SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1158 PM CST Tue Dec 01 2020 Valid 021200Z - 031200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ALONG THE MIDDLE AND UPPER TEXAS COAST...AND COASTAL SOUTHWESTERN LOUISIANA... ...SUMMARY... A couple of severe thunderstorms are possible over the middle/upper Texas Coast and far southwest Louisiana today. ...Synopsis... A large upper cyclone over the northeastern U.S. is forecast to shift northward into Quebec today, as a second evolves/lingers over the southern Rockies/southern Plains. Meanwhile, sharp ridging will remain over the West, though a short-wave trough is expected to approach the West Coast late in the period. At the surface, high pressure will prevail over much of the country, as the main surface low retrogrades across northern Quebec. A much weaker surface low is forecast to shift northeastward along the western Gulf Coast through the period. ...Middle and Upper Texas Coast to southwestern Louisiana... Showers and embedded thunderstorms are expected to evolve early this morning, prior to the start of the period -- largely over the western Gulf, but with elevated convection spreading northward across portions of East Texas by the start of the period. As a weak surface low shifts northeastward near or just offshore during the day, southeasterly/easterly low-level flow should permit some inland advance of a partially modified Gulf boundary layer. Combined with very limited heating, mixed-layer CAPE of 500 J/kg could evolve across portions of the Upper Texas Coast and eventually coastal southwestern Louisiana, with perhaps 1000 J/kg along the Middle Texas Coast. This will support an increase in convection during the late morning and into the afternoon, particularly over the immediate coastal counties. While relatively weak lapse rates aloft will likely limit hail risk, a couple of stronger storms will be capable of producing gusty/possibly damaging winds. Additionally, veering of the low-level flow with height near the low and attendant warm front, beneath strengthening mid-level west-southwesterlies will result in shear profiles supportive of updraft rotation -- including low-level circulations and possibly an inland tornado with one or two sustained storms. Limited severe risk may spread eastward across coastal southern Louisiana into the evening/overnight while gradually diminishing over coastal Texas, though more widespread/intense storms should remain largely offshore through the end of the period. ..Goss/Moore.. 12/02/2020
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