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Mesoscale Discussion 1974
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   MESOSCALE DISCUSSION 1974
   NWS STORM PREDICTION CENTER NORMAN OK
   1126 PM CST THU DEC 11 2014

   AREAS AFFECTED...PORTIONS OF SAN LUIS OBISPO...SANTA
   BARBARA...VENTURA...AND LOS ANGELES COUNTIES OF SOUTHERN CALIFORNIA
   INCLUDING THE ADJACENT COASTAL WATERS AND CHANNEL ISLANDS

   CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY 

   VALID 120526Z - 120800Z

   PROBABILITY OF WATCH ISSUANCE...5 PERCENT

   SUMMARY...A MARGINAL RISK FOR A SVR WIND GUST AND/OR A BRIEF/WEAK
   TORNADO WILL SPREAD ACROSS PORTIONS OF COASTAL CA AND VICINITY
   DURING THE REMAINDER OF THE OVERNIGHT HOURS. A WW WILL NOT BE
   NEEDED.

   DISCUSSION...RECENT MOISTURE-CHANNEL IMAGERY INDICATES A
   HIGH-AMPLITUDE TROUGH OFF THE PACIFIC COAST SLOWLY PROGRESSING EWD.
   AN EMBEDDED BASAL SHORTWAVE IMPULSE IS ABOUT 300 NM WSW OF POINT
   CONCEPTION...AND IS RAPIDLY SHIFTING ESEWD. AS THIS IS
   OCCURRING...SOME TRANSITION OF THE LARGE-SCALE TROUGH FROM SLIGHTLY
   POSITIVE TOWARD NEUTRAL TILT IS EVIDENT...WITH RELATED DEFORMATION
   PROCESSES SCULPTING THE PRECEDING WARM CONVEYOR INTO A SUBTLE
   BAROCLINIC LEAF STRUCTURE PER TRENDS IN THE DEEP MOIST PLUME.
   MEANWHILE...STRONG BAROCLINICITY WITHIN THIS PLUME IS SUPPORTING A
   LOW-TOPPED...N/S-ORIENTED BAND OF CONVECTION APPROACHING THE COAST
   OF SAN LUIS OBISPO AND SANTA BARBARA COUNTIES. KVBX BASE
   REFLECTIVITY AND SRM DATA INDICATE LEWP STRUCTURES EMBEDDED WITHIN
   THE CONVECTION...WITH INHERENT MESOVORTICES AT INFLECTION POINTS.
   THIS ACTIVITY WILL SPREAD ONSHORE DURING THE NEXT HOUR OR SO...AND
   THEN PROGRESS ACROSS COASTAL SRN CA THROUGHOUT THE OVERNIGHT HOURS.

   THE RECENT ABSENCE OF LIGHTNING STRIKES ACCOMPANYING THE
   AFOREMENTIONED ACTIVITY IS CONSISTENT WITH THE VERY-LOW-CAPE
   ENVIRONMENT PRECEDING THE CONVECTION. HOWEVER...GIVEN FURTHER
   TRANSITION OF THE TROUGH TOWARD A NEUTRAL TILT...AND PERHAPS
   SLIGHTLY NEGATIVE BY LATE IN THE NIGHT...GAINS IN NOCTURNAL STATIC
   STABILITY MAY BE SLIGHTLY OFFSET BY RELATED MID-LEVEL COOLING.
   AND...WITH SFC PRESSURE FALLS OF 2.0-2.5 MB PER 2 HOURS ANALYZED N
   OF POINT CONCEPTION...RELATED ISALLOBARIC FLOW WILL BE CONCOMITANT
   WITH AN INCREASE IN ONSHORE FLOW ACROSS THE REGION AS SFC WINDS
   GRADUALLY VEER. THIS WILL MANIFEST OCEANIC LATENT HEAT FLUXES INLAND
   ALONG THE COASTAL PLAIN TO SOME EXTENT...WITH SFC DEWPOINTS IN THE
   UPPER 50S ANALYZED INVOF THE CHANNEL ISLANDS. THE NET RESULT OF
   THESE PROCESSES MAY SUPPORT THE INLAND PENETRATION OF UP TO AROUND
   100 J/KG OF MLCAPE...PER MODIFICATIONS TO THE VBG 00Z RAOB.
   FURTHERMORE...THIS RAOB SAMPLED A PW AROUND 0.80 INCH -- I.E. AROUND
   THE 90TH PERCENTILE FOR THIS TIME OF YEAR -- WITHIN THE SRN EXTENT
   OF A POLEWARD-EXTENDING SUBTROPICAL MOIST PLUME HIGHLIGHTED BY GPS
   PW DATA. TURBULENT FLUXES RELATED TO THE STRONGLY SHEARED
   ENVIRONMENT WILL PROBABLY ESTABLISH THE ENHANCED PACIFIC MOISTURE
   ALOFT AT THE SFC...ALLOWING FOR NON-ZERO BUOYANCY TO EXTEND INLAND
   BY AROUND 25-35 MILES FROM THE COAST.

   DESPITE THE PAUCITY OF BUOYANCY...THE KVBX VWP SAMPLES ABUNDANT
   LOW-LEVEL SHEAR...WITH 60-KT SLYS JUST ABOUT THE SFC. CONVECTIVE
   MOMENTUM TRANSPORT WILL BE ENHANCED WITH BOWING SEGMENTS EMBEDDED
   WITHIN LEWP STRUCTURES...AND COULD YIELD A SVR WIND GUST. THE RISK
   FOR A BRIEF/WEAK TORNADO CANNOT BE RULED OUT...ESPECIALLY WITH
   INFLECTIONS/BREAKS IN CONVECTIVE SEGMENTS MOVING ONSHORE.
   HOWEVER...GIVEN THE DEARTH OF RICH BOUNDARY-LAYER MOISTURE YIELDING
   VERY SCANT BUOYANCY...A MORE SUBSTANTIAL SVR-TSTM RISK IS NOT
   EXPECTED TO ENSUE.

   ..COHEN/EDWARDS.. 12/12/2014


   ATTN...WFO...LOX...

   LAT...LON   34892075 35152078 35242055 34982015 34801949 34611886
               34191832 33901851 33881996 34052050 34492071 34892075 

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