MESOSCALE PRECIPITATION DISCUSSION 0790
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
901 AM EDT SAT SEP 09 2017

AREAS AFFECTED...ARIZONA

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 091300Z - 091700Z

SUMMARY...LOW END FLASH FLOOD THREAT REMAINS THROUGH
PRE-DAWN/EARLY MORNING REMAINS WITH SLOW MOVING CELLS.

DISCUSSION...GOES-W WV LOOP DEPICTS CLOSED LOW NEAR SAN FRANCISCO
BAY REGION WITH STRONG UP-STREM JET SET TO HELP SOUTHWARD AND EVEN
SLIGHT RETROGRADING MOTION OVER THE NEXT FEW HOURS.  ALONG THE
SOUTHEAST PERIPHERY A SHORT-WAVE TROF EXTENDS FROM S CA TOWARD THE
S COLORADO RIVER VALLEY PROVIDING AND EXPANDING REGION OF
DIFFLUENCE ACROSS SE CA/S NV AND W AZ.  RADAR REFLECTIVITY MOSAIC
/GOES-16 10.3UM IR SHOW A WEAK BAROCLINIC LEAF-LIKE FEATURE ACROSS
NW AZ WITH A SOUTHWARD EXTENDING BAND FROM YAVAPAI INTO W MARICOPA
COUNTY.  THIS BAND OF SCATTERED THUNDERSTORMS IS FORCED BY THE
CONFLUENCE OF SWLY AND SELY FLOW IN THE 8-7H LEVEL.  WEAK HEIGHT
FALLS AT THE EASTERN PERIPHERY OF THE APPROACHING SHORTWAVE HAS
ALLOWS FOR NEAR ZERO PROPAGATION VECTORS (SLIGHTLY E OR S) TO
EFFECTIVELY STALL THE ORIENTATION OF BEST REMAINING FORCING FOR
THE NEXT FEW HOURS WITHIN THE 15KT S TO N MEAN CLOUD BEARING/CELL
MOTIONS SUPPORTING TRAINING.

UTILIZING THE WELL OF WARM/MOIST LOW LEVEL AIR ACROSS THE SONORAN
DESERT...WITH MUCAPES TO 2000 J/KG AND MOISTURE ALONG THE TPW
GRADIENT UPWARDS OF 1.5" ...THERE IS A REMAINING CAPABILITY FOR
SCATTERED CONVECTION WITH MODEST UPDRAFT STRENGTH TO SUPPORT RATES
OF 1-1.25"/HR FROM FURTHER NORTH PARTICULARLY FURTHER SOUTH. 
GIVEN SLOW MOTIONS THERE IS SOME POSSIBILITY OF 1-2" TOTALS TO BE
EXPECTED THROUGH EARLY MORNING POSING AT LEAST A LOW END/LOCALIZED
FLASH FLOODING RISK ESPECIALLY IF UPSTREAM REDEVELOPMENT AND MEAN
FLOW SUPPORTS REPEATING OF DOWNDRAFT CORES. 

GALLINA

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

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

LAT...LON   36301243 35911166 35171140 34061143 33071166
            32731196 32551248 32671304 33121330 34121329
            35161371 36061337