SPC AC 151723

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1223 PM CDT Sat Aug 15 2020

   Valid 161200Z - 171200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM SOUTHWEST
   MN INTO PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS VICINITY...AS
   WELL AS ACROSS SOUTHERN AZ...

   ...SUMMARY...
   Clusters of strong to isolated severe thunderstorms are possible
   across portions of the central Plains into the southern High Plains
   and northern Texas. Additional strong storms are possible across
   southern Arizona. Locally damaging gusts and hail will be possible
   with the strongest storms.

   ...Synopsis...

   An amplified upper level pattern will exist over the CONUS on
   Sunday. An upper anticyclone centered over southern portions of the
   Great Basin and the attendant ridge extending well north into the
   Canadian Rockies. Meanwhile, an upper trough will be in place from
   the Hudson Bay vicinity south/southeastward through the upper Great
   Lakes to the Southeast. The northern branch of the eastern upper
   trough will pivot eastward across the upper Great Lakes vicinity
   toward NY and the Mid-Atlantic by the end of the period with the
   upper ridge persists across the West. 

   At the surface, a cold front will extend from far eastern Lower MI
   south/southwest into northern AR and arcing northwest toward the
   OK/KS border. A secondary cold front will be shifting southeast
   across the northern Plains during the morning, reaching southwest MN
   and central NE by mid-afternoon. The boundaries will be the main
   focus for strong to isolated severe thunderstorm development across
   portions of the central/southern Plains Sunday afternoon and
   evening. Further west, a seasonally moist environment will exist
   beneath the upper ridge across parts of southern AZ. Strong heating
   will lead to afternoon thunderstorm clusters which could produce
   strong winds. 

   ...Southwest MN into parts of the Central Plains vicinity...

   Thunderstorms are expected to develop along the surface boundary
   draped from southwest MN into southwest NE Sunday afternoon. A
   narrow corridor of low-to-mid 60s F dewpoints will be in place
   beneath modestly steep midlevel lapse rates. This will result in a
   corridor of moderate to strong instability ahead of the boundary.
   Forecast soundings indicated steep low level lapse rates, with LCLs
   near, to just above, 850 mb. As such, isolated convection will be
   rather high-based. Low to midlevel flow will not be particularly
   strong, but directional shear will provide effective shear values
   from 30-40 kt, supporting high-based supercells. Forcing for ascent
   will not be very strong, and only a narrow corridor of favorable
   conditions will exist from southwest MN into southwest NE/northwest
   KS. A few strong gusts and/or hail will be possible, but isolated
   coverage and somewhat short-duration of the threat will preclude
   higher probabilities. 

   ...Southern High Plains into North Texas...

   A surface boundary will sag southward across OK/AR through the
   morning hours and extend from near the Red River, arcing
   northwestward through the TX Panhandle. Resulting east/southeasterly
   low level flow will maintain upper 50s to mid 60s dewpoints across
   the region, and moderate to strong destabilization is expected by
   afternoon. Thunderstorms are expected to first develop over higher
   terrain across NM and CO and shift eastward over the High Plains in
   a weak-shear environment. High-based cells will generally be capable
   of strong downburst winds given very steep, sub-cloud layer lapse
   rates. Given steep midlevel lapse rates and effective shear values
   around 25-35 kt, an occasional hail producing storm is also
   possible. Additional thunderstorm development is expected along the
   instability gradient and surface boundary east/southeastward from
   the TX Panhandle into northern TX. Thunderstorm clusters will mainly
   be capable of wet microbursts. Some guidance suggest a
   westward-propagating cluster could develop via storm mergers/outflow
   interactions. If this occurs, it could increase the threat for
   strong/locally damaging gusts.

   ...Southern AZ...

   High-based thunderstorms are expected to develop during the
   afternoon in a weak to moderately unstable airmass. Steep low level
   lapse rates, with inverted-v low-level thermodynamic profiles
   evident in forecast soundings, suggest strong outflow winds may
   occur with this activity. Hi-res CAMs suggest a westward-propagating
   line of convection could move across the lower desert vicinity
   during the late afternoon. Strong to isolated severe gusts could
   occur with these storms.

   ..Leitman.. 08/15/2020

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