SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0636 AM CST Sat Jan 09 2021

Valid 091300Z - 101200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
Isolated thunder may occur this afternoon and evening over parts of
central/south Texas and New Mexico, and tonight over coastal areas
of northwestern California and southwestern Oregon.

...Synopsis/Discussion...
A complex, progressive, somewhat split-flow pattern will prevail in
mid/upper levels.  Continental/polar air in the boundary layer, left
behind the last substantial cold-frontal passage, will render the
low-level air mass by itself unsuitable for supporting thunderstorms
over the CONUS from the Rockies eastward.  However, isolated thunder
-- from convection rooted well above the surface -- may develop over
parts of the southern Rockies region (perhaps associated with winter
precip) and parts of central/south TX, this evening and overnight. 
Isolated thunder also is possible late tonight over parts of the OR
and nearby CA coastlines.

The main two upper-air features relevant to convective potential
will be:

1.  A strong shortwave trough now apparent in moisture-channel
imagery over the Great basin.  This feature will dig southeastward
across the Four Corners region today, evolving a closed (or very
nearly so) 500-mb low this evening and overnight over NM.  By 12Z,
the trough should extend from the central High Plains across far
west TX to north-central MX, with primary vorticity lobe likely
positioned between FST-ELP.  A preceding swath of strong large-scale
DCVA/lift in midlevels will cross NM overnight, beneath strongly
difluent mid/upper flow, steepening 700-500-mb lapse rate quickly to
near 8 deg C/km.  Where saturation occurs near the base of that
layer, CAPE may develop, straddling the -20 deg C isotherm. 
Forecast soundings indicate MUCAPE will remain less than 100 J/kg,
and any resulting thunder should be very sporadic.  This outlook
area appears the most marginal and uncertain of the two associated
with this perturbation, in terms of lightning-coverage potential,
due to a lack of more-robust theta-e beneath the zone of intense
midlevel cooling.

Meanwhile, a very immaturely modified, yet still marginally moist,
return-flow air mass will spread over the relatively stable boundary
layer across parts of southwest through central/southeast TX
tonight.  Lapse rates steepened by warm advection and moistened at
the base of the buoyant layer will yield 100-400 J/kg MUCAPE, rooted
largely in the 600-750-mb layer.  Buoyancy sill be activated by
parcels isentropically lifted to LFC, within a large area of precip
(some of it becoming snow in northwestern areas) and embedded
convection.

2.  A weaker shortwave trough that now forms the basal perturbation
of a synoptic cyclone over the northeastern Pacific, south of AK. 
The basal trough -- currently located west of CA between 143W-146W
-- is progged to split eastward from what is now the coldest, most
lightning-active part of the cyclone.  This southern lobe then
should penetrate the mean larger-scale ridge as it reaches the
northwestern CA/southwestern OR coastline around 09Z.  In the few
hours before this trough reaches the coast, sufficient midlevel
cooling may occur to foster MUCAPE up to about 200 J/kg, including
some icing layers potentially supporting lightning generation.  Any
thunder threat will be isolated, decreasing quickly with inland
extent.

..Edwards.. 01/09/2021

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