SPC AC 211256

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0756 AM CDT Sun Jul 21 2024

   Valid 211300Z - 221200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   CENTRAL AND SOUTHERN ARIZONA...AND FROM SOUTHERN COLORADO TO EASTERN
   NEW MEXICO...

   ...SUMMARY...
   Isolated, marginally severe thunderstorms are possible this
   afternoon into early evening across parts of central and southern
   Arizona, and from southern Colorado to eastern New Mexico.

   ...Synopsis...
   The mid/upper-level pattern will remain rather blocky over and near
   the CONUS through the period, with longwave ridging remaining over
   the West, and a persistent, positively tilted longwave trough from
   QC across the mid Mississippi Valley to central/south TX.  A series
   of mostly small shortwaves will traverse the modest northerly flow
   aloft, in between those features, over the Plains States and
   central/southern Rockies.  A long-lived anticyclone -- initially
   centered over the lower Colorado River Valley -- will continue to
   retrograde, drifting northwestward over parts of NV while expanding
   somewhat.

   Farther northwest, a strong shortwave trough was apparent in
   moisture-channel imagery over and offshore from the coastal OR/CA
   border.  This feature should accelerate northward through the period
   and weaken, being absorbed into the eastern semicircle of a massive,
   slow-moving cyclone centered over the Gulf of AK.  The trough will
   pass along and just off the OR/WA coastline today, and over
   Vancouver Island to coastal west-central BC by 00Z.

   At the surface, 11Z analysis showed a weak, muddled surface pattern
   north of an increasingly diffuse, wavy frontal zone that extended
   from the Hampton Roads region across the Tennessee Valley region to
   parts of southern OK and north TX.  That front should continue to
   demarcate richest Gulf moisture from a lesser-modified, more
   continental air mass to the north, and should remain
   quasistationary.

   ...South-central Rockies and adjoining High Plains...
   Widely scattered to scattered thunderstorms will form from midday
   through the afternoon and move generally southward, with local
   deviance to the southeast or southwest as boundary and outflow-
   driven processes force.  Weak low/middle-level winds will limit deep
   shear, with multicells the dominant storm mode.  Due to the lack of
   greater buoyancy, and high-elevation nature of the surface limiting
   boundary-layer depth in many instances, much of this convection
   should remain relatively shallow and struggle to organize.  However,
   the most vigorous cells, including those that move over valleys and
   lower Great Plains elevations with deeper subcloud layers and
   greater boundary-layer moisture content, will pose a threat for
   isolated severe gusts and marginal hail.  Some cold-pool aggregation
   also may occur, especially over Plains areas of east-central/
   northeastern NM and extreme southeastern CO.  300-800 J/kg MLCAPE
   should be common across this region in the preconvective air mass. 
   The overall strong/severe thunderstorm-wind potential should
   diminish through the evening as the foregoing boundary layer
   stabilizes diabatically, and stable-outflow coverage increases.

   ...AZ...
   Scattered thunderstorms should develop today, primarily as strong
   insolation preferentially erodes MLCINH amidst at least marginal
   moisture, over the Mogollon Rim and adjoining higher elevations of
   eastern/central/northern AZ.  With the retrogression of the mid/
   upper ridge, the mid/upper flow will be largely northerly, and
   through a fairly deep layer (roughly 700-300 mb, sometimes higher). 
   Activity therefore should move southward to southwestward across
   deeper mixed layers of the lower desert elevations, with at least
   isolated severe downdrafts possible.  Peak MLCAPE may reach the
   500-1000 J/kg range in some areas, and should be at least 250 J/kg
   over most of the outlook, with DCAPE commonly over 1000 J/kg. 
   Boundary mergers, orographic effects and storm-scale outflow
   aggregations may focus the severe threat locally within this broader
   "marginal risk" area.  Convection may go as far as the borderlands
   before weakening this evening -- hence the minor extension of the
   outlook area.

   ...Interior Northwest...
   Widely scattered to scattered, high-based thunderstorms are expected
   to develop this afternoon over much of inland OR, northeastern CA
   and northern NV, amid peripheral large-scale lift from the Pacific
   trough and strong surface diurnal heating.  This activity may
   produce locally strong, often dry downbursts -- indeed, scattered
   dry thunderstorms are forecast in the SPC fire-weather outlook. 
   Large dewpoint depressions (40-70 deg F) should result after a few
   hours of diurnal mixing, causing a drop in buoyancy, but with enough
   (200-600 J/kg MLCAPE) to support thunderstorms with subcloud/
   boundary-layer depths reaching into the 500-600 mb layer.  Lack of
   greater low/middle-level flow and buoyancy both preclude an
   unconditional severe outlook, but a downdrafts approaching severe
   limits cannot be ruled out.

   ..Edwards/Broyles.. 07/21/2024

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