MESOSCALE PRECIPITATION DISCUSSION 0432
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1046 PM EST WED DEC 03 2014
 
AREAS AFFECTED...NORTH-CENTRAL CA 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY 
 
VALID 040330Z - 040730Z
 
SUMMARY...PERSISTENT BAND OF HEAVY SHOWERS WILL CONTINUE TO FOCUS
A THREAT OF FLASH FLOODING THE NEXT FEW HOURS.

DISCUSSION...IR SAT IMAGERY SHOWS A BAND OF WARM-TOPPED SHOWER
ACTIVITY WHICH CONTINUES TO REPEAT GENERALLY OVER THE SAME AREA
FROM JUST EAST OF SACRAMENTO NORTHEASTWARD INTO THE FOOTHILLS AND
WESTERN SLOPES OF THE SIERRA-NEVADA INCLUDING AREAS AROUND BLUE
CANYON.

THE SHOWER ACTIVITY IS ASSOCIATED WITH AN AREA OF DEEPER LAYER
CONVERGENCE ACROSS THE NORTH-CENTRAL SACRAMENTO VALLEY AIMED INTO
THE SIERRA-NEVADA AROUND THE SOUTH SIDE OF A SHORTWAVE TROUGH
MOVING ACROSS SOUTHWEST OREGON ATTM. DESPITE PWATS THAT ARE RATHER
MODEST AT 1 INCH OR SO...AND VERY MEAGER INSTABILITY...WARM CLOUD
PROCESSES ARE ALLOWING THE SHOWER ACTIVITY TO PRODUCE SOME VERY
EFFICIENT RAINFALL RATES THAT ARE EXCEEDING 1 INCH/HR LOCALLY.
UPSLOPE FLOW/OROGRAPHICS ARE ALSO CONTRIBUTIGN TO THE HEAVIER
RAINFALL RATES AND SHOWER ORGANIZATION.

THE REPEAT NATURE OF THIS BAND OF SHOWER ACTIVITY HAS ALREADY
RESULTED IN AT LEAST LOCALIZED AREAS OF FLASH FLOODING...AND IT IS
EXPECTED THAT THE ACTIVITY WILL CONTINUE FOR A FEW MORE HOURS
UNTIL DEEPER LAYER FORCING ASSOCIATED WITH THE AFOREMENTIONED
SHORTWAVE FARTHER NORTH MOVES AWAY...AND DRIER MID LEVEL AIR ALOFT
ARRIVES FRO, THE WEST.

THE 12Z ARW AND 12Z NMMB ALONG WITH RECENT RUNS OF THE HRRR FAVOR
THIS BANDED PRECIP PATTERN CONTINUING THROUGH 09Z WITH THE
ACTIVITY SLOWLY SETTLING FARTHER SOUTHEAST AFTER ABOUT 06Z.
ADDITIONAL RAINFALL AMOUNTS OF 2 TO 3 INCHES...WITH ISOLATED
HEAVIER AMOUNTS WILL BE POSSIBLE. SO...WILL CONTINUE TO BE
CONCERNED FOR ADDITIONAL FLASH FLOODING THE NEXT FEW HOURS. WILL
CONTINUE TO MONITOR.

ORRISON

ATTN...WFO...REV...STO...

ATTN...RFC...CNRFC...

LAT...LON   39672062 39462031 38562063 38152096 38192138 
            38502148 38782136