MESOSCALE PRECIPITATION DISCUSSION 0621
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
250 PM EDT FRI AUG 04 2017

AREAS AFFECTED...SOUTHEAST NV...SOUTHERN UT...CENTRAL/NORTHERN AZ

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

VALID 041850Z - 050050Z

SUMMARY...SLOW-MOVING SHOWERS AND THUNDERSTORMS WITH LOCALLY HEAVY
RAINFALL RATES OF UP TO 2 INCHES/HR ARE EXPECTED. SOME FLASH
FLOODING WILL BE POSSIBLE.

DISCUSSION...THE LATEST GOES-16 0.64 MICRON/VIS IMAGERY SHOWS A
VERY WELL-DEFINED MCV OVER SOUTHEAST NV TO THE NORTHWEST OF LAS
VEGAS AND JUST EAST OF DESERT ROCK. THE MCV IS ALREADY HELPING TO
FOCUS SLOW-MOVING CONVECTION OVER THE NORTHERN PARTS OF CLARK
COUNTY. THE ENERGY IS EMBEDDED WITHIN A BROADER AREA OF MID LEVEL
VORTICITY THAT STRETCHES FROM SOUTHEAST NV EAST ACROSS SOUTHERN UT
AND ESPECIALLY CENTRAL/NORTHERN AZ WHERE THERE IS PROXIMITY OF
ANOTHER MCV TO THE EAST OF THE GRAND CANYON.

THE BROADER MID LEVEL VORTICITY COUPLED WITH STRONG DIURNAL
HEATING...OROGRAPHICS AND A MOIST VERTICAL COLUMN ASSOCIATED WITH
1.5 TO 1.7 INCH PWATS IS EXPECTED TO RESULT IN AN EXPANDING AREA
OF SCATTERED SHOWERS AND THUNDERSTORMS. ALREADY MLCAPE VALUES OVER
PARTS OF NV/AZ/UT TRI-STATE AREA ARE APPROACHING 2000 J/KG AND
THIS WILL BE CONDUCIVE FOR SOME LOCALLY VIGOROUS CONVECTIVE
GROWTH. THE MEAN STEERING FLOW IS QUITE WEAK OVER THE REGION AND
SO THE CONVECTIVE CELLS WILL BE QUITE SLOW MOVING. THIS COUPLED
WITH RAINFALL RATES OF UP TO 2 INCHES/HR WILL LIKELY RESULT IN
SOME LOCALLY ENHANCED FLASH FLOODING CONCERNS.

THE LATEST HRRR/HRRRX GUIDANCE SUPPORTS SOME SPOTTY 2 TO 3 INCH
TOTALS...AND SOME ISOLATED HEAVIER AMOUNTS CANNOT BE RULED OUT
GIVEN THE SLOW CELL MOTION. THE MORE SENSITIVE AREAS TO
RUNOFF/FLASH FLOODING WILL AGAIN BE THE AREA DRY WASHES/ARROYOS
AND ANY STEEP TERRAIN ALONG WITH ADJACENT SLOT CANYONS. WILL
CONTINUE TO MONITOR.

ORRISON

ATTN...WFO...FGZ...GJT...LKN...PSR...SLC...VEF...

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

LAT...LON   38501288 38281163 37531022 36510939 34790917
            33750958 33411059 33661171 34171299 34931407
            35281482 35651547 36391635 37461627 38231491