SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0654 AM CST Tue Nov 10 2020 Valid 101300Z - 111200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE MIDDLE MISSISSIPPI VALLEY AND SOUTHERN LAKE MICHIGAN REGION... ...SUMMARY... Thunderstorms capable of producing damaging to severe winds and a few tornadoes are possible this afternoon and evening across portions of the Middle Mississippi Valley and southern Lake Michigan region. ...Synopsis... The upper-air pattern will continue to feature mean/large-scale troughing over the Rockies, with its broad cyclonic-flow field enveloping most of the CONUS between the Pacific Coast and Mississippi River. That cyclonic flow will be traversed by several troughs of varying magnitudes, most notably: 1. A leading trough with a curving axis, presently extending from a vorticity max over the MT/ND border region across the NE Panhandle to another vorticity max near the KS/CO border, then southwestward across southern NM. The southern part of this perturbation will eject northeastward across the central Plains to the upper Mississippi Valley into this evening, while the northern part forms a weak closed low over ND. The southern lobe then may form a closed low as it continue to move northeastward, across northern WI and Lake Superior to adjoining parts of northern ON. 2. A complex trough accompanied by multiple vorticity lobes, and apparent in moisture-channel imagery from northeastern to southern BC, then southward over western/central WA and northern OR. At the surface, 11Z analysis depicted a slow-moving cold front from eastern Upper MI southwestward to a low over eastern IA and another over extreme eastern KS, with a faster-moving cold front across central OK, and west-central/southwest TX. The KS low will ripple northeastward along the boundary as the primary frontal-wave low, causing it to back westward temporarily as a warm front before the low passes this afternoon. The low will intensify, proceed northeastward over Lake Superior, and occlude as the southern vorticity lobe aloft catches up tonight. By 00Z the cold front should extend from the low across southeastern WI, central/eastern IL, southeastern MO, central AR, and south TX. By 12Z the front should reach central OH, western/mid TN, and LA. Meanwhile, T.S. Eta is forecast by NHC to begin moving northward from its present quasistationary presence off northwestern Cuba. The now-compact system should remain over the southeastern Gulf and too far from FL to contribute tornado potential this period. See latest NHC advisories a for tropical watches/warnings related to Eta, and track/intensity guidance. ...Mid Mississippi Valley, southern upper Great Lakes... A line of thunderstorms is expected to develop during mid/late morning either 1) along the cold front, or 2) just behind it, but soon catching up to the front. Either way, with access to increasingly surface-based effective-inflow parcels in the diurnally destabilizing warm sector, the convection should sweep east- northeastward to northeastward across the region in and near the outlook area, while backbuilding somewhat on the south end. The net effect should be a band of strong-severe thunderstorms shifting eastward over the 15%/slight and 5%/marginal wind areas, before weakening this evening. Additional convection may develop over eastern fringes of both the outlook area and modest supportive SBCAPE late this afternoon and this evening, in an initially separate warm-advection plume. This activity may be overtaken by the front (and any remaining frontal convective band) tonight, while its inflow layer becomes more elevated, as it outruns surface-based destabilization. The main concern across the region will be occasional damaging wind, along with the possibility of severe/50+ kt gusts in a few locales. A QLCS or embedded-supercell tornado threat may develop as well, maximized where the parameter-space overlap between diurnally aided max buoyancy and strong low-level shear across eastern IA, northeastern MO, northern IL and southeastern WI. This is east and northeast of the projected path of the deepening surface low, where both speed and backing of low-level winds should be relatively maximized with isallobaric forcing. 500-1000 J/kg peak preconvective MLCAPE and 45-55-kt effective-shear vectors are expected midday into early afternoon in that area. Deep-layer lift and low-level instability should lessen with eastward extent, but only gradually, as warm and moist advection help to offset nocturnal boundary-layer cooling. Given the strength of the ambient flow, progged presence of at least weakly unstable surface parcels as far east as portions of Indiana this evening, and potential for downward momentum transfer, both sets of probabilities have been expanded eastward spatially. Forecast soundings as far east as the IND area between 00-03Z indicate the potential for surface dew points in the 60s F to contribute to around 500 J/kg MLCAPE, amidst 30-40 kt of 0-1-km shear and 40-50-kt kt effective-shear vectors. Any remaining convection moving through this environment will offer at least a marginal severe threat. ..Edwards/Kerr.. 11/10/2020
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