SPC AC 170557

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1257 AM CDT Fri Jul 17 2020

   Valid 171200Z - 181200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF SOUTHEASTERN NORTH DAKOTA...

   ...SUMMARY...
   Severe thunderstorms are expected across portions of the northern
   Plains into the Upper Midwest Friday afternoon into early Saturday
   morning. Damaging wind gusts, large hail, and a few tornadoes will
   be possible.

   ...Synopsis...
   Severe-weather potential today will be tied to an active northern
   stream, with multiple impulses rotating around the base of a trough
   centered over the western half of Canada.  The longer-wave trough
   will aid in a gradually strengthening lee trough from the MT/ND
   border region south-southeastward through the eastern Colorado
   Plains.  Weak cyclogenesis should occur within the broader trough
   along the ND/SD border area by afternoon.  Meanwhile, a warm front
   will retreat northward and reach portions of central/northern North
   Dakota and northern Minnesota in the afternoon.  70s F dewpoints
   south of the boundary will retreat northward in tandem with the
   retreating warm front beneath very steep mid-level lapse rates,
   potentially creating a volatile environment for severe storms across
   the eastern Dakotas and western Minnesota.  Mid-level trough will
   also aid in development of a few high-based storms in
   central/southern Montana during the afternoon.

   ...Northern Plains and Upper Midwest...
   A few elevated storms may develop across portions of the discussion
   area in the morning, though these storms are not currently expected
   to have a tangible impact on the severe weather scenario for later
   in the afternoon.

   Through the afternoon, surface heating within the warm, moist
   airmass south of the retreating warm front will result in strong
   surface-based instability and minimal inhibition along and east of
   the surface trough over the western Dakotas.  The glancing influence
   of a shortwave trough traversing southern Saskatchewan/Manitoba will
   aid in development of isolated to scattered thunderstorms.  Large
   buoyancy (upper 60s F dewpoints beneath 8+ C/km mid-level lapse
   rates) and vertically veering kinematic profiles should support
   supercellular storms initially, with hail (perhaps significant) and
   damaging wind gusts being the primary threats.

   Toward evening, most CAMs/model guidance suggests that these initial
   storms will evolve into a substantial linear complex that will
   migrate eastward toward western Minnesota overnight.  Flow both at
   low- and mid-levels will gradually strengthen, with sufficiently
   strong low-level shear for maintenance of any ongoing linear
   complex(es) and an appreciable tornado threat with any isolated
   convection that can develop near the warm front, which should be
   located around northern ND/MN by 00Z or so.  Any linear complex
   should eventually reach the Mississippi River overnight, and a
   weakening trend should commence as storms become gradually elevated,
   though a few damaging wind gusts may still remain possible overnight
   as low 70s F dewpoints keep parcels nearly surface based.

   ...Central and southern Montana...
   Strong insolation will result in steep low-level lapse rates within
   a deeply mixed boundary layer tomorrow afternoon.  Additionally,
   lift/ascent associated with shortwave troughs on the southern
   periphery of the longer-wave western Canada trough should result in
   a few thunderstorms in the afternoon.  Strong flow aloft should
   reach the surface via downward momentum transfer processes near
   thunderstorms, which should pose an isolated and primarily diurnally
   driven wind-damage risk.

   ..Cook.. 07/17/2020

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