SPC AC 281630

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1130 AM CDT Sun Jun 28 2020

   Valid 281630Z - 291200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
   ACROSS EASTERN NY/SOUTHERN NEW ENGLAND AND FROM EASTERN TN INTO THE
   CAROLINAS...AND THIS AFTERNOON THROUGH TONIGHT ACROSS PARTS OF IA/MN
   AND THE NORTHERN HIGH PLAINS...

   ...SUMMARY...
   Scattered severe storms with a couple tornadoes, large hail, and
   damaging winds are possible from western Iowa to southern Minnesota
   during the late afternoon into tonight. Scattered severe storms with
   mainly a hail and wind threat are possible this afternoon and
   evening in the northern High Plains. Scattered damaging winds are
   possible across the southern Appalachians, as well from the Upper
   Delaware Valley across southern New England this afternoon.

   ...Eastern NY to southern New England this afternoon...
   As of midday, scattered thunderstorm development is underway across
   NY, in association with a diffuse speed max rotating around the
   southern periphery of a closed low over QC.  Storms will spread
   east-southeastward through the afternoon, as continued surface
   heating will boosts MLCAPE to near 1500 J/kg.  Effective bulk shear
   of 25-30 kt will support multicell clusters and low-end supercells,
   with attendant threats for damaging winds and marginally severe
   hail. 

   ...Eastern TN to the Carolinas this afternoon...
   A remnant cluster of overnight storms continues to move
   southeastward from KY to TN, while additional convection is
   beginning to develop over the southern Appalachians in association
   with daytime heating.  The combination of MLCAPE up to 2500 J/kg and
   a belt of 35-40 kt flow around 700 mb may support scattered
   strong/damaging gusts with the remnant cluster and the storms
   forming of ahead of it, as the convection spreads into the Carolinas
   this afternoon.

   ...IA/MN this evening into tonight...
   An MCV from overnight convection is moving north-northeastward over
   northwest IA as of late morning.  There will be some potential for
   thunderstorm development with the MCV into southern MN this
   afternoon.  However, the more substantial thunderstorm threat should
   be this evening in the warm advection zone immediately trailing the
   MCV, near the MN/IA border.  As a low-level jet strengthens, warm
   advection on the southwest flank of the MCV (rooted in the richer
   moisture/steeper lapse rates across eastern NE) will increase and
   become supportive of thunderstorm initiation.  Daytime heating and
   boundary-layer dewpoints of 70-72 F, beneath midlevel lapse rates
   greater than 8 C/km, will result in MLCAPE near 4000 J/kg, which
   will support intense updrafts.  A mesoscale region of enhanced
   low-level shear/hodograph curvature and deep-layer shear in that
   zone of warm advection will initially favor supercells capable of
   producing very large hail and a couple of tornadoes.  The threat for
   occasional large hail and damaging winds will persist overnight as
   convection grows upscale into an MCS.

   ...UT/WY to ND through tonight...
   A midlevel low will continue to deepen over the interior Pacific
   Northwest through this evening, with strengthening flow aloft
   expected atop a slow-moving baroclinic zone from northern UT across
   WY to western ND.  Though moisture is somewhat limited across
   northern UT/WY, 12z soundings suggest that surface heating will
   drive steep low-level lapse rates and inverted-v profiles with weak
   surface-based buoyancy this afternoon along the front.  High-based
   thunderstorm development is expected in clusters/bands along the
   front this afternoon, and storms will spread northeastward toward
   the northern High Plains this evening.  A few severe wind gusts will
   be the main threat with the high-based storms from UT into WY
   (greater threat with northeastward extent).  There is some potential
   for the outflow-driven convection to survive as far northeast as the
   lee trough and richer warm sector moisture into SD.  Some storm
   intensification may occur in this scenario, though convection will
   also encounter a strengthen cap with eastward extent later this
   evening into tonight.

   A separate band of storms is expected along the stalled front across
   ND this afternoon/evening.  Low-level ascent along the front, in
   combination with weakening convective inhibition with daytime
   heating, will support scattered thunderstorm development along the
   boundary by roughly 21-23z.  Though vertical shear will not be
   particularly strong (effective bulk shear of 25-30 kt along the
   front), a mix of multicell clusters and supercells will be capable
   of producing isolated very large hail and damaging gusts this
   evening.

   ..Thompson/Wendt.. 06/28/2020

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