SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0649 AM CST Fri Nov 13 2020

Valid 131300Z - 141200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
Severe thunderstorms are not expected through tonight.

...Synopsis/Discussion...
In mid/upper levels, the pattern over most of the CONUS will be
nearly zonal through the first half of the period.  Cyclonic-flow
amplification will occur in much of the West after 00Z, shifting
eastward to the High Plains late overnight.  This will occur as a
strong shortwave trough -- now apparent in moisture-channel imagery
just offshore from the Pacific Northwest and southern BC -- moves
inland after about 18Z.  Multiple closely associated vorticity
maxima will contribute to general strengthening toward more of a
synoptic-scale trough over the Rockies by 12Z tomorrow, extending
south-southeastward from a closed 500-mb low over southwestern SK.  

In the meantime, cold air aloft and related steepening of low/
middle-level lapse rates -- above a cool but favorable moist marine
layer -- are expected over coastal WA/OR.  This will contribute to
general, isolated thunder potential until the low-topped convection
leaves behind its supportive low-level air mass inland.  Cooling
aloft, steepening lapse rates and weak moisture also may contribute
to a nonzero potential for lightning over parts of the northern
Great Basin tonight, under the left-exit region of a cyclonically
curved upper speed max digging across OR to central parts of NV/UT. 
However, weak low-level theta-e should limit coverage/duration of
convection, and the potential appears too isolated/uncertain to
maintain an unconditional area. 

As heights fall tonight across much of the West and Plains, and the
center of a large surface anticyclone retreats slowly northeastward
up the Ohio Valley, a broad, strong swath of low-level warm
advection will spread northeastward over the southern Plains to mid
Mississippi Valley.  Accompanying moist advection/transport in the
700-850-mb layer will increase RH under steep midlevel lapse rates.
leading to the development of effectively uninhibited MUCAPE in the
100-500 J/kg range.  Parcels will be isentropically lifted to LFC. 
Forecast soundings suggest the buoyancy may extend into thermal
layers supporting lightning production in the deepest cells.  Lack
of both greater moisture and shear should limit organization/
intensity of this convection.

..Edwards/Leitman.. 11/13/2020

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