Valid 282000Z – 291200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS EVENING
ACROSS THE LOWER CO RIVER VALLEY…
…SUMMARY…
Isolated severe thunderstorms are possible this evening over the
Lower Colorado River Valley and vicinity.
…20Z Update…
Primary change is to remove severe probabilities along the southern
California coast as frontal band-related convection has moved
inland. Additional low-topped convection/isolated thunderstorms may
develop offshore in the wake of post-frontal stratiform rain/cloud
cover, but severe storms are unlikely. Elsewhere, other aspects of
the outlook remain on track; see prior discussion below.
..Guyer.. 11/28/2019
.PREV DISCUSSION… /ISSUED 1029 AM CST Thu Nov 28 2019/
…Southern CA to AZ through tonight…
A deep midlevel trough will move slowly eastward over
central/southern CA toward AZ by tonight. A focused band of ascent
along the primary baroclinic zone, now crossing the southern CA
coast, will spread eastward to the lower CO River Valley by this
evening, and continue eastward across AZ overnight. In response to
pressure falls across the lower CO River Valley this afternoon,
boundary-layer dewpoints in the low-mid 50s will spread northward
into southwest AZ. Though surface temperatures will remain
relatively cool, this moistening will be sufficient for weak
buoyancy rooted at the surface (MLCAPE of 250-500 J/kg).
Deep-layer vertical shear, characterized by long/curved hodographs,
will increase through the afternoon over the lower CO River Valley
as the midlevel trough moves eastward. The strong band of ascent
along the front will support storm initiation this afternoon/evening
near the CO River, with a possible mix of line segments and some
supercell structures. Weak buoyancy will be a limiting factor, but
isolated damaging gusts, hail, and a brief tornado will be possible
with any sustained supercells or embedded circulations with bowing
segments.
In the short term, there will be a window of opportunity for
low-topped rotating storms to reach the southern CA coast in the
band of ascent along the front, though surface-based buoyancy should
remain confined to the immediate coast.
…TX to AR through tonight…
Elevated convection with low-level warm advection (rooted near 700
mb per 12z FWD sounding) and an embedded midlevel speed max has
overspread southwest AR as of mid morning. This convection could
reach central AR prior to diminishing. A lull in the convection is
expected today across TX, though additional elevated convection will
be possible later tonight from the Edwards Plateau into northwest TX
as warm advection strengthens.
$$