Valid 281300Z – 291200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER THE LOWER
COLORADO RIVER VALLEY AND VICINITY…
…SUMMARY…
Isolated severe thunderstorms are possible this evening over the
Lower Colorado River Valley and vicinity.
…Synopsis…
In mid/upper levels, a slowly westward-moving anticyclone over the
Gulf of Mexico will anchor a longwave ridge extending northward
across the upper Great Lakes. Meanwhile, a large cyclone —
initially centered over coastal northwestern CA — should pivot
eastward gradually through the period, with the 500-mb low near RNO
by 12z. Progs indicate a shortwave trough developing today over the
Pacific and in the outer southwestern quadrant of the cyclone. This
perturbation should maximize intensity while moving across northern
Baja tonight.
At the surface, a wavy, quasistationary front was drawn from a low
offshore from Padre Island, across the north-central/east-central
Gulf to the Straits of FL, and northeastward over the Atlantic.
Little movement is expected for this boundary until day 2. A
Pacific cold front — related to the larger-scale cyclone and
approaching shortwave trough — will sharpen and move eastward over
southern CA today, then across western AZ and northwestern Sonora
tonight.
…Lower Colorado River Valley and vicinity…
Isolated to scattered thunderstorms are possible today, forming near
the Colorado River and perhaps in extreme southeastern CA, then
intensifying into AZ. The main concerns are damaging gusts and
isolated severe hail.
The strongest forcing for ascent, and most probable time window for
strong convective development, are expected to occur well after max
diurnal heating. Still, sufficient prefrontal theta-e advection
should occur up the northern Gulf of CA and lower Colorado River
Valley, from a source region of 60s F surface dew points now over
the central/southern Gulf of CA, to support surface-based inflow
parcels. Accordingly modified forecast soundings depict MLCAPE
300-800 J/kg in a narrow preconvective corridor. Vertical shear
will be favorable for supercells, though messier storm modes are
evident in convection-allowing guidance. A strong/50-60-kt LLJ will
contribute to effective SRH in the 200-400 J/kg range, with 50-60-kt
effective-shear magnitude developing amidst intensifying mid/upper
winds. Lack of more robust instability, especially in the boundary
layer, is the main factor keeping the outlook marginal.
…Southern NM, far west TX…
Farther east over southern NM, especially near the Rio Grande
Valley, and southward over the ELP area, some progs (most notably
the RAP) destabilize the boundary layer enough for a surface-based
effective-inflow layer this afternoon. Were that to occur, the
weak-MLCAPE/strong-shear parameter space would favor a marginal
severe threat with any convection that can take advantage. However,
the source for such destabilization is very unclear, with
observations of northerly surface components, temperatures and dew
points in the 30s and 40s, ongoing precip, and the nearest air mass
similar to the RAP’s forecast being located near the Sonoran coast,
hundreds of miles away and across several intervening mountain
ranges. Midlevel moisture is plentiful over this area, and should
become even more so through the evening, and warm advection will
modify the column enough to support elevated thunderstorms.
However, the potential for substantial boundary-layer
destabilization appears too low and conditional to introduce severe
probabilities at this time.
..Edwards/Kerr.. 11/28/2019
$$