SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

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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