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Sunday, October 10, 2021

SPC Oct 10, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0748 AM CDT Sun Oct 10 2021 Valid 101300Z - 111200Z ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF CENTRAL/EASTERN OKLAHOMA... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF NORTHERN MINNESOTA...AND SURROUNDING THE NESTED MODERATE AND ENHANCED AREAS OVER THE SOUTHERN PLAINS... ...SUMMARY... The greatest threat for severe weather will be over parts of Oklahoma and north Texas this afternoon and evening. Tornadoes (some capable of significant/EF2+ damage), destructive hail, and severe gusts all are possible. ...Synopsis... A mid/upper-level mean trough will remain over western North America through the period, with an active, progressive and intense synoptic- to subsynoptic-scale wave train fostering severe-storm potential. An initially low-amplitude shortwave trough -- evident in moisture-channel imagery over the northeastern Pacific southwest of the AK Panhandle -- will amplify considerably through the period as it moves rapidly southeastward to northern CA and OR. A large, related area of height falls will spread across the Great Basin and Intermountain West late in the period, portending the major mid/upper cyclogenesis discussed in the day-2 and day-3 outlooks. As that occurs, two strong mid/upper lows/troughs will eject northeastward away from the mean-trough position and directly influence today's and tonight's severe-weather episodes: 1. A leading, compact cyclone -- initially located over the eastern Dakotas and MN, and centered near the three states' common point. The associated 500-mb low should move to near HIB by 00Z, then eject northeastward over northwestern ON. 2. A well-defined, intense trough now apparent from west-central CO across the Four Corners to south-central/southeastern AZ. This feature will pivot eastward to the southern High Plains through 00Z, becoming less positively tilted. By 12Z, the trough should arc from an attached 500-mb low near MKC northwestward toward FSD, and southwestward to near DFW. At the surface, 11Z analysis showed a low over eastern ND, with occluded front southeastward over western/southern MN to a triple point near RST. The warm front extended from there east- southeastward toward TOL. The cold front was drawn across central/southwestern IA to south-central KS and northwestern OK, to a low over the eastern TX Panhandle, and further, to eastern NM. The northern low will remain nearly vertically stacked with respect to the mid/upper cyclone center, and move to northwestern ON through tonight. By 00Z, the occluded/cold front will advance to western Lake Superior, central IA, east-central KS, central/southwestern OK, and the TX Permian Basin, overtaking the dryline from north to south from then onward into tomorrow morning. Related to the southern mid/upper-level trough, the southern low will become better-defined this afternoon over northwest TX, moving northeastward across central/northeastern OK this evening and tonight while strengthening, then reaching west-central MO by 12Z tomorrow. By that time, the cold front should be near a line from GRB-MLI-SZL- FSM-CRS-SAT, then southwestward over northern Coahuila. ...Southern Plains... A classic "second season" severe event appears to be in order for parts of the Southern Plains this afternoon and evening, featuring tornadoes, large hail and severe gusts. The greatest threat will be bound on the north and west by the cold front and dryline, covering parts of OK and north TX. Severe potential should persist into tonight toward the Ozarks/Ouachitas, Arklatex region and parts of north and central TX. For much of the day, the boundary layer across the southern Plains near the front and dryline will remain capped, restricting substantial vertical mixing to the boundary layer, and permitting moist advection amidst heating/destabilization. This capping will be maintained into early/mid afternoon as a modest EML and some relatively warm midlevel temperatures move over the area, downstream from -2 to -4 deg C 500-mb temperatures analyzed over northwest through southwest TX and northern MX at 00Z last night. However, strong height falls and cooling aloft, along with strengthening mid/upper flow and deep shear, will occur mid/late afternoon ahead of the strong mid/upper trough. A north-south corridor of peak/ preconvective MLCAPE around 1500-2000 J/kg should develop from central TX northward to southwestern/central OK. Meanwhile frontal convergence/lift near and northeast of the dryline should act on diurnally weakening MLCINH. By the time of deep convective development (likely in the 22-00Z window), the parameter space over portions of southwestern/central and south-central OK ahead of the front and dryline will support supercells, with surface dew points into the mid 60s F, 50-60-kt southwesterly effective-shear vectors, and a strengthening LLJ that will enlarge hodographs. Effective SRH should be favorable from the outset of first storms' maturity, increasing into the 300-450 J/kg range this evening. This will support a cyclic tornado threat from any sustained, discrete supercells, which also will offer a damaging-hail and severe-gust hazard. The big hodographs also will persist eastward after activity evolves to more quasi-linear modes and backbuilds into parts of north TX, supporting continued tornado potential. Damaging-gust threats should expand eastward and southward as the convection does likewise this evening. ...Upper Great Lakes, Upper Midwest... The low- and middle-level air masses are expected to destabilize across extreme northern MN and parts of the Arrowhead behind earlier clouds/convection, as the mid/upper-level circulation approaches. A combination of DCVA/cooling aloft, and pockets or narrow ribbons of at least some diurnal heating near the occluded front, will steepen lapse rates. With minimal MLCINH anticipated, one or more arcs of convection should develop near and northeast through east of the surface low. This activity may pose a threat for a few tornadoes, as well as large hail and damaging gusts, before crossing the Boundary Waters region and entering ON. At this time, development appears probable south of the international border, potentially becoming supercellular as the convection interacts with backed low-level winds, related locally enlarged hodographs, and relatively maximized boundary-layer vorticity near the boundary. A narrow, generally front-parallel plume of 500-1200 J/kg MLCAPE may be available, with effective SRH in the 150-300 J/kg range. The environment farther southeast across parts of WI and upper MI will be at least marginally favorable from a parameter-space perspective. However, low-level lift will be weaker, and convective coverage uncertain but likely much less -- rendering the severe potential very conditional. Any sustained convection that does form in those areas will be capable of all severe modes, especially late afternoon into early evening. ..Edwards/Gleason.. 10/10/2021 Read more LIVE: spc.noaa.gov
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