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Monday, October 11, 2021

SPC Oct 11, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0740 AM CDT Mon Oct 11 2021 Valid 111300Z - 121200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE MID MISSISSIPPI VALLEY INTO PORTIONS OF THE UPPER GREAT LAKES... ...SUMMARY... Severe thunderstorms are possible today into this evening from the mid Mississippi Valley into portions of the upper Great Lakes, with damaging wind, isolated hail, and a few tornadoes possible. ...Synopsis... In mid/upper levels, a broad area of troughing over the western CONUS is accompanied by three primary shortwave to synoptic-scale perturbations that will influence the convective forecast: 1. A leading closed low over northwestern ON that leaves behind an influential low-level/surface frontal zone across parts of the mid/upper Mississippi Valley; 2. A compact, pronounced cyclone initially centered near the KS/MO border south of MKC, forecast to eject northeastward to near MLI by 00Z and central Lake Michigan around 12Z tomorrow; 3. A deepening trough -- now apparent in moisture-channel imagery over northern CA and the interior Northwest -- which will dig southeastward and form a strong, closed cyclone covering much of the West by the end of the period. At the surface, 11Z analysis showed an occluded low over northwestern ON, with a cold front across eastern WI and northwestern IL, becoming a quasistationary to warm front across northeastern/central MO to a low between JLN-MKC. The cold front arched from that low across southwestern MO, west-central AR, northwestern LA, and southeast/south-central TX. The southern low -- already nearly vertically stacked with the mid/upper cyclone center, likewise will move northeastward toward eastern WI or central Lake Michigan through the period. By 00Z, the front should extend across parts of northern/eastern IL, western portions of KY/TN, northwestern MS, central LA, the mid/upper TX coastal shelf waters, and deep south TX. By 12Z, the front should arc across southwestern Lower MI, central/eastern IN, west-central KY, and northern MS. The trailing southern/Gulf Coast frontal segment should return northward over south TX and the mid/upper TX/southwestern LA coastal plain overnight as a warm front. ...Mid Mississippi Valley to upper Great Lakes... Scattered thunderstorms are expected to form in at least a couple of arcs from mid/late morning through this afternoon, curved through the northeastern and eastern sectors of the ejecting cold-core circulation, near and northeast through east of the surface low and front. The convective arcs may contain supercells. A few tornadoes, along with sporadic large hail and damaging to severe gusts, will be possible. Widely scattered to scattered thunderstorms also are expected to develop near the front today from northern MI across the Lake Michigan region as well, offering a threat for all severe hazards. The latter activity should move northeastward in a favorably sheared environment with weaker midlevel lapse rates, but potentially more boundary-layer heating, than closer to the southern deep-layer low. The strongest deep-layer lift and low-level vorticity/shear should be near, and in the path of, the mid/upper vortex near the surface front. 40-55-kt effective-shear magnitudes are possible from eastern MO northeastward over western/central/northern IL toward southern WI, with effective SRH potentially reaching the 200-350 J/kg range. Abundant clouds and precip will temper surface heating substantially in most areas. Still, even where boundary-layer lapse rates remain only weakly unstable, sufficient low-level moisture and cooling aloft are expected to support surface-based effective-inflow parcels with MLCAPE in the 500-1000 J/kg range. Supercell numbers/ longevity and tornado potential are strongly conditional on mesobeta-scale processes, such as narrow corridors of localized diabatic heating, and both favorable and handicapping interactions with outflow boundaries and other convection. Given the conditionalities, will maintain a broad-brushed 5%-tornado/15% wind/hail area, with the idea that a greater concentration of severe activity may be possible within that, under optimal mesoscale conditions. ..Edwards/Gleason.. 10/11/2021 Read more LIVE: spc.noaa.gov