Graphic for MPD #0507

MESOSCALE PRECIPITATION DISCUSSION 0507
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
837 PM EDT TUE JUL 18 2017

AREAS AFFECTED...SOUTHEAST CA...CENTRAL/WESTERN AZ...SOUTHERN
NV...SOUTHWEST UT

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 190035Z - 190600Z

SUMMARY...SHOWERS AND THUNDERSTORMS WILL CONTINUE OVER THE NEXT
SEVERAL HOURS...WITH A GRADUAL WEAKENING TOWARD LATE EVENING.
HOWEVER...ADDITIONAL AREAS OF FLASH FLOODING ARE EXPECTED UNTIL
THEN.

DISCUSSION...NUMEROUS COLD-TOPPED SHOWERS AND THUNDERSTORMS
CONTINUE TO IMPACT A LARGE AREA OF AZ LATE THIS AFTERNOON...WITH
THE ACTIVITY MORE SCATTERED OVER SOUTHWEST UT...FAR SOUTHEAST NV
AND ALSO RECENTLY OVER PARTS OF SOUTHEAST CA.

THE ENVIRONMENT CONTINUES TO REMAIN VERY MOIST AND UNSTABLE WITH
PWATS AS HIGH AS 2.0 INCHES OVER SOUTHERN AZ AND ALSO NOW
EXTENDING WEST OVER INTO ADJACENT AREAS OF SOUTHEAST CA AS DENOTED
BY EXPERIMENTAL GOES-16 AND CIRA-LPW TPW DATA PRODUCTS. THIS IS
HIGHLY ANOMALOUS WITH THESE PWATS RUNNING 2.5 TO 3 STANDARD
DEVIATIONS ABOVE NORMAL. MEANWHILE...AS MUCH AS 2000 TO 2500 J/KG
OF MLCAPE IS IN PLACE LOCALLY...WHICH IS PROMOTING A MODERATE TO
STRONG LEVEL OF UNCAPPED INSTABILITY. THE COMBINATION OF THE TWO
CONTINUE TO FOSTER EXTREMELY HEAVY RAINFALL RATES THAT ARE LOCALLY
REACHING UP TO 2.5 INCHES/HR OR GREATER.

CONVECTION HAS CONTINUED TO EXPAND IN COVERAGE OVER THE LAST
COUPLE OF HOURS AND ASIDE FROM THE HIGHLY FAVORABLE THERMODYNAMICS
ENVIRONMENT...THERE IS ALSO PROXIMITY OF A WEAK WESTWARD MOVING
MID LEVEL TROUGH AXIS IN VICINITY OF THE CA/AZ BORDER. OROGRAPHIC
FORCING/UPSLOPE FLOW IS ALSO LOCALLY ENHANCING THE CONVECTIVE
DEVELOPMENT AND ALSO ENHANCING RAINFALL RATES FURTHER.

THE THUNDERSTORM ACTIVITY WILL CONTINUE TO PROPAGATE GRADUALLY
WEST AND WILL BECOME A BIT MORE FOCUSED OVER PARTS OF SOUTHEAST NV
AND SOUTHEAST CA OVER THE NEXT FEW HOURS. MEANWHILE...ADDITIONAL
ROUNDS OF THUNDERSTORMS WILL BE IMPACTING THE MOGOLLON RIM AREA
AND ADJACENT AREAS OF CENTRAL/WESTERN AZ. HOWEVER...AS THE
DIURNALLY DRIVEN INSTABILITY IS GRADUALLY EXHAUSTED THROUGH THE
EVENING HOURS...EXPECT THE CONVECTION TO BEGIN TO WEAKEN BY LATE
EVENING.

EXPECT RAINFALL AMOUNTS FOR THIS PERIOD AGAIN TO BE AS MUCH AS 3
TO 4 INCHES WITH ISOLATED HEAVIER AMOUNTS...AND THIS WILL LIKELY
RESULT IN FLASH FLOODING OF AREA DRY WASHES/ARROYOS AND WILL
RESULT IN ENHANCED RUNOFF/FLASH FLOODING CONCERNS INVOLVING HIGHER
TERRAIN AND ADJACENT CANYONS.

ORRISON

ATTN...WFO...EPZ...FGZ...PSR...SGX...SLC...TWC...VEF...

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

LAT...LON   38121425 37901267 36621105 35200992 33000903
            32020949 31471102 31911298 32441449 33401601
            34561684 35781711 37461622


Last Updated: 837 PM EDT TUE JUL 18 2017