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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