SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0159 PM CST Sat Dec 19 2020 Valid 192000Z - 201200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Thunderstorms will continue to shift eastward from the upper Texas Coast into Louisiana this afternoon into tonight. ...20z Update - Middle/Upper Texas Coast... A line of storms along the middle/upper TX Coast to southwest LA will continue to shift east/southeast the remainder of the afternoon. Thunderstorms will likely continue across southern portions of LA into tonight. Ongoing convection has at times shown some weak and transient areas of rotation embedded within the broader line, but has remained sub-severe. Weak instability and a bit of a warm layer in the 850-700 mb layer (contributing to poor lapse rates) is likely limiting stronger/more organized convection. The strongest portion of the ongoing line, near Galveston Bay as of 1945Z will continue to shift east/southeast and offshore the coast within the next 15-30 minutes. Thus, any low-end severe threat has moved offshore and will remain over the offshore waters of the Gulf, and the Marginal risk has been removed with this update. Additional strong cells are possible into parts of southwest LA, but this activity appears to be largely elevated and outpacing low level moisture return, and thus surface-based instability. As such, severe convection is not expected further east along the LA coast. ..Leitman.. 12/19/2020 .PREV DISCUSSION... /ISSUED 1030 AM CST Sat Dec 19 2020/ ...Middle/Upper Texas Coast... A shortwave trough over Texas/Oklahoma will continue to spread generally eastward and reach the Tennessee Valley and middle Gulf Coast late tonight. In advance of a southeastward-moving cold front across northeast/central Texas, multi-layer cloud cover remains prevalent late this morning as middle 60s F surface dewpoints slowly spread inland along the middle/upper Texas coast. Modest destabilization will steadily occur through early/mid-afternoon, but the persistent cloud cover and increasing showers/thunderstorms will probably limit the spatial extent of appreciable pre-frontal destabilization over inland areas. Regardless, an increase in surface-based deep convection is possible along the immediate coast this afternoon. Moderately strong deep-layer/low-level shear would support some supercells, with locally damaging winds and/or a brief tornado possible. It is probable that near/pre-frontal storm mergers across the coastal plain should lead to an offshore shift of a convective band by late afternoon/early evening. Accordingly, the most intense convection should be relegated to the western Gulf of Mexico tonight.
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