SPC AC 131247

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0647 AM CST Mon Dec 13 2021

   Valid 131300Z - 141200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   A few thunderstorms will be possible across the eastern Florida
   Peninsula, and over parts of the West Coast and northern California.

   ...Synopsis/Discussion...
   In mid/upper levels, currently near-zonal flow east of the Rockies
   will amplify anticyclonically through the period, as heights rise
   ahead of intensifying West Coast synoptic troughing.  A cyclone west
   of Vancouver Island and the WA coastline will pivot southeastward
   toward WA and OR, becoming a strong, open-wave trough near the
   coast, by the end of the period. Farther south, another strong
   shortwave trough will develop and dig southeastward into the base of
   the larger-scale cyclonic-flow field offshore from CA.  That
   perturbation will remain offshore until mid/late day-2.

   At the surface, a frontal zone initially located over south FL and
   the east-central to northwestern Gulf will lose definition gradually
   through the period, while a lee trough becomes better defined just
   east of the central/northern Rockies (under southwest flow aloft). 
   In the meantime, a few weak thunderstorms may move off the Gulf
   Stream to the east coast of FL.  A cold front will move ashore on
   the Pacific Coast today from northwest to southeast, arriving in
   northern CA late this afternoon into evening.

   ...Northern CA...
   Isolated lightning may occur in well-forced frontal-band convection
   this afternoon and evening inland, as well as in both the band and
   post-frontal convection near the coastline.  Strong low-level shear
   will be present over parts of northern CA -- in particular the
   Sacramento Valley and Bay Area -- during midday and afternoon, with
   near-surface flow in the valley backed by orographic forcing.  As
   such, storm-scale cyclonic shear/rotation cannot be ruled out even
   in non-lightning-producing showers.  However, severe potential
   appears too conditional and low to warrant an outlook area at this
   time, given:
   1. Poor midlevel lapse rates in modified forecast soundings inland,
   with coldest air aloft remaining offshore until late overnight and
   day-2, and
   2. Very limited opportunity for substantial diabatic destabilization
   in the spatiotemporally narrow slot between the ongoing precip plume
   and a frontal convective band in the afternoon/early evening.
   These factors will keep buoyancy minimal at best.

   ..Edwards/Leitman.. 12/13/2021

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