SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0754 AM CDT Fri Nov 01 2024
Valid 011300Z - 021200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE SOUTHERN HIGH PLAINS...
...SUMMARY...
Isolated severe hail and/or gusts are possible across parts of the
southern High Plains, from this evening into the overnight.
...Synopsis...
Today will mark the start of a substantial mid/upper-level pattern
amplification over the CONUS. This will be mainly related to a
strong shortwave trough now digging southeastward over the Pacific,
offshore from OR, and south of a cyclone now covering the BC Coast
and adjacent waters. The cyclone will move inland and devolve to an
open-wave trough, but also, will phase better with the amplifying
shortwave trough. By the end of the period, this should result in a
major synoptic-scale trough from southern BC over the Pacific Coast
States and offshore from Baja. Associated cyclonic flow then will
cover nearly all the CONUS from the High Plains westward, with
height falls from the northern/central Rockies to the southern High
Plains.
At the surface, 11Z analysis showed a low over southern QC, with a
cold front southwestward across western portions of NY/PA/WV,
eastern KY, middle TN, northern MS, to east and south-central parts
of TX. The western part of this frontal zone will decelerate today,
become more diffuse, and merge with an inland-shifting, older,
marine/warm frontal zone now lying quasistationary near the LA
coastline and over deep south TX. The latter boundary will
demarcate the northern extent of optimal Gulf moisture, and should
shift northward/inland through tonight, but remain southeast of west
TX and eastern NM until day 2. Another low, with a weak cold front
arching southward over the central High Plains, will dissipate
through the period, as lee troughing intensifies just to the west
(in advance of the strengthening mid/upper trough).
...Southern High Plains...
Isolated to widely scattered thunderstorms are expected to form over
portions of far west TX and southeastern NM from late afternoon into
evening, as gradually increasing boundary-layer moisture becomes
available to diurnally heated higher terrain in the region.
Thunderstorm coverage should increase from late this evening through
the overnight hours, and begin training within a southwest/
northeast-oriented plume from far west TX to southern KS. This
convective growth will be related to strengthening low-level theta-e
and moisture, WAA-related large-scale ascent, and weakening MUCINH.
The most intense cells embedded in this regime may produce severe
hail or gusts, but the overall areal severe threat still appears to
be on the marginal side, due to lack of greater
instability/buoyancy.
In response to trough amplification aloft over the West, low-level
warm advection and moisture transport will persist and enlarge into
the southern High Plains region, with a 30-40-kt LLJ progged to
develop overnight. However this will be an early stage of the
return-flow process under modest lapse rates aloft, with
considerable residual continental trajectories still involved in low
levels, and the most-favorable, Gulf-modified parcels not forecast
to reach the region until day-2 and beyond. Though upper 50s to low
60 F surface dewpoints should spread into the Permian Basin region
of west TX and southeastern NM tonight, the area of strongest lift
farther northwest will have less moisture. MUCAPE of 500-800 J/kg
is expected around the time of most of the convective development,
increasing to the 1000-1500 J/kg range in non-convective inflow
sector by the end of the period. A layer of weaker midlevel flow
above the LLJ will restrict vertical shear, keeping effective-shear
magnitudes under 30 kt over most of the area. As such, any
supercell processes should be isolated and transient in character,
especially considering the anticipated upscale evolution to a
relatively dense convective-precip corridor.
...TX Rio Grande Valley between DRT-LRD...
Widely scattered to scattered thunderstorms should develop this
afternoon as the easterly/upslope component of flow impinges on
higher terrain of the Serranias del Burro of northern Coahuila, and
adjoining higher slopes in the direction of the Rio Grande.
Isolated large hail and damaging gusts are possible, but
predominantly on the Mexican side of the border. The boundary layer
in the valley will be strongly heated and richly moist, with
dewpoints from the upper 60s to low 70s F. Given the lack of
stronger midlevel flow, however, two related factors render a highly
conditional potential on the TX side:
1. Modest deep shear, despite strong veering with height from
surface into midlevels. Forecast soundings suggest just 30-35-kt
effective-shear magnitudes.
2. Uncertainty about whether orographically initiated convection
can remain organized/severe long enough to reach the border.
Early-stage supercells, if any, may not survive far enough eastward
in that mode, but outflow-dominant/forward-propagating multicellular
clusters might. As such, a very conditional severe-gust threat may
reach TX. Given the absence of a robust EML and of related MLCINH,
deep convection also may develop in the warm sector east of the Rio
Grande, amid strong diurnal heating and rich moisture, but the foci
for lift are quite unclear at this time. With all these
uncertainties, will refrain from an unconditional severe area for
this outlook cycle.
..Edwards/Mosier.. 11/01/2024
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