SPC AC 261252

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0752 AM CDT Sun Sep 26 2021

   Valid 261300Z - 271200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS SOUTHERN
   PARTS OF ARIZONA AND NEW MEXICO...

   ...SUMMARY...
   Isolated severe hail and/or gusts are possible today across southern
   parts of Arizona and New Mexico.

   ...Synopsis...
   A progressive synoptic-scale pattern will be noted through the
   period, as the eastern trough deamplifies and shifts east, and so
   will the ridging now over the Intermountain West and Rockies. 
   Upstream, a strong cyclone was evident in moisture-channel imagery
   over the northeastern Pacific, centered near 49N142W, with troughing
   extending approximately 1,000 nm southward.  This system will shift
   eastward through the period and remain offshore, while amplifying
   southward, with the parent cyclone west of the BC coastline by 12Z
   tomorrow.  Still, a broad area of associated cyclonic flow and
   height falls aloft will spread inland tonight ahead of the trough,
   and across northern CA and the Pacific Northwest.  As that occurs, a
   longstanding cut-off now -- now over northern Baja -- will eject
   northeastward over southwestern through east-central AZ.

   At the surface, 11Z analysis showed a cold front offshore most of
   the Atlantic Coast except central FL, where it became
   quasistationary and extended westward over the Gulf.  A field of
   Pacific moisture was manifest at the surface over southern AZ, with
   dew points commonly in the mid 50s to low 60s F (with slight
   decrease due to mixing expected today).  PW commonly around 1.25-1.5
   inches was noted at 12Z over southern AZ, based on the TUS sounding,
   limited GPS PW sampling, and objectively analyzed RAP fields.

   ...AZ/NM...
   Convective coverage is expected to increase from midday through the
   afternoon over the region as the boundary layer destabilizes and
   MLCINH becomes negligible.  By midafternoon, widely scattered to
   scattered thunderstorms are expected across much of southern AZ and
   far southwestern NM, offering isolated gusts near severe limits and
   marginally severe hail.  Convection subsequently should spread in a
   fan-shaped pattern (strongly resembling the difluence of mean-wind
   and mid/upper flow vectors) northward and northeastward toward both
   the high country north of the Mogollon Rim, and toward south-central
   NM.  The threat for isolated severe may be maintained as far as near
   the southern NM I-25/Rio Grande corridor before evening weakening
   takes place, and the outlook has been somewhat enlarged eastward
   accordingly.  Intensity and coverage should diminish this evening
   and overnight as activity encounters a more-stable boundary layer,
   both from outflows and nocturnal diabatic cooling.

   Forecast soundings suggest that the preconvective environment will
   be characterized by:
   1.  Somewhat greater low-level moisture content and steeper midlevel
   lapse rates westward contributing to 1000-1500 J/kg peak MLCAPE, but
   weaker low/middle-level flow/shear, and
   2.  Steeper low-level lapse rates with larger DCAPE and stronger
   mid/upper winds eastward.  These factors only overlap diffusely, but
   will contribute to potential for at least brief/localized
   organization with attendant marginally severe potential.

   ..Edwards/Jewell.. 09/26/2021

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