MESOSCALE PRECIPITATION DISCUSSION 0361
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
559 PM EDT SAT JUL 18 2015
 
AREAS AFFECTED...SOUTHERN CA AND ADJACENT DESERTS/SOUTHERN
NV/SOUTHWESTERN UT/AZ... 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY 
 
VALID 182159Z - 190259Z
 
SUMMARY...MODERATE TO HEAVY CONVECTION CONTINUES TO STREAM THROUGH
THE DESERT SOUTHWEST WITH THE STRONGEST ACTIVITY CURRENTLY OVER
THE DESERT REGION. THIS SYSTEM ALREADY HAS A HISTORY OF PRODUCING
FLASH FLOODING WITH SUCH CONDITIONS LIKELY TO PERSIST GIVEN
SLOW-CELL MOTIONS AND TREMENDOUS MOISTURE CROSSING THE
SOUTHWESTERN U.S.

DISCUSSION...RADAR IMAGERY THIS AFTERNOON SHOWED QUITE THE AREAL
COVERAGE OF CONVECTION SPREADING ACROSS THE COASTLINE OF SOUTHERN
CA EXTENDING EASTWARD ACROSS THE DESERTS AND INTO SOUTHERN
NV/SOUTHWESTERN UT/WESTERN TO CENTRAL AZ. THIS IS WORKING UNDER
THE INFLUENCE OF A PRONOUNCED SURGE OF TROPOSPHERIC MOISTURE PER
THE BLENDED-TPW PRODUCT AND ACCOMPANYING 12Z RAOBS. IN THE PAST
SEVERAL HOURS...PWATS HAVE JUMPED ROUGHLY 0.40 INCHES BRINGING
MANY VALUES TO THE 2 INCH MARK ACROSS SOUTHEASTERN CA. WHILE THE
ELEVATED INSTABILITY WHICH WAS IN PLACE ACROSS THE SOUTHERN CA
COAST HAS BEEN ERODED FROM PREVIOUS CONVECTION...THERE APPEARS TO
BE A LOT OF SURFACE-BASED INSTABILITY STRETCHING FROM SOUTHWESTERN
UT DOWN TO SOUTHWESTERN AZ. THE 21Z RAP OBJECTIVE ANALYSIS PLACES
AROUND 2000 TO 3000 J/KG OF SBCAPE WITH VERY LITTLE CAPPING WHICH
ALLOWED THE REGION TO FREELY CONVECTION THIS AFTERNOON. THIS SEEMS
CONSISTENT WITH THE 18Z SOUNDING OUT OF YUMA AZ. HOURLY RATES IN
THIS SECTOR HAVE RANGED FROM 1 TO 2 INCHES PER DUAL-POL
ESTIMATIONS. A WEAK STEERING FLOW HAS GENERALLY KEPT CONVECTIVE
TOWERS IN PLACE WHICH HAS LED TO FLASH FLOODING. WFO LAS VEGAS
ALREADY NOTED FLASH FLOODING IN SAN BERNARDINO COUNTY A COUPLE
HOURS AGO. GIVEN THE HISTORY OF SUCH ISSUES...FLASH FLOODING WILL
REMAIN LIKELY...PARTICULARLY ACROSS SOUTHERN UT DOWN TO
WEST-CENTRAL AZ.

AS USUAL...IT SEEMS THE CONVECTION-ALLOWING MODELS ARE STRUGGLING
WITH THE INTENSITY OF THE EXISTING ACTIVITY. HOWEVER...CURRENT
TRENDS IN RADAR/SATELLITE/OBSERVATIONS SUPPORT 1 TO 2 INCH HOURLY
RATES IN THE MORE ROBUST CONVECTION. WILL LET THE NIGHT SHIFT
RE-ASSESS THE SITUATION AFTER 03Z.


RUBIN-OSTER

ATTN...WFO...FGZ...HNX...LOX...PSR...SGX...SLC...TWC...VEF...

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

LAT...LON   37851403 37161264 36391102 35281080 34291097 
            33111062 31611065 31121167 31321369 32251480 
            32531601 33131665 33431738 33761883 34451921 
            34981910 35641827 36351716 36941622 37811503