SPC AC 091730

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1230 PM CDT Thu Jul 09 2020

   Valid 101200Z - 111200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE FRIDAY
   AFTERNOON AND EVENING ACROSS PARTS OF SOUTHWESTERN NORTH DAKOTA
   THROUGH MUCH OF CENTRAL SOUTH DAKOTA...

   ...SUMMARY...
   One or two clusters of severe thunderstorms are possible across
   parts of the western Dakotas through the middle Missouri Valley
   region late Friday into Friday night, accompanied by a risk for
   strong wind gusts and large hail.

   ...Synopsis...
   Mid-level subtropical ridging, centered along a major axis from the
   southern California international border vicinity through the
   southern plains Red River Valley, may become increasingly prominent
   through this period.  This may include further building along an
   axis northward through the eastern Great Basin/northern Rockies,
   while ridging also builds within the mid-latitude westerlies to the
   north, into the lee the Canadian Rockies, downstream of a
   significant mid-level low turning eastward toward the British
   Columbia coast.

   In response to the upstream amplification, it appears that one
   fairly significant short wave trough, initially rounding the crest
   of the ridging across the northern Rockies, will dig southeastward 
   across the Dakotas Friday night.

   Farther downstream, generally weak cyclonic mid-level flow is
   expected to persist east of the Mississippi Valley into the western
   Atlantic.  A number of smaller-scale short wave perturbations will
   continue to progress through this regime, including one migrating
   north-northeastward, near/east of the northern Mid Atlantic coast,
   toward southern New England.  Spread among the various model output
   is sizable concerning this feature, and an associated developing
   surface cyclone, which could impact at least portions of the Greater
   New York City Metropolitan area/Long Island into portions of
   southern New England.

   ...Northern Great Plains/Missouri Valley...
   Downstream of the short wave impulse forecast to cross Montana
   during the day Friday, models indicate considerable warming in the
   700-500 mb layer across much of Nebraska and Kansas into the
   middle/lower Missouri Valley.  

   Stronger mid-level height falls may approach the deepening surface
   troughing near the eastern Montana/Dakota border vicinity by the
   21-23Z time frame, preceded by boundary-layer moistening including
   surface dew points increasing into the middle upper 60s in at least
   a narrow corridor.  Beneath steep mid-level lapse rates, it appears
   that the moisture return will contribute to large CAPE on the order
   of 2000-3000+ J/kg, which is expected to support the initiation of
   vigorous thunderstorm development as inhibition weakens.  Aided by
   moderate to strong deep-layer shear, beneath a 30-50 kt westerly jet
   associated with the mid-level impulse, a couple of supercells are
   possible initially.  

   Activity probably will tend to grow upscale fairly quickly, aided by
   large-scale ascent associated with lower/mid tropospheric warm
   advection.  This, coupled with veering of mid-level flow from
   westerly to northwesterly, is expected to contribute to the
   southeastward propagation of an increasingly organized convective
   system across the middle Missouri Valley through Friday evening,
   accompanied by severe wind gusts.

   This activity, and associated convective outflow, may tend to
   progress southeastward toward the lower Missouri Valley overnight,
   while gradually weakening.  However, model output suggests new
   convective development is possible late Friday evening into the
   overnight hours across the lower Missouri Valley, near the nose of a
   nocturnally strengthening central/southern plains low-level jet.  In
   the presence of large CAPE and moderate shear, this activity could
   pose a risk for severe hail and wind.

   ...Northern Mid Atlantic/southern New England...
   In addition to sizable spread evident among the various model
   output, there is little currently evident in guidance to suggest an
   appreciable severe weather risk across or inland of coastal areas
   during this period.  However surface cyclone development will need
   to continue to be monitored, and there probably is at least a
   conditional risk for convection capable of producing a tornado or
   locally damaging wind gusts.

   ..Kerr.. 07/09/2020

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