MESOSCALE PRECIPITATION DISCUSSION 0475
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
204 AM EDT WED JUL 18 2018

AREAS AFFECTED...CTRL/SRN AZ...FAR SERN CA

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

VALID 180603Z - 181100Z

SUMMARY...THUNDERSTORMS WERE REDEVELOPING ACROSS PORTIONS OF
CENTRAL AND SOUTHERN ARIZONA LATE THIS EVENING AND MAY LAST INTO
THE NIGHT. THE ENVIRONMENT WILL CONTINUE TO SUPPORT RAIN RATES OF
1-2 IN/HR IN THE STRONGEST STORMS...WHICH COULD LEAD TO FLASH
FLOODING.

DISCUSSION...GOES-15 IR SATELLITE TRENDS SHOWED SEVERAL BURSTS OF
CONVECTION THAT INITIATED IN THE 04-06Z TIMEFRAME. THE FIRST WAS
JUST TO THE NORTH OF A +PV ANOMALY (EVIDENT IN WATER VAPOR
CHANNELS) THAT WAS MAKING STEADY PROGRESS WEST THROUGH NORTHERN
MEXICO. THE SECOND WAS ACROSS PARTS OF CENTRAL ARIZONA. HI-RES
MODELS GENERALLY DO NOT SEEM TO BE HANDLING THIS SCENARIO
WELL...WITH ONLY THE NSSL WRF AND NCEP WRF-ARW2 SHOWING VIGOROUS
CONVECTION AT THIS TIME OF NIGHT. THAT BEING SAID...THE
ENVIRONMENT IS GENERALLY FAVORABLE FOR CONVECTION...AND HEAVY
RAINFALL IF CONVECTION CAN BE SUSTAINED. THE LARGEST IMPEDIMENT TO
SUSTAINED CONVECTION WOULD BE A LACK OF FORCING. HOWEVER...THE
AFOREMENTIONED PV ANOMALY SHOULD PROVIDE SOME FORCING FOR ASCENT
ALONG THE SOUTHERN BORDER OF ARIZONA OVERNIGHT. ADDITIONALLY...RAP
ANALYSIS SHOWED INCREASING LOW-LEVEL MOISTURE CONVERGENCE ACROSS
PARTS OF SOUTH-CENTRAL ARIZONA. IT IS STILL UNCERTAIN WHETHER THIS
WILL PERSIST OVER THE NEXT SEVERAL HOURS. HOWEVER...THE AREAS OF
EXISTING CONVECTION SHOULD BE SUFFICIENT TO PRODUCE LOCALLY HEAVY
RAINFALL FOR AT LEAST ANOTHER HOUR OR TWO. THE DEVELOPING
CONVECTION WAS ALSO GENERALLY ALIGNED WITH THE MEAN WIND...NOT
PERFECTLY...BUT ENOUGH TO LEAD TO SOME TRAINING OF INDIVIDUAL
CELLS AND CLUSTERS.

AVAILABLE INSTABILITY AND SHEAR SHOULD BE SUFFICIENT FOR ORGANIZED
CONVECTION (1000-2000 J/KG MUCAPE AND 30 KNOTS OF EFFECTIVE DEEP
LAYER SHEAR)...AND THE INCREASED SHEAR AROUND THE N/NW FLANK OF
THE PV ANOMALY COULD AID IN MAINTENANCE OF THE CONVECTION.
PRECIPITABLE WATER VALUES ABOVE THE 90TH PERCENTILE WILL SUPPORT
THE HEAVY RAIN RATES...AND SEVERAL RECENT GPS-PW OBSERVATIONS FROM
FAR SOUTHWEST ARIZONA WERE AROUND 2.1 INCHES WHICH IS SOMEWHAT
CLOSE TO THE HIGHEST VALUES OBSERVED AT THE NEARBY YUMA SOUNDING
LOCATION. THE OUTLINED AREA FOR THIS DISCUSSION WAS EXPANDED A BIT
TO ACCOUNT FOR UNCERTAINTIES IN CONVECTIVE PROPAGATION AND
EVOLUTION... PARTICULARLY TO THE WEST INTO SERN CALIFORNIA WHERE
THE NSSL WRF AND NCEP WRF-ARW2 SHOW A SWATH OF HEAVY RAIN PRIOR TO
12Z...AND THE VALID TIME WAS ALSO EXTENDED TO 11Z EVEN THOUGH
THERE IS MORE UNCERTAINTY IN THE MAINTENANCE OF CONVECTION BEYOND
08-09Z.

LAMERS

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

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

LAT...LON   35291119 34681083 33901120 33101132 32201155
            31571219 32001371 32471510 33061582 33841569
            34451466 34891355 35181243