SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0742 AM CDT Wed Oct 07 2020 Valid 071300Z - 081200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS TODAY OVER PORTIONS OF NEW YORK AND NEW ENGLAND... ...SUMMARY... Isolated marginally severe gusts are possible with thunderstorms later today from New York into New England. ...Synopsis... In mid/upper levels, a large, well-defined cyclone will meander around the northern fringes of Hudson Bay, anchoring a broad cyclonic-flow regime that covers the north-central to northeastern CONUS. Embedded within that flow, a strengthening shortwave trough was apparent in moisture-channel imagery the last few hours over southern ON and Lake Huron. This perturbation should amplify further and pivot rapidly eastward across western/central/northern NY and much of New England through 00Z, then move offshore before 06Z. At the surface, 11Z analysis showed a wavy frontal zone from offshore the Carolinas across northern FL and the northern Gulf shelf waters. This boundary may continue to drift northward in segments through the period. A well-defined low -- related to the northern-stream shortwave trough -- was drawn over the ON/QC border northeast of Lake Huron, with cold front across western Lower MI, central Lake Michigan, southeastern MN, and southeastern ND, becoming quasistationary from there to near the northeastern corner of MT. The low should deepen and move eastward over southern QC to near the northern border of ME by 00Z, then across parts of the Gulf of St. Lawrence overnight. The cold front will proceed southeastward and eastward over the Mid-Atlantic region and New England through the period. Meanwhile, NHC forecasts depict that Hurricane Delta and associated wind fields supporting any tornado potential will remain well south of the west-central Gulf Coast through this period. ...NY/New England... Widely scattered thunderstorms are possible during the daylight hours, sweeping roughly eastward across the outlook area. Isolated severe/50-kt+ gusts are possible, as well as sporadic gusts that are subsevere but still locally damaging. Large-scale ascent is forecast to increase across the outlook area today ahead of the mid/upper trough, both as warm advection and DCVA, while diurnal heating occurs erratically amidst cloud breaks. Resulting deep-layer destabilization (increasing lapse rates in the midlevel and boundary layer) will combine with meager but sufficient low-level moisture to yield surface-based buoyancy and minimize MLCINH ahead of the main UVV plume. A swath of 200-600 J/kg MLCAPE (decreasing southward with weaker midlevel lapse rates) should develop across northern NY and shift eastward, in the presence of strong deep-tropospheric wind fields beginning just above the surface. The nearly unidirectional nature of vertical wind profiles will limit bulk shear somewhat; however, sufficient speed shear still will develop as the trough approaches to yield 35-50-kt effective-shear magnitudes. The main change to the outlook for this cycle is to add some more of ME to the severe probabilities, given forecast soundings and planar fields reasonably showing enough diurnal destabilization to permit the preconvective field of around 500 J/kg MLCAPE to extend over this area briefly. Some consideration was given to a 15% area, given the strength of ambient flow. However, lack of greater low-level instability/moisture, and associated potential for convection to be rather shallow and skeletal, still indicates organized wind potential is rather conditional at this stage. ..Edwards/Goss.. 10/07/2020
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