MESOSCALE PRECIPITATION DISCUSSION 0426
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
341 PM EDT MON JUL 09 2018

AREAS AFFECTED...NW/CTRL AZ...SE NV...SW UT

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

VALID 091940Z - 100130Z

SUMMARY...SCATTERED THUNDERSTORMS WERE BEGINNING TO INITIATE ALONG
TERRAIN FEATURES AROUND MIDDAY FROM ARIZONA INTO SOUTHERN UTAH.
THESE STORMS SHOULD BUILD INTO LOWER ELEVATION AREAS LATER IN THE
AFTERNOON...ESPECIALLY TO THE WEST INTO SOUTHERN NEVADA...FAR
SOUTHWEST UTAH...AND NORTHWEST AND CENTRAL ARIZONA. BECAUSE OF THE
SCATTERED NATURE OF THE STORMS...HEAVY RAINFALL SHOULD BE
LOCALIZED...BUT RAIN RATES OF 1-2 IN/HR WILL BE POSSIBLE WHICH
COULD LEAD TO FLASH FLOODING. THE THREAT OF FLASH FLOODING SHOULD
INCREASE AFTER 3 PM PDT.

DISCUSSION...GOES-15 VISIBLE SATELLITE AND REGIONAL RADARS SHOWED
CONVECTION INITIATING ALONG THE ARC OF HIGHER TERRAIN FROM THE
MOGOLLON RIM IN ARIZONA...NORTH INTO THE RANGES OF CENTRAL UTAH.
THIS CONVECTION SHOULD GENERALLY CONTINUE TO BE FOCUSED ALONG
TERRAIN FEATURES FOR THE NEXT COUPLE HOURS. TO THE WEST...AN
INSTABILITY AXIS STRETCHED FROM NRN LINCOLN COUNTY NV...SSE TO
CENTRAL MOHAVE COUNTY AZ...TO NEAR THE PHOENIX METRO AREA. THIS
WAS A REGION OF HIGHER PRECIPITABLE WATER VALUES (1.5+ INCHES PER
CIRA BLENDED TPW)...BUT 19Z RAP ANALYSIS STILL SHOWED SOME
RESIDUAL CINH (AROUND -30 TO -70 J/KG) THAT HAD YET TO ERODE.
ADDITIONAL INSOLATION SHOULD FURTHER ERODE THAT CINH OVER THE NEXT
SEVERAL HOURS AND ALLOW CONVECTION TO BEGIN TO BUILD INTO LOWER
ELEVATION AREAS LATER IN THE AFTERNOON. MLCAPE VALUES ANALYZED BY
THE RAP WERE AROUND 1500 J/KG IN A POCKET OF SE NEVADA AND NW
ARIZONA...AND THIS SHOULD SUPPORT SOME VIGOROUS CONVECTION CAPABLE
OF REALIZING 1-2 IN/HR RAIN RATES GIVEN THE ATMOSPHERIC MOISTURE
LEVELS.

THE THREAT OF FLASH FLOODING IS EXPECTED TO INITIALLY BE QUITE
ISOLATED AND LOW...AND CONFINED MAINLY AROUND TERRAIN
AREAS...PRIOR TO 22Z. AFTER THAT TIME...ENOUGH CONVECTIVE
DEVELOPMENT SHOULD OCCUR TO ALLOW FOR CELL MERGERS AND OUTFLOW
BOUNDARY COLLISIONS TO BRIEFLY ENHANCE RAINFALL IN LOCALIZED AREAS
AND PROMOTE A GREATER FLASH FLOOD THREAT. SOME AREAS NEAR THE
AZ-NV BORDER MAY SEE BETTER CHANCES FOR FLASH FLOODING AFTER THE
TIME FRAME OF THIS DISCUSSION...BUT THEY WERE INCLUDED TO
ENCOMPASS THE ENTIRE INSTABILITY AXIS. MEAN WINDS (850-300MB) ARE
WEAKER FROM SRN NV INTO CTRL UT NEAR THE UPPER LEVEL RIDGE
AXIS...SO THIS MAY FAVOR SLOWER MOVING CONVECTION IN THOSE AREAS.
THE MEAN FLOW IS STRONGER (FROM THE ENE) IN PORTIONS OF
ARIZONA...WHICH MAY ALLOW FOR MORE FORWARD PROPAGATION...BUT THERE
IS ENOUGH OF A HI-RES MODEL SIGNAL FOR LOCALLY HEAVY RAINFALL FROM
NW-CTRL AZ THAT SOME RISK OF FLASH FLOODING IS ALSO EXPECTED
(ALTHOUGH IT MAY ARISE MORE FROM COLLIDING OUTFLOW OR STORM
MERGERS).

AN EXISTING SLIGHT RISK IN SOUTHERN ARIZONA WAS EXPANDED NORTHWEST
TO INCLUDE MUCH OF NORTHWEST AND CENTRAL ARIZONA...AND INTO THE
SOUTHERN TIP OF NEVADA...ON THE NEW DAY 1 EXCESSIVE RAINFALL
OUTLOOK.

LAMERS

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

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

LAT...LON   39401119 38051140 36251212 35071059 33861043
            33061188 33451329 35111480 36911579 38321500
            38921322