Day 1 Severe Weather Outlook

Valid 221300Z – 231200Z


Thunderstorms are possible across the Southwest.

In mid/upper levels, rather distinct northern and southern streams
will persist over the CONUS, as split flow occurs around a
pronounced cyclone now centered off the southern CA coast. The
associated 500-mb low is forecast to move east-northeastward across
southern CA and northern AZ to the Four Corners area by the end of
the period. By than time a trough should extend southwestward from
the low diagonally across AZ, past the YUM area, and over northern
Baja. Meanwhile, the northern stream will become less zonal as
shortwave troughs amplify toward the WA coastline and over
northeastern ND/northwestern MN.

At the surface, a major continental/polar anticyclone will cover the
southeastern CONUS, with associated ridging to northeastern MX.
This pattern will substantially impede a still-incompletely modified
Gulf boundary layer’s moisture return to land areas east of the

Isolated to widely scattered thunderstorms are possible through the
period across a vast swath from southern CA to the southern Rockies.
Strong thunderstorms with hail and gusts approaching severe limits
are possible this afternoon across parts of western to central/
northern AZ, as the leading rim of the cyclone’s cold-core region
overlies a narrow plume of destabilization related to diabatic
surface heating and warm advection. At this time, severe potential
still appears too isolated and conditional for an outlook upgrade,
though one may be needed in a succeeding update if 12Z and later
models, combined with mesoscale trends, more robustly suggest
buoyancy/shear parameter spaces would verify on the upper end of
what has been progged.

The afternoon destabilization should occur behind a plume of precip
— with isolated, embedded, brief thunderstorms — now spreading
across most of central and southern AZ, and forecast to shift
northeastward in a warm-advection regime, and eastward in phase with
the cyclone’s translation. MLCAPE 200-400 J/kg (locally/briefly
near 500 J/kg) may develop. Time series of forecast soundings
indicate that destabilization of the low/middle-level air mass — in
the narrow corridor following morning clouds/precip — generally
will coincide with reduction in low-level hodograph area, though
deep shear will be favorable. This offset, combined with the
marginal nature of the buoyancy, support the lack of persistent
supercellular convection evident in convection-allowing guidance,
and suggest taking a restrained approach to outlining severe
potential for this cycle.

..Edwards/Broyles.. 02/22/2020


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