MESOSCALE PRECIPITATION DISCUSSION 0505
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1115 AM EDT TUE SEP 22 2015
 
AREAS AFFECTED...PORTIONS OF AZ 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 221514Z - 222114Z
 
SUMMARY...SHOWERS AND THUNDERSTORMS ARE EXPECTED TO INCREASE IN
COVERAGE/INTENSITY AFTER 16Z.  FLASH FLOODING IS POSSIBLE DUE TO
CELL TRAINING.

DISCUSSION...A LOW- TO MID-LEVEL CYCLONE ACROSS NORTHERN AZ IS
MOVING SLOWLY EASTWARD, WITH COMMA HEAD CLOUDINESS TO ITS NORTH
AND WEST ACROSS NORTHWEST AZ AND SOUTHERN CA.  BREAKS IN THE
STRATUS DECK HAVE BEEN APPARENT ACROSS SOUTHERN AND EAST-CENTRAL
AZ OVER THE PAST FEW HOURS.  PRECIPITABLE WATER VALUES OF
1.5-1.75" LIE AT LOWER ELEVATIONS, WHILE 0.75" AMOUNTS ARE SEEN
NEAR THE MOGOLLON RIM AND 1-1.25" AMOUNTS LIE ACROSS THE SOUTHEAST
MOUNTAINS OF AZ PER GPS INFORMATION.  GOOD INFLOW AT 700 HPA IS
SEEN ACROSS SOUTHEAST AZ, ON THE ORDER OF 20-25 KTS FROM THE
WEST-SOUTHWEST.  WESTERN AZ DISPLAYS A DECENT AMOUNT OF VERTICAL
WIND SHEAR BETWEEN 850-700 HPA, INDICATING 20-25 KTS OF CLOUD BASE
INFLOW.  LOW-LEVEL INFLOW/VERTICAL WIND SHEAR IS ROUGHLY THE
MAGNITUDE OF THE 850-400 HPA MEAN WIND.

THE EXPECTATION IS THAT INSTABILITY SHOULD BUILD ACROSS PORTIONS
OF SOUTHERN AND EASTERN AZ THROUGH THE DAY TODAY.  WHILE
THUNDERSTORMS DEVELOP DOWN SOUTH AS CIN ERODES AFTER 16Z, COMMA
HEAD CLOUDINESS COULD TRY TO THIN LATER THIS MORNING, WHICH WOULD
ALLOW INSTABILITY TO BE INTRODUCED TO AREAS BORDERING THE WESTERN
MOGOLLON RIM WHERE CELL MOTIONS WOULD BE SLOWER TOWARDS THE END OF
THE MPD WINDOW.  CELL TRAINING APPEARS TO BE THE MAIN FLASH FLOOD
THREAT IN SOUTHEAST AZ.  THE 06Z SSEO NEIGHBORHOOD PROBABILITIES
SHOW A SIGNIFICANT SIGNAL FOR EXCEEDANCE OF THE THREE HOURLY FLASH
FLOOD GUIDANCE VALUES, SHOWING 50+% CHANCE OF EXCEEDANCE FROM 19Z
ONWARD.  THE WETTER MESOSCALE GUIDANCE (00Z NMMB, 00Z ARW, 11Z
EXPERIMENTAL HRRR, AND 00Z NSSL4WRF) SHOW LOCAL AMOUNTS OF ROUGHLY
3", WHICH WOULD BE A ONCE IN 100 YEAR EVENT WITHIN A 3-6 HOUR TIME
FRAME FOR PORTIONS OF CENTRAL AZ IN THE NEIGHBORHOOD OF PHOENIX. 
FLASH FLOODING, WITH ASSOCIATED DEBRIS FLOWS, IS POSSIBLE.

ROTH

ATTN...WFO...FGZ...PSR...TWC...VEF...

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

LAT...LON   36841245 36651193 35741143 34931073 34400914 
            32790912 31970943 31341035 31311080 31361135 
            31731253 32001336 32031337 32031337 32741280 
            33831246 34001256 34711359 35931403 36791347