Severe Storm Outlook Narrative (AC)
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000
ACUS11 KWNS 120526
SWOMCD
SPC MCD 120526
CAZ000-120800-

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

...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...

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

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




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