SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0642 AM CST Sun Nov 08 2020

Valid 081300Z - 091200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER THE
SOUTHERN FLORIDA PENINSULA AND KEYS...

...SUMMARY...
A tornado threat may develop from late this afternoon through
tonight across south Florida and the Keys, as Tropical Storm Eta
approaches.

...Synopsis...
A high-amplitude mid/upper-level pattern will exist over the CONUS
through the period, anchored by a big, complex, cold-core cyclone
over the West, and an anticyclone moving slowly east-southeastward
over/off the Mid-Atlantic region.  To the south and southwest of the
anticyclone, an elongated, quasistationary mid/upper trough and
cyclonic-shear zone is evident in moisture-channel imagery from the
Arklatex across the central Gulf to near Tropical Storm Eta.  The
Gulf/Arklatex part of this perturbation will weaken through the
period, as the eastern part merges with Eta.  The resultant combined
cyclone will become the dominant vorticity component of the trough,
effectively forming the southern, cut-off member of a Rex-like
pattern configuration, in tandem with the synoptic anticyclone to
the north-northeast. 

At the surface, 11Z analysis showed a pronounced cyclone over the
SD/MT/WY border area, with cold front south-southwestward across
east-central CO, central NM, and northwestern MX.  The cyclone
should move northeastward across Lake Manitoba late tonight, as the
front proceeds eastward across the High Plains and portions of the
Upper Midwest.  By 12Z, the front should extend near a line from
INL-ONL-GLD-TCC-ELP, generally moving slower with southward extent. 
A dryline now developing over western portions of NE/KS/TX will
sharpen gradually, but also, will be overtaken in the central High
Plains by that cold front.  General thunderstorms are possible late
in the period near the front, in the outlined area, but with
inadequate moisture/instability for an organized severe threat. 

...Eta: South FL, Keys...
Increasingly influenced by (and part of) the aforementioned mid/
upper perturbation, T.S. Eta is forecast by NHC to exit central Cuba
later this morning and turn northwestward to westward over the
Straits and Keys of FL through the period.  As it does so, this
cyclone is forecast to spread a strengthening low-level wind field
(in space and time) across south FL and the Keys, with enlarging
low-level hodographs.  Under current NHC progs, the most-favored
sector of the vortex, north through northeast of center, will shift
from east-west across the outlook area this evening and overnight,
apace with the westward component of Eta's translation vector. 
Hodographs will enlarge and become more-favorably oriented during
that process.  Still, concerns about low-level instability and
convective character -- especially with a good deal of dry air aloft
being entrained into the blended system per moisture-channel imagery
-- preclude more than a marginal unconditional probability of
tornadoes at this juncture. 

Refer to the latest NHC advisories and maps for tropical-related
watches/warnings, as well as track/intensity forecasts for Eta.

..Edwards/Kerr.. 11/08/2020

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