SPC AC 011257

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0757 AM CDT Tue Sep 01 2020

   Valid 011300Z - 021200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
   SOUTHERN PLAINS TO LAKE ERIE AND VICINITY...

   ...SUMMARY...
   At least isolated, marginally severe thunderstorms are possible
   today from the southern Plains to Lake Erie and vicinity.

   ...Synopsis...
   In mid/upper levels, a strong synoptic cyclone will cover much of
   mainland Nunavut and Hudson Bay in Canada.  To its south, broadly
   cyclonic flow will cover much of the north-central CONUS and Great
   Lakes, with several embedded shortwaves.  One such perturbation --
   evident in moisture-channel imagery over central/southern SK -- is
   forecast to dig southeastward over ND and northwestern MN by around
   00Z.  The perturbation then should pivot eastward across WI, upper
   MI and Lake Superior by 12Z tomorrow.  Farther south, a breakaway
   trough will maintain positive tilt, as it moves from current
   position over southern WY and eastern UT southeastward to west TX
   and southeastern NM.  Downstream, an MCV was moving from
   northeastern OK into the Ozarks of southwestern MO and northwestern
   AR, and should reach the lower OH Valley by mid/late afternoon.

   At the surface, the 11Z analysis showed a cold to quasistationary
   front from a low over southwestern Lower MI southwestward across
   western IL, central MO, southeastern KS, northwestern OK, and
   east-central NM.  The front should move little through the period,
   with the southern Plains and MO portions remaining substantially
   behind several areas of convective outflow.  A separate cold front
   -- related to the mid/upper-level northern-stream trough over SK --
   will move eastward across the northern Plains and Upper Midwest.

   ...Southern Plains to Mid-South...
   Scattered to locally numerous thunderstorms in clusters are expected
   to continue, and to develop anew through tonight, southeast of the
   front from the TX Panhandle and north TX to the Mid-South.  Isolated
   to widely scattered convection should occur mainly this afternoon
   into early evening in a strongly heated, favorably moist air mass
   over parts of west-central/southwest TX and across the Rio Grande
   into higher terrain of northern Coahuila.  Isolated severe gusts/
   hail are possible, with hail potential mainly over the Panhandle to
   Rio Grande corridor in TX where midlevel lapse rates will be
   greatest, along with increasing large-scale forcing for ascent ahead
   of the southern Rockies shortwave trough.

   Moisture will be rich over much of the region, with low/mid 70s F
   surface dew points quite prevalent except over the post-frontal High
   Plains area.  Lift will arise from a combination of the frontal
   zone, prefrontal outflow/differential-heating boundaries, and
   upslope flow and elevated low-level warm advection behind the front.
   Forecast soundings suggest peak MLCAPE away from ongoing convection
   and outflow pools could reach 3000-4000 J/kg over the northwest TX
   to Rio Grande prefrontal corridor, 1000-1500 J/kg behind the front
   where low-level lapse rates will be weaker, and  and 1500-2500 J/kg
   from northeast TX across the Mid-South where midlevel lapse rates
   will be weaker.  Deep shear generally will be modest, within the
   spectrum supporting multicells.  However, some parts of the Mid-
   South may have more-favorable deep shear under flow enhancements
   aloft compelled by the MCV, with at least transient,
   messy/heavy-precipitation supercells possible.

   Localized concentrations of severe-gust potential may develop this
   evening and overnight, where convective clusters can merge enough to
   foster upscale growth and sustained cold-pool organization.  Any
   such processes will be strongly tied to mesobeta- and smaller-scale
   boundary and convective processes that remain too uncertain to etch
   a specific 15% unconditional line into the outlook at this time.

   ...Ohio Valley to Lake Erie and vicinity...
   Isolated to scattered thunderstorms are possible along/east of the
   front, mainly this afternoon into early evening.  The best-organized
   of these may pose a damaging-wind hazard.  A tornado cannot be ruled
   out.

   Deep shear should be stronger over this region than the southern
   Plains prefrontal sector, mainly due to closer proximity to the
   northern-stream flow belt aloft and approaching shortwave trough,
   but also with some influence of the approaching MCV late this
   afternoon and evening over the lower Ohio Valley.  The main limiting
   factor will be lack of robust instability, related both to modest
   mid/upper-level lapse rates and some impediments to boundary-layer
   heating imposed by cloud cover spreading over some of the area. 
   Surface dew points in the warm sector should range from the low 60s
   north to the low 70s south, supporting MLCAPE in the 500-1500 J/kg
   range, with 30-40-kt effective-shear magnitudes (locally higher).

   ..Edwards/Kerr.. 09/01/2020

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