Valid 031300Z – 041200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE NORTHERN PLAINS…
…SUMMARY…
Isolated strong to severe thunderstorms are possible this afternoon
over the northern High Plains.
…Synopsis…
The upper-air pattern will continue to feature a stout anticyclone
centered over the southern Rockies, with ridging northward across
the central Rockies and northern High Plains. A strong shortwave
trough — apparent in moisture-channel imagery over the Canadian
Rockies — should move eastward across much of SK by the end of the
period. A lesser perturbation — initially over central/eastern MT
— will move eastward through the mean ridge to central ND by 00Z.
At the surface, 11Z analysis showed a quasistationary frontal zone
from south-central New England across central PA, becoming diffuse
over much of the Ohio Valley, then better-defined again from the
Ozarks to the central Dakotas. A moist axis was drawn just to the
west of the boundary across the central/northern Plains. This
boundary should remain near its present locations except where
overtaken tonight by a cold front moving southward from QC across NY
and New England.
…Northern Plains…
Isolated to widely scattered thunderstorms are expected to develop
this afternoon near the ND/MT line and south-southwestward over
northeastern WY, in a zone of favorably strong surface heating, and
near a surface trough located west of the moist axis. Activity
should move eastward to southeastward into stronger moisture for a
few hours, with isolated large hail or strong-severe gusts possible.
The minor perturbation and associated cloud cover should pass over
the area before peak afternoon destabilization. This will render
two somewhat counterbalancing effects:
1. Relatively unimpeded, strong surface diabatic heating, producing
steep low-level lapse rates in an environment of at least marginally
favorable low-level moisture, with MLCINH essentially eliminated in
some spots.
2. Weak large-scale subsidence, or at best, near-neutral vertical-
motion fields in midlevels by late afternoon. Height falls related
to the peripheral influence of the Canadian trough should wait unto
tonight, and will be subtle.
These factors call into question the coverage of convection, through
buoyancy should be favorable, with surface dew points 60s F
contributing to a narrow corridor of MLCAPE in the 1000-2000 J/kg
range (locally/briefly higher). Effective-shear magnitudes 30-35 kt
are forecast with nearly straight hodographs, indicating a blend of
multicellular and splitting/brief supercell modes possible.
Convection should diminish markedly after dark as appreciable
large-scale forcing for ascent, preceding the major shortwave
trough, remains north of the international border.
…Southern OK to central TX…
Isolated hail briefly near severe levels cannot be ruled out in the
next couple hours from a core or two within a lengthy plume of
convection training southward across southern KS and eastern/
southern OK into north TX, until the supportive warm-advection
regime weakens diurnally. The potential appears too isolated/
conditional for an outlook area.
Isolated to widely scattered thunderstorms may develop this
afternoon across southern OK and north TX, a few of which may
produce strong gusts. The most favorable instability is expected
south and west of the early cloud cover over parts of central TX, as
strong surface heating weakens MLCINH and deepens/mixes the boundary
layer. However, low-level forcing and convective coverage in that
regime appears questionable. Meanwhile, extensive cloud cover is
evident in satellite imagery and should linger over much of the area
of greatest deep-layer support. As such, an unconditional outlook
is not being re-introduced at this time due to lingering concerns
over convective coverage/organization, as well as the lack of more-
substantial vertical shear. However, the well-mixed boundary layer
will offer some cause for concern if enough storm coverage can
accumulate to organize cold pools. Mesoscale trends will be
monitored for a better-organized, stronger wind threat than
currently evident.
…Eastern PA to southern New England…
Widely scattered thunderstorms are possible this afternoon in a
west-southwest/east-northeast corridor along and south of the front.
This area will reside within the northern rim of a corridor of
favorable low-level moisture and diurnal heating south of the
boundary. Surface dew points commonly in the mid/upper 60s F in
lower elevations, and MLCAPE near 1500 J/kg may support a few strong
multicells with downdrafts approaching severe levels. However, weak
low/middle-level winds will substantially limit vertical shear.
While minor damage cannot be ruled out in the most intense cores,
the potential for severe/50 kt wind appears too small and
conditional for a categorical area at this time.
..Edwards/Broyles.. 08/03/2019
$$