MESOSCALE PRECIPITATION DISCUSSION 0773
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
748 AM EST MON NOV 21 2016
 
AREAS AFFECTED...NORTHWEST AZ...SOUTHERN UT 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 211248Z - 211648Z
 
SUMMARY...INCREASED CONVECTIVE ACTIVITY RESULTING IS AN ISOLATED
FLASH FLOOD RISK ACROSS PORTIONS OF NORTHWEST AZ AND FAR SOUTHWEST
UT.

DISCUSSION...A COMPACT SHORTWAVE FEATURE IS CURRENTLY MOVING
ACROSS SOUTHERN CALIFORNIA. STEEPENING LAPSE RATES AND INCREASED
SYNOPTIC SUPPORT AHEAD OF THIS FEATURE HAS ALLOWED FOR CONVECTION
TO RAPIDLY DEVELOP ACROSS PORTIONS OF FAR SOUTHEAST NEVADA AND
NORTHWEST ARIZONA. THIS CONVECTIVE ACTIVITY IS ALSO LIKELY BEING
AIDED BY LEFT EXIT JET STREAK DYNAMICS AND STRONG 850 MB MOISTURE
CONVERGENCE. RADAR AND AUTOMATED GAGES SHOW RAINFALL AMOUNTS UP TO
0.75" IN AN HOUR WITH THIS CONVECTIVE LINE. QUICK STORM
MOTIONS...WITH MEAN STORM LAYER WINDS OF 40-50 KTS...SHOULD
GENERALLY KEEP HOURLY AMOUNTS MAXING IN THIS 0.5"-0.75" RANGE.
RECENT RUNS OF THE HRRR TIME LAGGED ENSEMBLE DO INDEED SHOW A HIGH
PROBABILITY OF 0.5" IN AN HOUR ACROSS PORTIONS OF NORTHWEST
AZ...BUT LITTLE TO NO PROBABILITIES OF 1" IN AN HOUR. GIVEN THE
QUICK DURATION OF HEAVY RAINS...SOME OF THE FLASHIER BASINS ACROSS
THE AREA COULD SEE A FLASH FLOOD RISK...ALTHOUGH THE QUICK STORM
MOTIONS SHOULD LIMIT THE OVERALL THREAT.  

PRECIPITABLE WATER VALUES ARE RUNNING AROUND THE CLIMATOLOGICAL
90TH PERCENTILE...ALTHOUGH A SLIGHT DECREASING TREND OF THESE
VALUES IS EXPECTED THROUGH THE MORNING HOURS. MUCAPE IS
MARGINAL...BUT VALUES UPWARDS OF 500 J/KG HAVE BEEN ENOUGH TO
MAINTAIN THIS CONVECTION. IN GENERAL RECENT RAP RUNS SUGGEST THIS
CONVECTION WILL MOVE INTO AN AREA OF EVEN LESS MUCAPE ACROSS
NORTHERN AZ AND SOUTHERN UT. GIVEN THE GENERAL TREND OF DECREASED
PWATS...MOISTURE CONVERGENCE AND INSTABILITY AS THE CONVECTION
MOVES EAST NORTHEASTWARD...WOULD GENERALLY EXPECT TO SEE A
WEAKENING TREND THROUGH THE MORNING HOURS. THE HRRR TIME LAGGED
ENSEMBLE DOES SHOW A DECREASED CHANCE OF 0.5" IN AN HOUR AS THE
CONVECTION MOVES EAST. THUS OVERALL WOULD EXPECT TO SEE A
DECREASING FLOOD RISK THROUGH THE MORNING. HOWEVER WITH PWAT
VALUES REMAINING 1-2 SD ABOVE AVERAGE...AND SOME WEAK INSTABILITY
LIKELY TO REMAIN AHEAD OF THE SHORTWAVE...SOME HEAVIER CONVECTIVE
ELEMENTS WILL LIKELY CONTINUE...WHICH MAY MAINTAIN A WIDELY
ISOLATED FLASH FLOOD RISK EAST INTO NORTHERN AZ AND FAR SOUTHERN
UT THROUGH THE MORNING HOURS.

CHENARD

ATTN...WFO...FGZ...PSR...SLC...VEF...

ATTN...RFC...CBRFC...

LAT...LON   37131336 36971237 36321226 36071228 34751232 
            34361240 34091278 34071323 34551365 35231394 
            35601420 36031426 36551424 37101410