Valid 281200Z – 291200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE LOWER
CO RIVER VALLEY INTO SOUTHWESTERN AZ…
Isolated severe storms are possible from the lower Colorado River
Valley into southwestern Arizona later this evening and overnight.
Large-scale height falls are forecast to spread across the
southwestern US during the latter half of the period in response to
a strong upper trough that is digging across CA. Latest model
guidance suggests 100kt 500mb speed max will rotate into the base of
the trough before translating across the northern Baja Peninsula by
29/12z. In response to this digging trough, boundary-layer moisture
should advance north across the Gulf of CA/northwestern Mexico into
the lower CO River Valley. While some boundary-layer heating is
expected across the southwestern deserts, cooling profiles
aloft/steepening lapse rates will contribute to destabilization from
southern CA into AZ. Forecast soundings suggest surface-based
parcels become sufficiently buoyant for thunderstorms after 23z.
With strengthening wind fields and strong shear forecast, a few weak
supercells could ultimately develop within the MRGL Risk outlook.
Hail and locally severe wind gusts would be the primary threats.
Downstream across TX/OK…weak elevated convection is expected along
a corridor from northeastern Mexico – Concho Valley – southern OK
due to strengthening isentropic ascent amidst moistening/cooling mid
levels. However, weak instability will preclude severe threat across