Valid 251300Z – 261200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE OZARKS
WESTWARD OVER PARTS OF OKLAHOMA AND KANSAS…AS WELL AS PARTS OF
ARIZONA AND SOUTHEASTERN CALIFORNIA…
…SUMMARY…
Isolated severe winds and hail are possible this afternoon and
evening from the Ozarks westward over parts of Oklahoma and Kansas,
as well as parts of Arizona and southeastern California.
…Synopsis…
In mid/upper levels, broadly cyclonic flow will cover the northern
CONUS from the northern Rockies across much of the Midwest and Great
Lakes. This flow belt, and a series of embedded shortwaves, will
reside largely on the poleward side of a related surface front
described below. A cut-off low — initially located over the
northern Baja/northern Gulf of CA area — is expected to meander
erratically near its present location through today, then move
slowly northward up the lower Colorado River area overnight.
At the surface, 11Z analysis showed a cold front from central Lake
Superior across central WI, southern IA, and central KS, to a weak
low over southwestern KS. By 00Z the front should reach Lake Huron,
northern/central IN, southeastern/south-central MO, and north-
central OK, then into an elongated area of low pressure covering
southern CO, northeastern NM and the TX Panhandle. A separate,
prefrontal surface trough and wind-shift line, likely related to an
area of outflow, may develop from the TX Panhandle across central/
eastern OK. By 12Z, the front should reach Lake Ontario, the upper
Ohio Valley, and eastern KY, becoming quasistationary across the
southern Ozarks into central/western OK and the TX Panhandle.
…Ozarks and vicinity to western/central OK…
Isolated to scattered thunderstorms, mainly in clusters, should
develop over the region today, likely focused most densely around
the front, outflow boundaries and differential-heating zones.
Damaging to severe gusts and large hail are possible beneath the
most vigorous cells.
Latest low/middle-level satellite imagery and composited radar
animations reveal an MCV over northeastern OK. This feature, and
the associated swirl of clouds/precip and sporadic convection,
should move eastward to east-southeastward across the southern
Ozarks region through the day. A related outflow boundary,
initially curving from northeastern OK south of OUN to near CHK and
over west-central OK, should lose some definition through the day
and retreat northward somewhat. Meanwhile, the front will
decelerate over the MO Ozarks and southeastern KS. Given the likely
plethora of subtle foci, strong mesoscale dependence of the risk,
and related uncertainties still lingering at this hour, strong-
severe storm concentration is expected to be highly inconsistent/
erratic across the outlook area. Away from patches of sustained
clouds/precip, diurnal heating and rich low-level moisture (upper
60s to low 70s F surface dew points) will support MLCAPE of 2500-
4000 J/kg, but with only 25-30 kt effective-shear magnitudes for
most of the afternoon due to modest low/middle-level flow.
Convection initiation over central/northern/northwestern OK, and a
nearby sliver of KS containing sufficient moisture, is highly
uncertain due to weak low-level flow and related lack of boundary-
layer forcing, as well as somewhat weaker theta-e near the front
itself. Most progs develop little, if any, sustained convection
across this corridor. Still, conditional severe potential exists,
given the presence of strong warm-sector diurnal heating, ample
low-level moisture, locally strong buoyancy, well-mixed boundary
layer, winds veering with height, low-level shear marginally
extending into supercellular parameter spaces by around 00Z, and
vorticity-concentrating boundaries.
…Southern AZ/CA…
Widely scattered thunderstorms are expected to develop across
portions of south-central/central AZ and Sonora, pivoting
cyclonically about the northern semicircle of the mid/upper vortex.
Some of this activity ultimately may move westward or even
southwestward over southeastern CA. Any sustained convection over
southeastern AZ also may move northward to northeastward in a regime
of strongly difluent mid/upper-level winds and largely meridional
mean deep-layer flow. These thunderstorms will be capable of
isolated severe hail and gusts.
The organized severe threat still appears too low/conditional for
more than a marginal area, given the modest magnitudes of both deep
shear and max preconvective buoyancy. Still, the most intense cells
may produce large hail and damaging gusts, given the proximity of
the cold-core low, sufficient residual moisture for surface-based
storms, and enough diurnal heating to build a well-mixed, inverted-v
thermodynamic profile in the boundary layer.
..Edwards/Leitman.. 09/25/2019
$$