SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0654 AM CST Sun Dec 20 2020 Valid 201300Z - 211200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ALONG PARTS OF THE COASTAL WESTERN FLORIDA PENINSULA... ...SUMMARY... Isolated severe thunderstorms may affect parts of the coastal western Florida Peninsula late this afternoon and evening. ...Synopsis... In mid/upper levels, broadly cyclonic flow will cover much of the central and eastern CONUS this period, with several embedded shortwave troughs. Among them, a leading perturbation -- initially located over the southern Appalachians to western FL Panhandle, will weaken and eject east-northeastward away from the ongoing convective regime extending southwest/northeast across the Gulf. A stronger shortwave trough -- apparent in moisture-channel imagery from central KS to west TX -- is forecast to dig southeastward through the period, reaching portions of the Mid-South and LA around 00Z, and extending from the Carolinas to northern FL around 12Z tomorrow. At the surface, an elongated area of low pressure was analyzed from near the mouth of the Mississippi River eastward across shelf waters south of PNS. A warm front was drawn southeastward across the eastern Gulf and Keys, and a cold front southwestward to the west- central Gulf Coast of MX. The warm front will move eastward diffusely across the northeastern Gulf and into parts of west- central FL this evening, while the cold front crosses the central/ northeastern Gulf. This front may catch up to a currently primary/ prefrontal band of broken convection extending northeast-southwest across much of the southwestern/central Gulf. The front should cross northwestern FL this evening and tonight, reaching east- central/southwestern FL by 12Z. ...FL Gulf Coast... Thunderstorms from the Gulf may cross parts of the FL coastline from midday through this evening. Marginal threat for a tornado and/or damaging gusts is apparent for a short-lived, late-afternoon/evening episode over parts of coastal west-central FL. The north and east sides of a marginally thermodynamically favorable warm sector will brush across the southward-extending AAF area of the southeastern FL Panhandle today, and just onshore the west- central FL Peninsular coastline late afternoon into evening. A strong storm cannot be ruled out in the small notch of warm sector briefly passing over the AAF area; however, flow is progged to veer in the warm sector as the low passes and before cold frontal passage, while destabilization is limited and uncertain due to ongoing/foregoing antecedent precip. Potential in between the two areas -- around northern Apalachee Bay and northern parts of the FL coastal bend, still appears lower and too conditional for an outlook area. Farther south, a window of opportunity onshore may develop near the warm-frontal zone, where flow veers enough to advect better-modified marine air inland, but before too much westerly near-surface component reduces lift, shear and storm-relative flow in the boundary layer. Upper-60s F surface dew points will represent enough boundary-layer moisture to offset modest midlevel lapse rates, for supporting a deep buoyant profile (but with low CAPE density). Time series of forecast soundings suggest potential for 500-1000 J/kg MLCAPE along the immediate coastline area and 40-50 kt effective-shear magnitudes, with hodographs supporting 150-300 J/kg effective SRH during the early part of the most-buoyant period, prior to the further veering. The main uncertainty is how much mature prefrontal convection can cross this environment while it lasts. As convection reaches the coast farther southward past about the FMY area, both deep shear and low-level hodographs will be shrunken relative to near Tampa Bay. ..Edwards/Smith.. 12/20/2020
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