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Tuesday, October 12, 2021

SPC Oct 12, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0246 PM CDT Tue Oct 12 2021 Valid 122000Z - 131200Z ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS NORTHEAST TEXAS PANHANDLE...FAR NORTHWEST OKLAHOMA AND THE EASTERN OKLAHOMA PANHANDLE...AND SOUTHWEST KANSAS... ...SUMMARY... Scattered to numerous severe thunderstorms are expected to begin late this afternoon, peak this evening, and continue into tonight across parts of the central and southern Great Plains. Several tornadoes, some of which may be strong, along with destructive hail and wind are possible. ...Discussion... Forecast thinking from the previous outlook is on track regarding the overall scenario, and confidence has increased for several long-track supercells capable of a couple strong and perhaps an intense tornado this evening/tonight. As a result, this outlook update is introducing a 15-percent tornado probability and an associated Moderate Risk for portions of the northeast TX Panhandle and far northwest OK, and areas to the north-northwest into southwest KS. Storm-simulated model guidance continues to show quasi-discrete convective mode with storms that develop initially near the dryline and move east into richer low-level moisture and high SRH this evening. Forecast soundings strongly favor discrete supercells and long-track, fast-moving supercells appear likely to traverse across the Moderate Risk. Linear forcing and slightly drier/less unstable air will probably favor upscale growth into a band of storms in northwest KS and northeast CO, limiting the tornado risk. Have adjusted the southeastern bound of the 10-percent tornado area to be farther northwest and mainly be confined to western and northwestern OK. ..Smith.. 10/12/2021 .PREV DISCUSSION... /ISSUED 1130 AM CDT Tue Oct 12 2021/ ...Central/southern Great Plains... Thunderstorms should initially develop during the late afternoon/early evening over the central High Plains. 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. Numerous supercells are expected, some discrete or nearly so with significantly large hail, severe gusts, and tornadoes most probable in a corridor from western KS to the eastern Panhandles and western OK. A couple supercells may pose a threat for significant tornadoes. This will be of greatest concern during a time window around 01-05Z. Warm-sector hodographs will be enlarging substantially with the influence of a LLJ strengthening to 50-65 kt. The leading edge of the rich boundary-layer moisture plume (characterized by mid 60s surface dew points) has reached the Permian Basin/Big Country/Low Rolling Plains regions of west to northwest TX. This moisture plume will continue to advect north, likely spreading into southern KS by later this evening. Guidance has spatiotemporal timing differences which should be crucial to the degree of surface-based CAPE/CIN this evening into tonight within the most favored area for significant tornado potential. Recent RAP runs are consistently more moist, while the 12Z NAM is consistently slower and colder especially compared to the RAP and even the 00Z ECMWF. As such, NAM soundings indicate more neutral near-surface lapse rates until late evening and the overnight. However, 16Z surface observations suggest warmer/moister surface conditions are running ahead of schedule across southwest KS through the TX Panhandle. An upgrade to a tornado-driven Moderate Risk may be needed in later outlooks should this trend continue given the spread in guidance with regard to near-surface thermodynamics and longevity of discrete supercell structures. As the cold front surges out farther east and southeast, impinging on the increasing boundary-layer moisture, a transition to mostly linear mode is expected from north to south with damaging gusts becoming the main severe mode. Some continued tornado threat should persist overnight in parts of KS/OK from embedded supercells, bows, and QLCS mesovortices. The resulting convective band should outpace the supportive surface-based parcels in the moist/warm sector overnight 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... Isolated severe thunderstorms will be possible this afternoon into tonight, but mostly anticipated this evening. A series of weak perturbations emanating northeast from eastern Pacific Hurricane Pamela should aid in multiple bouts of convective potential through the period. While the low-level forcing mechanisms should remain nebulous and mid-level lapse rates will not be as steep as a typical severe setup, very rich boundary-layer moisture will remain prevalent and maintain a broad plume of 2000-3000 J/kg MLCAPE through dusk, before waning tonight. Effective bulk shear will be adequate for a conditional supercell risk, perhaps somewhat more favorable in the Edwards Plateau region this evening. If confidence increases in a relative concentration of severe threat within this regime, an upgrade may be needed in a succeeding outlook. Read more LIVE: spc.noaa.gov
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