MESO: Mesoscale Discussion 1746

ACUS11 KWNS 211931
SWOMCD
SPC MCD 211930
NVZ000-CAZ000-ORZ000-212200-

MESOSCALE DISCUSSION 1746
NWS STORM PREDICTION CENTER NORMAN OK
0230 PM CDT WED AUG 21 2013

AREAS AFFECTED...PORTIONS OF NRN CA...NWRN NV...EXTREME SRN ORE

CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY

VALID 211930Z - 212200Z

PROBABILITY OF WATCH ISSUANCE...5 PERCENT

SUMMARY...AN ISOLATED SEVERE THUNDERSTORM POTENTIAL MAY DEVELOP
DURING THE NEXT FEW HOURS AND COULD PERSIST INTO THE EVENING.

DISCUSSION...VISIBLE SATELLITE IMAGERY INDICATES DEEPENING
CONVECTION OVER THE CNTRL/NRN SIERRA. THIS IS WITHIN AN ARC OF
ENHANCED MID-LEVEL MOISTURE COILING AROUND THE NERN SEMICIRCLE OF A
CYCLONE CURRENTLY POSITIONED OFF THE SAN FRANCISCO COAST -- PER
MOISTURE-CHANNEL IMAGERY. ISOLATED LIGHTNING STRIKES HAVE ALREADY
BEEN DETECTED WITH THIS CONVECTION S OF RENO...AND ADDITIONAL
CONVECTION HAS RECENTLY BEEN DEVELOPING FARTHER N TOWARD THE HIGHER
TERRAIN OF NRN CA INCLUDING THE KLAMATH MOUNTAINS.

FURTHER DIURNALLY-DRIVEN DEEPENING/STRENGTHENING OF OROGRAPHIC
CIRCULATIONS SHOULD FOSTER AN INCREASE IN THE COVERAGE OF
THUNDERSTORMS THROUGH THE REMAINDER OF THE AFTERNOON...WITH STORMS
BECOMING SCATTERED TO LOCALLY NUMEROUS IN COVERAGE. MID-LEVEL HEIGHT
FALLS/ADIABATIC COOLING PRECEDING THE CYCLONE...WHOSE CENTROID WILL
SLOWLY EDGE NNEWD WITH TIME...SHOULD ALSO CONTRIBUTE TO MAINTAINING
AN ENVIRONMENT FAVORABLE FOR DEEP CONVECTION AMIDST MODEST
TROPOSPHERIC MOISTURE. THIS MOISTURE IS CHARACTERIZED BY PW VALUES
AROUND 0.7-1.4 INCHES PER GPS DATA AND SFC DEWPOINTS IN THE MIDDLE
40S TO THE MIDDLE 50S. GIVEN THE COUPLING OF THIS MOISTURE WITH
DIABATIC SFC HEATING/RELATIVELY STEEP MID-LEVEL LAPSE RATES...MLCAPE
ON THE ORDER OF 1000-2000 J/KG WILL BE MAINTAINED AND SUPPORT A FEW
VIGOROUS UPDRAFTS INTO THIS EVENING.

VISIBLE SATELLITE IMAGERY SUGGESTS FAIRLY WELL-SCULPTED...DOWNSHEAR-
DIRECTED ANVILS AROUND THE CNTRL SIERRA...WHICH ARE LIKELY
RESPONDING TO MODEST HIGH-LEVEL FLOW. HOWEVER...RELATIVELY WEAK
MID-LEVEL FLOW DISPLACED FROM THE CYCLONE CENTER IS LIKELY
MINIMIZING DEEP SHEAR...SUCH THAT CONVECTION MAY INITIALLY STRUGGLE
TO EXHIBIT SUBSTANTIAL ORGANIZATION. HOWEVER...A MODEST INCREASE IN
MID-LEVEL FLOW MAY OCCUR AS THE HEIGHT GRADIENT TIGHTENS LATER THIS
AFTERNOON...AND THIS COULD SUPPORT ENOUGH DEEP SHEAR FOR A FEW
SUSTAINED MULTICELL STRUCTURES INTO THIS EVENING.

THE AFOREMENTIONED FACTORS MAY YIELD THE POTENTIAL FOR ISOLATED
MARGINALLY SVR WIND...WITH LARGE SFC TEMPERATURE-DEWPOINT SPREADS
OVER 30F ACROSS THE LOWER ELEVATIONS ENHANCING SUBCLOUD EVAPORATIVE
COOLING/SFC-GUST POTENTIAL. SPORADIC...MARGINAL SVR HAIL CANNOT BE
RULED OUT IN CLOSER PROXIMITY TO THE CYCLONE OVER NRN CA.
HOWEVER...THE LACK OF STRONGER DEEP SHEAR/RICHER MOISTURE WILL
LIKELY LIMIT THE OVERALL SVR THREAT.

..COHEN/CORFIDI.. 08/21/2013

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

ATTN...WFO...LKN...REV...MFR...STO...EKA...

LAT...LON 40622183 40492246 39962281 39942333 40592390 41512382
42032297 42202143 41981972 40631882 39021856 38331891
38281968 38692031 39842095 40622183

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