500 J/kg MLCAPE, amidst roughly
35 kt effective-shear magnitudes.
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z