SPC AC 031725

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1125 AM CST Sun Jan 03 2021

   Valid 041200Z - 051200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   Scattered thunderstorm activity is possible along Pacific Northwest
   coastal areas, mainly late Monday afternoon, with lightning also
   possible along the northern Sierra Nevada.

   ...Discussion...
   Much of North America will remain under the influence of branching
   westerlies, downstream of a persistent strong zonal jet across the
   southern mid-latitude and subtropical Pacific, which may nose as far
   east as 140-145 W latitude during this period.  Stronger surface
   cyclogenesis associated with this feature will remain focused across
   the northeastern Pacific, but an emerging mid-level perturbation and
   associated occluding surface front are forecast to progress inland
   of the U.S. Pacific coast Monday through Monday night.  The front
   appears to be preceded by a plume of moisture emanating from the
   subtropical latitudes, and models suggest that moisture influx will
   become maximized across the northern California coast into the
   northern Sierra Nevada during the day Monday, before becoming
   cut-off by the inland advancing cold front.

   Downstream, it appears that mid-level troughing may undergo some
   further amplification east of the Mississippi valley into the
   western Atlantic.  This will include a number of embedded short wave
   perturbations, including one advancing east of the New England
   coast, where it will provide support for further deepening of an
   offshore surface cyclone.  As this occurs, the trailing surface cold
   front, and narrow plume of moisture return emanating from the
   southern Gulf of Mexico, are expected to shift south of the Florida
   Peninsula and Keys, with considerable further mid/low-level drying
   across much of the Gulf of Mexico.  This will contribute to the
   maintenance of generally dry and/or stable conditions across most
   areas east of the Rockies.

   ...Pacific Coast states...
   Models suggest that the development of thermodynamic profiles most
   conducive to convection capable of producing lightning will be late
   Monday afternoon, around 05/00Z (plus or minus an hour or two),
   along the Oregon into Washington coast.  It appears that this will
   take place within the post-frontal regime, and coincident with the
   inland progression of the mid-level thermal trough axis (with temps
   at or below -30C at 500 mb).

   Otherwise, models suggest that mid-level cooling may steepen lapse
   rates sufficiently to support convection capable of producing
   lightning within the moist pre-frontal regime across northern
   California coastal areas Monday afternoon.  Convection capable of
   producing lightning may also develop into the northern Sierra
   Nevada, aided by intensifying pre-frontal lift supported by
   low/mid-level warm advection and orography, which may become focused
   near or to the west of the Tahoe Valley vicinity by 05/00Z.

   ..Kerr.. 01/03/2021

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