SPC AC 130553

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1253 AM CDT Mon Jul 13 2020

   Valid 141200Z - 151200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
   THE MID-MO AND UPPER MS VALLEY...

   ...SUMMARY...
   Strong to severe storms are possible on Tuesday from the central and
   southern High Plains vicinity to the upper Mississippi Valley.

   A broad upper trough over the north-central U.S. will pivot eastward
   toward the upper Great Lakes on Tuesday. Several shortwave impulses
   will rotate through the larger-scale trough, providing a belt of
   strong southwesterly flow across portions of the central Plains to
   the Great Lakes, and aiding in focused upper forcing for ascent. The
   main surface low attendant to the upper trough will be located over
   northern Manitoba Tuesday morning, with a surface trough extending
   southward into the upper MS Valley southwestward toward a lee low
   over the southern High Plains. A cold front will drop southward
   across the central Plains and eastward across the Upper Midwest
   during the period. Thunderstorms will likely be ongoing Tuesday
   morning across portions of the Upper Midwest vicinity as one
   shortwave impulse ejects northeast. Additional development is
   expected near the front during the afternoon near far eastern NE
   into IA/MN/WI. Isolated to scattered storms also are expected over
   the higher terrain of CO/NM and could pose a marginal severe threat
   during the evening as they move eastward across western portions of
   the central/southern Plains, possibly interacting with the
   southward-advancing cold front.

   ...Mid-MO Valley to Upper MS Valley vicinity...

   Convection will likely be ongoing across parts of MN/WI and perhaps
   IA as a decaying MCS shifts eastward across the region early in the
   period. This could have some impact on destabilization during the
   afternoon, but most guidance suggests the air mass should remain
   intact, or recover enough for redevelopment during the
   afternoon/evening along the cold front from southern MN into western
   IA. Southerly low level flow will maintain moist advection across
   the region, and coupled with evapotranspiration influences,
   dewpoints in the upper 60s to near 70 F appear likely. A plume of
   steeper midlevel lapse rates is expected to spread across IA toward
   southeast MN and far southwest WI during the afternoon, further
   aiding in development of strong instability. Warm temperatures in
   the 850-700 mb pose some capping concern, but strong frontal forcing
   should be sufficient to overcome whatever modest capping remains by
   late afternoon. 

   Storm mode is also somewhat uncertain. Supercell wind profiles are
   forecast across the region, with backed low level winds resulting in
   enlarged, curved low level hodographs. However, upward development
   into clusters or bowing segments could occur rather quickly as the
   cold front surges east/southeast. If discrete convection can be
   maintained initially, supercells capable of all severe hazards will
   be possible. As a southwesterly low level jet increases during the
   evening, upscale development into an eastward propagating MCS is
   expected. 

   ...Central/Southern High Plains vicinity...

   Thunderstorms will develop over the higher terrain of CO into
   northeast NM during the afternoon and spread eastward over the
   adjacent Plains by evening. Coverage of convection remains somewhat
   in question given the very warm and deeply-mixed boundary layer that
   will be in place. There is also uncertainty related to timing of the
   southward-surging cold front and what impact that may have on both
   storm development and maintenance. Most guidance does develop at
   least isolated, high-based cells capable mainly of strong downburst
   winds. A weakness in midlevel winds is evident in forecast
   soundings, however vertically veering wind profiles and increasing
   southwesterly flow above 700 mb will result in supercell wind
   profiles with 40+ kt effective shear amid very steep midlevel lapse
   rates. As a result, a few organized cells could also produce large
   hail. Given a large degree of uncertainty across the area, a
   Marginal risk will be maintained, but upgrades may be needed in
   subsequent outlooks.

   ..Leitman.. 07/13/2020

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