Valid 200100Z – 201200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS EVENING
ACROSS PARTS OF SOUTHERN ARIZONA…
…SUMMARY…
Strong storms could still impact portions of southern Arizona
through mid to late evening, and perhaps pose at least some risk for
severe hail and locally strong wind gusts.
…01Z Outlook Update…
…Southwest…
Observational data suggests that strongest storms have remained well
south of the international border, across parts of eastern Sonora
into western Chihuahua. However, a narrow corridor of moderate
boundary layer CAPE has developed as far north as the south central
Arizona international border vicinity, where deep layer shear is
strong beneath the exit region of a 70 kt south-southwesterly 500
mb jet streak now beginning to nose east of the Baja Spur. As the
jet streak and increasingly deformed associated upper impulse
accelerate northeastward across Sonora this evening, there remains
at least some possibility for isolated supercell development which
could pose a risk for severe hail and wind.
Otherwise, increasing thunderstorm development remains possible
later this evening into the overnight hours, within an evolving
deformation zone across the lower Colorado Valley. However, a
combination of generally weak shear, modest lapse rates and weak
CAPE seem likely to limit the risk for severe weather.
Farther north, destabilization and forcing for ascent associated
with an initial digging short wave impulse has supported scattered
weak thunderstorms north and northeast of the Tahoe Valley and Reno
vicinity. It appears that this activity could continue into the
02-04Z time frame, before perhaps gradually redeveloping to the west
and south of Tahoe Valley later this evening and overnight, as a
much stronger perturbation (initially including a 100 kt 500 mb
jet) digs southward along the central into southern California
coast. Within the exit region of this jet, mid-level cooling and
forcing for ascent could support convection capable of producing
lightning near coastal areas south of San Francisco Bay into the
Point Conception vicinity by late tonight.
..Kerr.. 11/20/2019
$$