SPC 1300Z Day 1 Outlook
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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