SPC AC 061248
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0748 AM CDT Tue Jun 06 2023
Valid 061300Z - 071200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE INTERMOUNTAIN WEST...SOUTHERN ROCKIES TO FAR WEST TEXAS...SOUTH
FLORIDA...AND CAROLINAS...
...SUMMARY...
Isolated strong to severe thunderstorm gusts and hail are expected
today over parts of the Intermountain West, southern Rockies to far
west Texas, south Florida, and Carolinas.
...Synopsis...
In mid/upper levels, a blocking pattern will remain in place through
the period, as in previous days, anchored by a stout mean ridge up
and down the Great Plains, with a high over the northern Plains.
This period, the high will weaken and shift westward very slightly,
as a synoptic cyclone now centered over coastal NS retrogrades
northwestward toward northeastern ME. That, along with a series of
shortwaves pivoting through the western flow field of the cyclone,
will lead to a broad area of height falls from the Great Lakes
across the Appalachians to much of the Atlantic Coast. A
subtropical jet in upper levels will extend from central MX across
the southern Gulf and Yucatan, to the Keys and south FL, south of a
lingering field of cyclonic flow covering the Gulf Coast States and
much of the northern Gulf.
Farther west, a 500-mb low now over the SBA area will drift
erratically through the period, with very slight eastward motion
possible into more of southern CA through the period. Associated
mid/upper difluence, between the cyclone's eastern northern rim and
the mean ridge, will cover much of the Rockies and Intermountain
West. A weak, quasistationary trough aloft will extend across the
central Rockies/Plains and Arklatex to the Gulf of Mexico troughing.
At the surface, 11Z analysis showed a cold front over Atlantic
waters east of the Delmarva Peninsula, decelerating to
quasistationary across northern/western NC, the length of TN, and
northern parts of AR/OK. The boundary over and east of the
Appalachians will move southward again today as a cold front,
beneath the height falls aloft. Farther west, a dryline has backed
well into the higher terrain of central/northern MX and
southern/central NM.
...Intermountain West...
Widely scattered thunderstorms are expected to develop across
southern parts of this region and fan northward out of northern UT,
and northwestward to westward out of northern NV, amid the strong
difluence. Isolated severe gusts and hail near severe limits are
possible, despite a lack of substantial deep shear. A belt of
low-level trough/convergence along the outlook corridor, with local
orographic and diurnal-heating support, will contribute to lift over
the region. Deep/well-mixed subcloud layers should develop over
lower elevations, with very steep lapse rates, and enough remaining
moisture to support areas of 500-1000 J/kg MLCAPE, with even larger
DCAPE values evident in forecast soundings. Associated "inverted-V"
thermodynamic profiles will support deep downdraft
cooling/accelerations in the most intense cells.
...Southern Rockies to far west TX...
Isolated to scattered thunderstorms are expected to develop this
afternoon, predominantly over higher terrain on either side of the
Rio Grande Valley in NM, and southeastward across parts of far west
TX and adjoining Chihuahua. Strong/isolated severe gusts and hail
are possible for a few hours into early evening, before a
combination of environmental diabatic cooling and spreading outflow
air curtail the convection. Activity should form amid strong
heating and favorable moisture at high elevation, then move eastward
into adjoining valleys where 500-1000 J/kg MLCAPE and deeply mixed
subcloud layers can support strong downdraft production/
acceleration. Organization of the activity will be limited by lack
of vertical shear; however, some forward-propagational clustering
may boost severe potential on small scales.
...SC/NC...
Isolated to scattered thunderstorms are expected to develop along/
ahead of the front from eastern SC southwestward over SC, beneath
the southwestern rim of stronger cyclonic flow aloft related to the
major/retrograding low over western parts of the Canadian Maritimes.
The most intense cells will be capable of damaging, perhaps
marginally severe gusts, as well as isolated hail near severe
limits. An eastward-tapering corridor of favorable low-level
moisture and strong diurnal heating will develop ahead of the front
and west of the sounds over SC and parts of central/southern NC,
supporting MLCAPE in the 1500-2000 J/kg range atop steep low-level
lapse rates and a suitably deep, well-mixed boundary layer for
downdraft acceleration. Vertical shear will be weak except through
the entire cloud-bearing layer, given the majority westerly
directional wind component throughout the troposphere.
...South FL...
Scattered thunderstorms will form through afternoon as outflow from
ongoing convection, as well as the Gulf and Atlantic sea breezes
penetrate inland and interact, and on subsequent outflow boundaries
and intersections. The most intense cells will be capable of severe
hail or damaging to marginally severe gusts from water-loaded
downdrafts. Strong diurnal heating and rich low-level moisture
(e.g., surface dewpoints commonly in the low 70s F), along with weak
MLCINH, will offset modest mid/upper-level lapse rates in supporting
development, with preconvective MLCAPE reaching as much as 2000 J/kg
locally. Although near-surface flow and boundary-layer shear should
be weak overall, the presence of light easterlies under the
subtropical jet will lead to somewhat favorable deep/cloud-layer
shear for convective organization. Activity should weaken
substantially late afternoon into evening with outflow spread and
diabatic surface cooling.
..Edwards/Smith.. 06/06/2023
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