SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0749 AM CDT Tue Oct 12 2021
Valid 121300Z - 131200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS EVENING
AND TONIGHT...FROM WESTERN/CENTRAL KANSAS TO WESTERN OKLAHOMA AND
THE EASTERN TEXAS PANHANDLE...
...SUMMARY...
Severe thunderstorm gusts, large hail and tornadoes are possible
mainly this evening into tonight, across parts of the central and
southern Plains.
...Synopsis...
The dominant mid/upper-level feature for convective/severe potential
today will be a synoptic-scale cyclone initially centered over the
UT/AZ border region. The associated 500-mb low is forecast to eject
northeastward through the period, reaching northwestern/north-
central CO by 00Z, with the trough across south-central/southeastern
CO to northern/western NM. By 12Z, the low aloft should move to
near CDR, with 500-mb trough arching southeastward then
southwestward over the central plains and southern High Plains.
At the surface, 11Z analysis showed an area of low pressure over
central/western CO and eastern UT, with cold front southward across
western NM and southeastern AZ. A dryline -- initially developing
over the lower Pecos Valley and Permian Basin regions of west TX on
the northwestern rim of substantial Gulf moisture return -- should
sharpen from south to north today across the TX/OK Panhandles and
into western KS. This will occur as low-level moist advection
occurs out of a source region of 60s to 70s F surface dew points
initially located south of about an SJT-ACT-SHV line.
Meanwhile, the primary surface low related to the cyclone aloft
should reorganize/consolidate and deepen to the lee of the Rockies
near BFF by 00Z. The cold front by then should arch across eastern
CO, the western OK Panhandle, northwestern TX Panhandle, and
southeastern NM. The front will overtake the dryline rapidly from
north-south thereafter, as the front advances and dryline retreats
nocturnally. By the end of the period, the low should be over the
SD Badlands, becoming better aligned vertically with the cyclone
center aloft. The cold front should arc southeastward over east-
central NE, south-central KS, western OK, and northwest through far
west TX.
...Central/southern Plains...
Thunderstorms should develop initially late this afternoon over the
central High Plains, then as deep-layer forcing for ascent and
low-level moisture each increase with southward extent after sunset,
additional convection should form southward over more of western KS
into the eastern TX Panhandle and western OK. Supercells are
possible in the first few hours of convective activity across a
given mesobeta-scale area -- some discrete or nearly so with large
to very large hail, severe gusts and tornadoes most probable in a
corridor from western KS to the eastern Panhandles and western OK.
A supercell or two may pose a threat for significant (EF2+ damage)
tornadoes, in addition to very large hail. This will be of greatest
concern during a time window this evening defined by:
1. Warm-sector hodographs enlarging substantially with the
influence of an LLJ strengthening to 50-65 kt,
2. Moist/warm advection near the surface maintaining surface-based
effective inflow parcels to partly offset gradual diabatic cooling,
and
3. Convection isn't yet solidly linear and lacking embedded
supercells.
Forecast soundings indicate such conditions at least briefly
collocated with effective SRH of 300-500 J/kg and effective-shear
magnitudes around 50-60 kt over the hatched areas on the graphics.
Even the eventual QLCS will be moving into a high-SRH environment
over parts of KS and western OK, maintaining tornado potential into
part of the overnight time frame.
As the cold front surges out farther eastward and southeastward,
impinging on increasing boundary-layer moisture, quick transition to
quasi-linear mode is expected, with damaging gusts becoming the main
severe mode, isolated hail still possible, and some continued
tornado threat from embedded supercells, bows and QLCS mesovortices.
The resulting convective band should outrun the supportive surface-
based parcels in the moist/warm sector late tonight from north to
south, as it approaches the Missouri Valley and eastern KS. Extent
of both evening linear backbuilding into TX is uncertain, as is
potential dryline development south of about the latitude of (and
east of) LBB, but at least marginal unconditional probabilities will
be maintained for isolated potential.
...South TX...
Some concern also exists for convection to develop in the high
terrain (Serranias del Burro and vicinity) of northern Coahuila late
this afternoon or early evening, and subsequently spread east-
northeastward to northeastward across the adjoining Rio Grande
Valley, into portions of south TX near severe levels. At least
isolated severe hail/gusts are possible in a CAPE/shear parameter
space favorable for either supercells or upscale-expanding clusters.
Thick mid/upper-level cloud cover should spread northeastward across
this region today and especially this afternoon/evening, with
increasing contribution to that moisture from eastern Pacific
Hurricane Pamela. While this should limit intensity of diurnal
heating, a weak perturbation now apparent in moisture-channel
imagery north/northeast of Pamela and northern Coahuila may become
enhanced convectively over western/northern MX today. This
perturbation then may move northeastward into the southeastern
fringe of the favorable mid/upper flow, supplying large-scale lift
aloft. Meanwhile, evening dryline retreat and related moist
advection will lead to increasing low-level theta-e over higher
terrain, along with weaker MLCINH and storm-initiation potential.
Forecast soundings suggest 2000-3000 J/kg preconvective MLCAPE and
40-45 kt effective-shear magnitude.
Since this threat still appears uncertain in terms of both coverage
and storm mode, the unconditional probabilities are kept at marginal
levels, but expanded to account for the possible spatial extent/
penetration into TX. If confidence increases in a relative
concentration of severe threat with such a regime, an upgrade may be
needed in a succeeding outlook.
..Edwards/Gleason.. 10/12/2021
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