SPC 1630Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1121 AM CDT Wed Apr 14 2021

Valid 141630Z - 151200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON AND
EVENING ALONG THE COASTAL PLAIN OF SOUTHWEST LA AND THE MIDDLE-UPPER
TX COAST...

...SUMMARY...
Large hail and isolated damaging winds will be possible this
afternoon/evening across the coastal plain of southwest Louisiana
and the middle-upper Texas coast.

...Northwest Gulf coast through early tonight...
Widespread elevated convection has been ongoing over LA through the
morning, to the northeast of a rain-reinforced outflow/effective
front just inland from the LA coast.  The convection is linked to
low-level warm advection and a subtle midlevel perturbation now
approaching the Sabine River.  The 12z LCH sounding revealed MUCAPE
greater than 2500 J/kg, though buoyancy decreases with northeastward
extent away from the boundary, and as a result of substantial
convective overturning.  The greater threat for a few severe storms
should be focused along the surface boundary (near the
southwest/south central LA coast), where another cluster of storms
is expected, which will likely keep the boundary near the coast. 
Occasional large hail and/or damaging gusts will be possible with
the stronger embedded storms/supercells through the day.

Farther west along the TX coastal plain, 12z soundings reveal that
low-level moisture depth has increased and cap strength has
decreased over the past 12-24 hours.  Lapse rates above the 700 mb
level remain quite steep (> 8 C/km), and boundary-layer dewpoints of
72-74 F are driving MLCAPE of 2500-3000 J/kg.  Given the weakened
cap and the approaching of another subtle midlevel perturbation (now
south of the Big Bend), the chance for surface-based storms has
increased along a slow-moving front this afternoon/evening. 
Assuming a few storms form along the middle and upper TX coastal
plain, the large CAPE and effective bulk shear of 30-40 kt will
support a mix of organized clusters and supercells capable of
producing large hail and damaging winds.

...Southern VA/northern NC late this afternoon/evening...
A subtle shortwave trough now over western KY/TN will progress
eastward toward VA/NC by this evening, around the southeastern
periphery of the midlevel low over the upper Great Lakes.  A weak
surface cyclone is expected to form across the Piedmont in advance
of the midlevel trough, and then move eastward through this evening.
Modest low-level moisture return across NC into southern VA
(boundary-layer dewpoints now in the mid 50s) and afternoon
temperatures of 75-80 F will contribute to some destabilization this
afternoon.  However, there is uncertainty regarding how much
vertical mixing will offset moisture advection and limit buoyancy. 
Even with just a few hundred J/kg of MLCAPE, long hodographs with
some low-level curvature will support a threat for somewhat
high-based/low-topped supercells capable of producing a few damaging
gusts and marginally severe hail for a few hours late this
afternoon/evening.

..Thompson/Kerr.. 04/14/2021

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