SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0745 AM CDT Mon Aug 02 2021

Valid 021300Z - 031200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
CENTRAL/EASTERN OREGON...

...SUMMARY...
Strong to isolated severe downburst winds will be possible over
central Oregon this afternoon and early evening.

...Synopsis...
In mid/upper levels, a large-scale western-ridge/eastern-trough
configuration will persist over the CONUS.  Height falls will occur
across the Southeast as a series of mostly low-amplitude shortwaves
traverse the cyclonic flow aloft, while a strong shortwave trough
crosses, then departs, New England.  West of the mean ridge, a
shortwave trough -- apparent in moisture-channel imagery over
northern CA -- will move slowly northward over OR today, while
weakening.

At the surface, 11Z analysis showed a cold front from near the
southern NC Outer Banks southwestward over north-central GA,
becoming wavy and slow-moving to quasistationary over northern MS,
southern AR, southeastern OK, north-central TX (behind outflows),
and a cold front again across the Big Bend/Trans-Pecos region of
southwest TX.  This boundary will undergo a net southward drift
through the period, but very unevenly in the absence of substantial
forcing aloft.  A weak frontal/wave low may ripple northeastward
across parts of GA/SC tonight in response to shortwave activity in
the area of cyclonic flow and height falls aloft.

...OR...
Isolated to widely scattered, high-based thunderstorms are expected
to form over portions of central OR today as:
1.  Large-scale lift increases somewhat with the approaching
shortwave trough, and
2.  Strong surface heating/mixing remove MLCINH, mixing lower-
elevation dew points from the 50s and 60s into the 30s and 40s F,
while leaving just enough low/middle-level moisture to support the
convection.

Modified RAOBs and forecast soundings suggest the mixed layer may
extend as high as around the 600-mb level, with MUCAPE in the
250-500 J/kg range, atop deep inverted-v thermodynamic profiles. 
Though low/middle-level winds will be modest, with weak deep shear,
isolated downdrafts may accelerate to severe limits.  This regime
also will pose a threat for fire starts from dry lightning; see the
SPC day-1 fire-weather outlook for more details.

...Gulf Coast States to southern Atlantic Coast...
Widely scattered to scattered thunderstorms are expected to develop
episodically throughout the period, with greatest coverage from
mid-afternoon into early evening, following the weakest MLCINH of
the diurnal cycle.  Activity should form in the frontal zone and
southward, including along Gulf Coast and FL sea-breeze boundaries
and convective outflows.  Rich low-level moisture, strong surface
heating, and high tropopause/equilibrium levels will contribute to
areas of 1500-3000 J/kg MLCAPE, despite modest midlevel lapse rates.
 Vertical shear and low-level winds will be weak.  In such an
environment, brief water-loaded downdrafts near severe limits cannot
be ruled out anywhere from south TX to FL and the southern Atlantic
Coast.  Severe potential appears too isolated, conditional and
disorganized for a categorical outlook area at this time.

..Edwards/Leitman.. 08/02/2021

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