SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0745 AM CDT Wed Oct 28 2020

Valid 281300Z - 291200Z


Hurricane Zeta may spawn a few tornadoes later today through tonight
over parts of the central Gulf Coast States.

In mid/upper levels, a strong cyclone is forecast to eject
east-northeastward from its current central position over
southeastern NM, with the 500-mb low over the CDS area by 00Z, and
reaching southeastern OK by 12Z.

At the surface, 11Z analysis showed a weak low near DUJ with arctic
cold front southwestward across eastern portions of KY/TN, coming
quasistationary over northeastern or central MS.  The boundary then
curved across southwestern LA and the waters offshore from BRO.  As
the mid/upper cyclone proceeds across the southern Plains, and
shortly behind the passage of Zeta across the central/north-central
Gulf, the boundary will move eastward across the northwestern Gulf
as a cold front.  Portions well inland over MS/TN may move slightly
northwestward before Zeta passes those latitudes.

...Southeastern LA to western GA and FL Panhandle...
The NHC forecast depicts Hurricane Zeta's center moving ashore
southeastern LA late this afternoon.  That will be preceded from
midday through afternoon by increasingly favorable low-level winds
near the coast, enlarging hodographs and SRH values.  The favorable
kinematic field will spread northeastward across the outlook area
this evening and tonight as Zeta rapidly translates across southern
MS and AL to northwest GA.  The circulation will become embedded in
a regime of increasing ambient deep shear, ahead of the ejecting
southern Plains cold-core cyclone.  The main concerns for the
tornado threat instead are related to instability and convective

Convection not part of Zeta's circulation envelope has been noted
during the past several hours, moving north-northwestward off the
Gulf to between GPT-MOB and over portions of southeastern MS.  A few
cells therein exhibited horizontal cyclonic shear despite 1) the
lack of substantial apparent surface boundaries, and 2) the modest
to weak SRH apparent in 06Z RAOBs and VAD wind profiles for observed
cell motions.  Ambient hodographs have increased slightly since,
based on the 12Z LIX RAOB, and as they continue to grow today, any
such convection remaining in the area later this morning, or forming
over the Gulf in a weak-CINH setting as broader low-level
convergence increases ahead of Zeta, may pose a marginal tornado
threat before Zeta's directly related outer bands begin to affect
the coastal areas this afternoon.

A favorably high-theta-e air mass, with low/mid-70s F surface dew
points, has advected inland across parts of the central Gulf Coast
and lowest Delta regions, gradually diffusing an old frontal zone 
that had been in place for a few days, and effectively displacing it
inland in the direction of the arctic front described above.  That
process will remain incomplete, however, and both surface theta-e
and SBCAPE will continue to diminish northward through AL/GA from
the optimal CAPE/shear parameter space forecast to affect the
5%/"Slight" area.  With its expected acceleration and fast forward
speed, Zeta should outrun the greatest buoyancy inland tonight.  As
that occurs, the tornado threat will ramp down gradually, but still
be nonzero well inland, across the "Marginal" area, where a tornado
cannot be ruled out in what still will be a large-hodograph

..Edwards/Mosier.. 10/28/2020

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