MESOSCALE PRECIPITATION DISCUSSION 0317
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
608 PM EDT MON AUG 25 2014
 
AREAS AFFECTED...CENTRAL AZ 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 252208Z - 260138Z
 
SUMMARY...FLASH FLOODING IS POSSIBLE THROUGH AT LEAST 01Z...AS
SLOW MOVING AND ORGANIZED THUNDERSTORMS FEED UPON A MODEST INFLUX
OF MOIST AND UNSTABLE AIR FROM THE LOW DESERT REGION.

DISCUSSION...AT MID AFTERNOON THUNDERSTORMS AND NORTHWEST AZ WERE
BEHAVING AS WOULD BE EXPECTED WITH MODERATE DEEP LAYER SHEAR AND
UNUSUALLY LARGE MIXED LAYER CAPE IN PLACE. SOME SUPERCELL BEHAVIOR
WAS NOTED... INCLUDING AN EARLY SPLIT ACROSS NW YAVAPI COUNTY.
SYNOPTIC LIFT ACROSS THIS REGION WAS ALMOST NEUTRAL...BUT THERE
WAS A HINT OF A 700-600 MB TROUGH PASSING THROUGH IN THE RAP AND
GFS OUTPUT. FAIRLY RICH MOISTURE WAS IN PLACE...WITH DEWPOINTS
ONLY MIXING OUT TO THE LOW AND MIDDLE 60S IN THE LOW DESERT. IN
RESPONSE TO AN ANOMALOUSLY DEEP UPPER TROUGH IN NEVADA...ENHANCED
LOW LEVEL SOUTHERLY WINDS WERE IN PLACE ACROSS WRN AND NRN AZ.
THESE FACTORS SUGGEST THUNDERSTORMS THAT WERE THRIVING IN NW AZ AT
22Z MAY PERSIST AT LEAST A FEW HOURS.

MODELS WERE NOT HANDLING THE SCOPE OF THE CONVECTION VERY
WELL...BUT THE SIGNAL IN THE HRRR AND 12Z NSSL WRF WAS TO KEEP
THIS EVENT CONFINED IN AREAL COVERAGE...GIVEN STORM MOTIONS TO THE
NORTH AND EAST...WHILE REDEVELOPMENT IS FAVORED ALONG THE
SOUTHWEST FACING OUTFLOWS. WITH PW VALUES RANGING FROM NEAR 1.5
INCHES WEST OF PRESCOTT TO JUST BELOW 1 INCH AROUND
FLAGSTAFF...AND GIVEN THE EXPECTATION OF SLOW MOVEMENT AND
TRAINING...TWO INCHES OR MORE OF RAIN MAY FALL IN ISOLATED SPOTS
OVER 1-3 HOURS TIME...RAISING THE POSSIBILITY OF FLASH FLOODING.
LONGEVITY OF THE EVENT MAY DEPEND ON THE ABILITY OF CONVECTION TO
BUILD A LARGE ENOUGH COLD POOL...GIVEN THAT A SOURCE OF
INSTABILITY OVER THE LOW DESERT WILL BE PRESENT INTO THE EVENING.

BURKE

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

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

LAT...LON   35441145 34581147 33761251 33411374 33931417 34801378
            35591279 35441145