SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0635 AM CST Sat Nov 07 2020

Valid 071300Z - 081200Z


Strong to isolated severe wind gusts may accompany thunderstorms
across parts of eastern Arizona and western New Mexico later this
afternoon and early evening.

In mid/upper levels, a high-amplitude pattern is expected through
this period, anchored by longwave troughing over the western CONUS. 
Within the associated cyclonic flow, a complex synoptic to shortwave
evolution is expected.  A strong synoptic cyclone -- now centered
over northern CA -- is forecast to pivot eastward over the Sierra
today, reaching northern NV, then weakening overnight.  The
associated/basal shortwave trough will split eastward over southern
CA to central AZ by 06Z.  This perturbation then should eject
rapidly northeastward over the Four Corners to central CO by 12Z,
while weakening rapidly.  Meanwhile, a strong trough -- now evident
in moisture-channel imagery over central/southern BC -- will dig
southward across the Pacific Northwest as a closed low.  This
feature will become the main anchoring synoptic cyclone, centered
over southeastern OR by the end of the period, with associated
cyclonic flow covering most of the West and Great Plains. 

Elsewhere, a broad area of negatively tilted troughing, with several
embedded vorticity maxima, will persist from the Arklatex across the
central/southeastern Gulf, influencing both general thunderstorm
potential in parts of the Southeast and the winding path predicted
for what now is T.D. Eta.  Supercell/tornado-favorable wind fields
related to Eta, juxtaposed with favorable convection, are not
forecast to expand into south FL and the Keys until day 2.  Refer to
NHC advisories for latest tropical watch/warning info and track/
intensity forecasts for Eta. 

...Southwest CONUS...
A loosely organized belt of somewhat low-topped convection, with
isolated to widely scattered embedded thunderstorms, is forecast to
develop mid/late afternoon over southeastern AZ, extending northward
over higher altitudes north of the Mogollon Rim as it shifts
eastward over the outlook area.  The main concern will be strong to
isolated severe gusts.

A combination of gradual diurnal/diabatic heating and low-level warm
advection ahead of the basal shortwave trough will contribute to
destabilizing the boundary layer throughout the day.  Moisture
remains rather scant across much of the outlook area; however,
available surface obs to the south in MX, as well as GPS PW data,
indicate at least marginally favorable moisture will advect across
the region as well, ahead of a cold front now located across
southeastern portions of NV/CA.  Relatively maximized low-level
convergence along/ahead of the front, impinging upon the moistening/
destabilizing layer, should compel convective development by mid-
late afternoon.  Pockets of 100-400 J/kg MLCAPE are possible in and
near the outlook area.

Strengthening deep-layer winds -- aligned just to the right of the
frontal zone -- will boost speed/bulk shear steadily through the
day.  Time series of forecast soundings and isotach cross sections
show intense mid/upper winds lowering to just a few km AGL before
frontal passage. This supports the potential for evaporationally
aided downdrafts to convey momentum from those faster-flow areas
through a well-mixed layer to the surface, in a few locales. 

Though minimal, surface-based buoyancy above an inverted-v boundary-
layer thermodynamic profile may develop as far north as the Four
Corners region late this afternoon into evening, in a brief time
window before nocturnal surface cooling at those relatively high
altitudes remove the SBCAPE.  The wind potential there, while
nonzero, still appears too conditional and short-lived to place in a
categorical outlook.  The threat also will diminish with eastward
extent into central NM for similar reasons.

..Edwards/Kerr.. 11/07/2020

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