Valid 221200Z – 231200Z
…NO SEVERE THUNDERSTORM AREAS FORECAST…
Isolated thunderstorms are possible across parts of the southwestern
United States. Very small hail is possible from southern California
into the lower Colorado Valley.
…Discussion for the Southwest…
An upper low will move east across southern CA during the day, with
an 80 kt midlevel speed max into central AZ by 00Z. North of the
speed max, cooling aloft and lift will be maximized, with low
pressure and a wind shift crossing the lower CO Valley by 21Z.
Ahead of the upper low, an extensive moist plume will nose into AZ
during the day, producing widespread precipitation, with upslope
translating east along the Rim. By afternoon, some embedded
thunderstorms are possible over central and southern AZ as minimal
instability develops. Deep-layer shear will be strong, and may favor
a strong storm along the Rim where lift will be maximized. However,
models differ on how much instability will be present.
The greatest instability will likely develop north of the midlevel
temperature gradient, and downstream of a pocket of steep low-level
lapse rates which will develop over southern CA and into the lower
CO Valley. The result is likely to be a few strong storms over far
southern NV and into northwest AZ. Here, small hail appears likely
with any cellular activity given cold temperatures aloft, though
severe hail is not anticipated. A few strong wind gusts are also
Elsewhere, westerly winds into the coastal range of southern CA
beneath cold temperatures aloft may allow for small cells capable of
graupel/very small hail after 18Z.