SPC AC 211615

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1115 AM CDT Sun Oct 21 2018

   Valid 211630Z - 221200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON ACROSS PARTS OF CENTRAL AND EASTERN ARIZONA...

   ...SUMMARY...
   A couple of strong storms may impact portions of central and
   southern Arizona this afternoon, accompanied by some risk for severe
   hail and locally strong wind gusts.

   ...Synopsis...
   Within the main branch of westerlies emanating from the mid-latitude
   Pacific, amplified large-scale ridging appears likely to rebuild
   across western Canada and the Pacific Northwest/northern
   intermountain region/Rockies.  Downstream troughing east of the
   Plains will persist, but the cyclonic regime is expected to become
   less amplified, as a strong embedded short wave trough continues
   accelerating east-northeast of the northern Mid Atlantic/southern
   New England coast.

   Models indicate that a significant cold front associated with the
   short wave will advance southward through the remainder of the
   Florida Peninsula/Keys by 12Z Monday.  However, the southwestern
   flank of the frontal zone is forecast to stall and return northward
   across the western Gulf of Mexico, and possibly into parts of Deep
   South Texas.  This is expected as broad upper troughing, and an
   embedded mid-level cyclonic circulation, gradually progress inland
   of the California coast, within a belt of westerlies emanating from
   the southern mid-latitude/subtropical eastern Pacific.

   ...Great Basin/Southwest...
   Moisture return on southerly low-level flow has contributed to
   sufficient destabilization for scattered ongoing thunderstorm
   activity across parts of southern Nevada/northwest Arizona and
   southwestern Utah.  This has been supported by forcing for ascent
   associated with one short wave impulse embedded within the
   large-scale troughing, now progressing northward into the Great
   Basin.

   In its wake, another perturbation appears in the process of
   progressing northeastward toward the lower Colorado Valley.  Models
   suggest that this could be accompanied by initiation of scattered
   thunderstorm activity across parts of central/southern Arizona as
   early as mid afternoon.  This is where/when higher low/mid-level
   moisture content (characterized by precipitable water up to around 1
   inch) may contribute to CAPE up to around 1000 J/kg, beneath at
   least modestly steep mid-level lapse rates.

   Lower/mid-tropospheric wind fields appear likely to remain rather
   modest to weak in strength.  However, perhaps aided by 50+ kt
   southwesterly flow in the 300-250 mb layer, shear through the depth
   of the convective layer will be strong, and may contribute to
   potential for sustained/organized storms accompanied by at least
   some risk for severe hail and locally strong surface gusts through
   early evening.

   ..Kerr/Karstens.. 10/21/2018

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