SPC AC 191959

   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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