SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0655 AM CST Fri Nov 27 2020 Valid 271300Z - 281200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM SOUTH TEXAS TO SOUTHERN MISSISSIPPI... ...SUMMARY... Isolated, marginally severe thunderstorms are possible from South Texas to southern Mississippi today. ...Synopsis... The mid/upper-tropospheric pattern over the CONUS features a positively tilted synoptic trough, extending from far northwestern ON across MN, much of NE, through a col over central CO, to a closed cyclone over central AZ. That portion of this trough located poleward of the col will split eastward rapidly through the period, reaching portions of New England and the coastal Mid-Atlantic by 12Z. Meanwhile the closed cyclone -- though not completely cutoff -- will move eastward far slower, its center reaching east-central NM by 12Z. At the surface, 11Z analysis showed a cold front from Lake Michigan across central/southwestern MO, northeast/central TX, and the TX Big Bend area. This front is forecast to move southeastward by 00Z to OH, middle TN, central MS, southwestern LA, the middle/upper TX Coast, and deep south TX. By 12Z the front should reach central/ southern GA, southern LA, the northwest Gulf just offshore from the mid/upper TX Coast, and slightly southward through deep south TX. In doing so, it will overtake the western segment of a diffuse, stationary to warm frontal zone analyzed initially across southern GA, extreme southern portions of MS/AL, south-central LA, and east TX. ...South TX/Rio Grande Valley to southern MS... Scattered showers and widely scattered to scattered embedded thunderstorms are expected through this evening -- mainly ahead of the front but also along and behind it. Isolated, marginally severe hail/gusts are possible, with the gust threat being along and ahead of the front. The strongest convection should shift offshore along/ahead of the front, and any prefrontal outflow pushes, late this afternoon into this evening. Regional radar mosaics, satellite imagery and lightning data reveal a broad, gradual increase in convective coverage and areal extent the past few hours from Coahuila to the Mississippi River portion of the MS/LA border. This activity is building within a low-level warm-advection plume, with rich boundary-layer moisture theta-e over south TX to southwestern LA. Weak boundary-layer lift in the warm sector appears to be keeping surface-based development subsevere in character, despite the presence of 40-50-kt effective-shear vectors and MLCAPE estimated in the 1000-2000 J/kg range. Abundant, multi- level cloud cover also is apparent in IR imagery. That cloudiness will impede diurnal heating substantially today, with peak MLCAPE not much higher than at present. Still, with large-scale ascent forecast to increase gradually ahead of the slow-moving AZ/NM cyclone, and a favorable parametric juxtaposition of shear, moisture and buoyancy across the region, maintaining a marginal risk appears justified for this outlook cycle. Upscale aggregation/clustering of convection is forecast by some convection-allowing guidance across southeast TX near or just ahead of the cold front; such a process may boost wind potential locally should a sufficiently strong cold pool arise. Otherwise, the gust threat will be brief and with downbursts in the most intense multicells. Hail near severe limits is possible, but its size and frequency will be limited by lack of stronger 1) Deep-layer lapse rates, and 2) Low-level shear for developing and maintaining low-level mesocyclones, though transient supercell characteristics are possible. ..Edwards/Broyles.. 11/27/2020
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