SPC AC 041256

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0656 AM CST Mon Jan 04 2021

   Valid 041300Z - 051200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER A SMALL
   PART OF NORTHERN CALIFORNIA...

   ...SUMMARY...
   Isolated strong to damaging gusts and/or a tornado may occur this
   afternoon over a small part of northern California.

   ...Synopsis...
   In mid/upper levels, a progressive and fairly highly amplified
   pattern will persist over the CONUS.  A strong, slightly negatively
   tilted trough will amplify and move southeastward from its initial
   position over the Upper Midwest.  This perturbation should extend
   from Lake Michigan to Georgia by 12Z tomorrow.  As that occurs, a
   compact mid/upper cyclone currently centered over extreme southern
   IL will eject eastward and deamplify, crossing the southeastern
   VA/northeastern NC coast around 06Z.  Ahead of these features, a
   relatively dry and stable continental/polar boundary layer will
   cover the central and eastern CONUS behind a cold front now over the
   Bahamas. 

   Farther west, moisture-channel imagery indicates a cyclone over the
   northeastern Pacific around 52N140Z, with synoptic trough
   southeastward through a vigorous, basal shortwave perturbation
   around 40N132W, west of northern CA.  This shortwave feature is
   progged to amplify somewhat, become somewhat negatively tilted, and
   cross northern CA around 00Z, reaching portions of UT and northern
   AZ overnight.  Lightning has been common with the basal perturbation
   for much of its eastward progress across the north-central Pacific
   over the last 24 hours. 

   ...Northern CA...
   Isolated thunder may occur in a low-level frontal cloud/precip band
   preceding the shortwave trough today, with isolated to widely
   scattered, low-topped thunderstorms spreading/developing inland
   behind that.  Though the time and space window for favorable
   ingredients to juxtapose will be narrow, a mini-supercell or two may
   form in this regime, with the threat for a damaging gust or small
   tornado before activity passes into the higher terrain and weakens.

   The perturbation aloft times well with respect to the potential for
   some diabatic destabilization beneath the strongest large-scale
   forcing for ascent aloft, during mid/late afternoon.  With the
   midlevel height and thermal troughs passing overhead around 00Z, a
   narrow zone of favorable midlevel cooling aloft will immediately
   precede the trough, with 500-mb temperatures decreasing to around
   -25 deg C.  This may occur during a 2-3-hour period before about 00Z
   characterized by: 
   1.  Still-sufficient deep shear for convective organization,
   including orographically forced backing of surface winds just west
   of the northern Sierra foothills, in the northern Sacramento Valley.
   This may support well-curved hodographs with 0-1-km SRH around
   150-200 J/kg.
   2.  Afternoon surface heating possible beyond the near-frontal
   cloud/precip plume, steepening the low-level lapse rates and
   contributing to MLCAPE in the 200-500 J/kg range in forecast
   soundings.

   ..Edwards.. 01/04/2021

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