MESOSCALE PRECIPITATION DISCUSSION 0789
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
333 AM EDT SAT SEP 09 2017

AREAS AFFECTED...WESTERN AZ & VICINITY

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

VALID 090732Z - 091332Z

SUMMARY...AN INCOMING UPPER LOW IS TAPPING AN AREA OF INSTABILITY
WITHIN A MOIST ENVIRONMENT.  HOURLY RAIN TOTALS TO 1.5" WITH LOCAL
AMOUNTS TO 3" ARE POSSIBLE.

DISCUSSION...RECENT INFRARED SATELLITE AND RADAR IMAGERY HAS
SHOWED RENEWED CONVECTION ACROSS PORTIONS OF WESTERN AZ.  THIS IS
AHEAD OF AN UPPER LOW LOCATED NEAR SAN FRANCISCO, WHICH IS TAPPING
MU CAPE VALUES OF 500-2000 J/KG.  INFLOW AT 700 HPA ACROSS WESTERN
AZ IS CONVERGENT AT ~15 KTS, WHICH APPEARS TO MANIFEST AS A ZONE
OF CYCLONIC WIND SHEAR AT THE SURFACE.  EFFECTIVE BULK SHEAR OF
20-25 KTS LIES ACROSS THE REGION, WHICH WHEN COMBINED WITH THE
UNIDIRECTIONAL FLOW WITH HEIGHT IS CREATING SHORT TRAINING BANDS. 
PRECIPITABLE WATER VALUES ARE 1-1.7" PER RECENT GPS DATA.

THE MESOSCALE GUIDANCE SHOWS SOME UPTICK IN CONVECTION INTO THE
MORNING HOURS, WHICH APPEARS TO BE SUBSTANTIATED BY RECENT
SATELLITE AND RADAR TRENDS.  THERE IS A GOOD SIGNAL FOR LOCAL
AMOUNTS OF 2-3".  HOURLY RAIN TOTALS COULD REACH 1.5" WITHIN THE
TRAINING BANDS.  THIS COULD LEAD TO ISSUES WITHIN WASHES AND URBAN
AREAS.

ROTH

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

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

LAT...LON   36931294 32711143 31861267 32651534 33871560
            36521441