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Tuesday, October 12, 2021

SPC Oct 12, 2021 1300 UTC Day 1 Convective Outlook

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 Read more LIVE: spc.noaa.gov
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