SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0755 AM CDT Tue Jul 27 2021 Valid 271300Z - 281200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM WESTERN NEW YORK ACROSS PARTS OF NEW ENGLAND...AND OVER PORTIONS OF THE UPPER MISSISSIPPI VALLEY REGION... ...SUMMARY... Severe thunderstorms are possible today from western New York across parts of New England, and mainly tonight over portions of the upper Mississippi Valley region. ...Synopsis... In mid/upper levels, slight amplification is expected in the western-ridge/eastern-trough pattern that will continue through the period. The closed 500-mb anticyclone over the central/southern Rockies and adjoining Plains will expand in size and intensify gradually, with the high strengthening to around 597 dm overnight over the central High Plains. Meanwhile, height falls should occur over much of the Northeast, both with gradual amplification of the slow-moving synoptic trough, and assorted vorticity maxima and shortwaves pivoting through the associated cyclonic-flow field. The most pertinent such perturbation for this forecast appears to be an MCV -- now apparent in composited radar imagery crossing from northern/central Lower MI to Lake Huron. This feature, preceded by a field of large-scale lift and somewhat enhanced deep shear, should move east-southeastward across southern ON through 18Z, and NY/New England through this evening. At the surface, 11Z analysis showed a cold front from northern ME across southern ON to a weak frontal-wave low over northern Lower MI -- which also may be a surface manifestation of an MCV nearby. The cold front should move slowly southeastward through the period into northern NY and more of northern New England. An outflow boundary was drawn across central Lower MI, southern portions of Lake Michigan and WI, to central MN. This boundary should move southward more over southern WI and Lower MI before stalling later today. ...NY/New England... Surface-based thunderstorms are expected to develop by midday over parts of western NY and/or adjoining ON, along and south of the front. Activity should increase in coverage and move mostly eastward across central/eastern portions of NY and south-central New England through the afternoon, its movement aligned with an instability gradient and boundary-layer moist axis. Damaging to locally severe wind gusts will be the main concern. Confidence has increased that enough large-scale lift (related to the MCV) and diurnal low-level destabilization will occur to support maintenance of the threat eastward into New England. The 15% wind area has been extended accordingly. Surface dewpoints generally in the low/mid 60s F will be common in the preconvective environment across the region, with minimal MLCINH. Accordingly modified RAOBs and forecast soundings suggest that -- despite weak midlevel lapse rates -- MLCAPE around 500-1000 J/kg will develop (locally/briefly higher), atop a reasonably well-mixed boundary layer with steep low-level lapse rates. A roughly unidirectional deep-layer wind profile is expected, with effective-shear magnitudes in the 30-40-kt range. Given the weak capping and progressive regime of ascent preceding the MCV, some cold-pool aggregation may occur in upscale growth of convection, enabling forward propagation to impinge on the destabilized boundary layer, potentially as far east as the MA and perhaps NH coastlines given the westerly flow that can advect favorable boundary-layer theta-e that far. ...Upper Mississippi Valley region... Isolated severe gusts/hail may be noted from a cluster of thunderstorms that developed over the cold pool, and has been moving southeastward over west-central WI. See SPC mesoscale discussion 1363 for near-term details. Severe hail and gusts will be expected from any thunderstorms that can develop from late this afternoon through tonight over this corridor. The MCS that crossed WI last night produced a pronounced outflow boundary whose backing cold pool since has been reinforced by rounds of elevated convection to its north, including the ongoing activity. The associated baroclinic zone -- both at the surface and elevated over the cold pool to the boundary's north -- will be the main focus for additional convective potential through the period. Greater certainty exists in evening convective development in and near the somewhat-expanded eastern parts of the outlook area, where the boundary will be stronger and CINH weaker. A preponderance of numerical guidance -- from synoptic models to assorted CAMs, support this general concept, even if differing in timing/location details on the mesoscale as should be expected. Farther west, the outlook becomes more conditional -- with respect to both coverage and timing of storms -- in a volatile near-boundary environment that will favor severe with any sustained convection that can develop. The southern MN part of the outlook in particular is more of a "boom or bust" scenario with convective-development/maintenance uncertainties in a thermodynamically very favorable parameter space (e.g., 70s F surface dew points beneath steep midlevel lapse rates, 3500-5000 J/kg MLCAPE, and 30-40-kt effective-shear magnitudes supporting organized potential). Additional development is possible late tonight into early tomorrow morning over parts of west-central/ southern NM as well, with coverage/intensity uncertainties. ..Edwards/Gleason.. 07/27/2021
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