SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0748 AM CDT Tue Oct 27 2020

Valid 271300Z - 281200Z


Severe thunderstorms are not expected across the contiguous United
States through tonight.

The main upper-air feature influencing this period's forecast is a
well defined synoptic-scale cyclone, currently centered over
southern AZ per moisture-channel imagery.  The associated 500-mb low
is forecast to pivot roughly eastward and reach southwestern NM by
00Z, then move to near ROW by 12Z tomorrow.  An extensive fetch of
cyclonic flow aloft will accompany this feature from the
southeastern Great Basin and Colorado Plateau across northern MX,
the southern Rockies, and the southern Great Plains.

At the surface, 11Z analysis showed a wavy, quasistationary frontal
zone across northern FL, extreme southeastern LA, and the TX shelf
waters of the Gulf.  A slow-moving but strong cold front was drawn
from the southern Appalachians across western MS, western LA, the
middle TX coastal waters, and deep south TX, merging with part of
the southern baroclinic zone over the northwestern Gulf.  The
western Gulf part of the combined boundary will move little for most
of the period, while the eastern part remains stationary to
northward drifting.

...Southern Plains...
Large-scale forcing for ascent (elevated low-level warm advection
already in place at first, then DCVA aloft shifting in from the
west), juxtaposed with a favorable corridor of moisture transport,
will spread eastward over the southern Plains through the period. 
These will contribute to multiple episodes of thunderstorms rooted
atop the elevated frontal surface, beginning with the ongoing widely
scattered thunderstorms embedded in a large area of convective
precip offering winter mixed precip across parts of OK and northwest
TX.  Another major precip episode in the form of a coarsely north-
south convective plume is likely late overnight across northwest,
central and north-central TX into OK, also containing sporadic
thunder. By that time, isolated thunderstorms may also begin across
east TX and the Arklatex region, in a moistening regime of marginal
buoyancy between the southern Plains plume and the Gulf activity
discussed below.

...Gulf Coast States, Southeast...
Tropical Storm Zeta (as of this writing) is forecast by NHC to
reinvigorate to hurricane strength over the Gulf north of the
Yucatan Peninsula today, its path curving toward a day-2 landfall
across the southeastern LA area.  Associated wind fields favorable
for potentially tornadic supercells are expected to remain offshore
through this period, but will affect portions of the Gulf Coast east
of its track on Wednesday; see the SPC day-2 convective outlook for
more details.

In the meantime, the boundary-layer response to the approaching
hurricane -- and indirectly, the ejecting Southwestern cold-core
cyclone -- will foster early stages of an inland spread of
increasingly well-modified return-flow air, both at and above the
surface.  This will support at least isolated general thunder
potential over a broad swath of the Gulf Coast States and Southeast,
in addition to antecedent potential across the FL Peninsula. 
Weaknesses of lapse rates, buoyancy and shear will preclude
organized severe.  See NHC advisories for latest track/intensity
guidance and tropical watches/warnings related to Zeta.

..Edwards/Mosier.. 10/27/2020

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