MESOSCALE PRECIPITATION DISCUSSION 0172
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
239 PM EDT TUE JUL 23 2013
 
AREAS AFFECTED......NEVADA/CALIFORNIA/ARIZONA... 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 231839Z - 232339Z
 
SUMMARY...DEEP MOISTURE NEARING 200 PERCENT OF NORMAL WILL COMBINE
WITH INCREASING CONVECTIVE
INSTABILITY AND WEAK FLOW TO PRODUCE RAINFALL RATES OF 0.75 TO 1.5
INCHES/HR AS SCATTERED 
CONVECTION FORMS THROUGH THE AFTERNOON...POSSIBLY CAUSING FLOODING.

DISCUSSION...A REMNANT UPPER LOW NEAR THE CENTRAL SIERRAS AND DEEP
MOISTURE NEARING 200 PERCENT OF NORMAL ARE NEARLY COINCIDENT WITH
VISIBLE SATELLITE IMAGERY SHOWING RAPIDLY FORMING CUMULUS CLOUDS
ALONG OROGRAPHICALLY-FAVORED DIFFERENTIAL HEATING ZONES...WITH
RADAR BEGINNING TO SHOW THE EARLY STAGES OF CONVECTION.  GOES
SOUNDER ESTIMATES AND RAP ANALYSIS SHOW MOST UNSTABLE AND MIXED
LAYER CAPES FROM AS LOW AS 600 J/KG TO AS HIGH AS 1800 J/KG...WITH
SEVERAL OR MORE HOURS OF INSOLATION REMAINING.  THE 12Z FLAGSTAFF
AND LAS VEGAS RAOBS SHOW WEAK CLOUD LAYER FLOW AVERAGING 10 TO 20
KNOTS...WITH NEARLY UNIDIRECTIONAL INFLOW OF THE SAME MAGNITUDE
SUGGESTING THAT THE ADVECTIVE COMPONENT OF THE CELLS WILL BE
NEARLY OFFSET BY PROPAGATION...RESULTING IN SLOW OR STATIONARY
MOTION.  GIVEN THE INCREASING CAPE VALUES AND ANOMALOUSLY HIGH
MOISTURE-LADEN AIR...CONVECTION THAT DEVELOPS THROUGH THE
AFTERNOON WILL BE CAPABLE OF PRODUCING RAIN RATES OF 0.75 TO 1.5
INCHES/HR...WHICH WILL EXCEED HOURLY FFG VALUES AND MAY CAUSE
FLASH FLOODING.  SOMEWHAT UNCLEAR IS THE POTENTIAL FOR MID/UPPER
LEVEL DRIER AIR ON THE SOUTHERN PERIPHERY OF THE UPPER LOW THAT
COULD SUPPORT CONSIDERABLE ENTRAINMENT OF DRIER AIR WITH
DEVELOPING DOWNDRAFTS...THUS CAUSING ERRATIC THUNDERSTORM MOTION. 

JAMES

ATTN...WFO...VEF...LKN...REV...HNX...STO...

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

LAT...LON   35231781 36951934 37602006 38492070 39991996 40441731
            39591541 38021424 35541418 34591546 35231781