MESOSCALE PRECIPITATION DISCUSSION 0366
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1256 PM EDT SUN JUL 19 2015
 
AREAS AFFECTED...PORTIONS OF SOUTHERN & CENTRAL CA 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 191700Z - 192300Z
 
SUMMARY...THUNDERSTORMS WITH HEAVY RAINFALL ARE EXPECTED TO
DEVELOP WITHIN AN ANOMALOUSLY MOIST AND UNSTABLE AIRMASS.  FLASH
FLOODING, WITH DEBRIS FLOWS, IS POSSIBLE.

DISCUSSION...PRECIPITABLE WATER VALUES OF GENERALLY 1.25-1.75",
WITH 1.75-2.25" VALUES CLOSE TO VANDENBERG AIR BASE PER GPS
INFORMATION, HAVE INVADED MUCH OF SOUTHERN AND CENTRAL CA DUE TO
THE COMBINATION OF AN EXPANDING SUBTROPICAL RIDGE ACROSS THE
SOUTHERN ROCKIES AND THE APPROACH OF POST-TROPICAL CYCLONE DOLORES
FROM THE SOUTH.  THESE VALUES ARE 2-4 SIGMAS ABOVE THE MEAN OR
NEAR THE RECORD FOR MID-JULY.  INFLOW AT 700 HPA AND 850 HPA IS
STRONGEST NEAR THE COAST, CLOSE TO 20 KTS.  THE LAST OF THE
OVERNIGHT CIN IS ERODING AT THIS TIME, WHICH SHOULD LEAD TO
THUNDERSTORM INITIATION ACROSS THE COASTAL RANGES AND SIERRA
NEVADA SHORTLY.  A COUPLE POCKETS OF 500-1000 J/KG MLCAPE HAVE
DEVELOPED IN THE SOUTHERN SIERRA NEVADA RANGE AND ACROSS SOUTHERN
CA.  EARLY MORNING ACTIVITY ALONG THE CENTRAL CA COAST DISPLAYED
RAINFALL RATES ABOVE 1.25" PER HOUR AT PASO ROBLES, WHICH COULD BE
A SIGN OF THINGS TO COME.

THE 06Z SSEO NEIGHBORHOOD PROBABILITIES HINT THAT THUNDERSTORM
DEVELOPMENT AND COVERAGE EXPANDS  AROUND 20Z, WITH THE GREATEST
CHANCE OF EXCESSIVE RAINFALL IN THE TERRAIN EAST OF LOS ANGELES
AND SAN DIEGO BEYOND 22Z AND WITHIN THE SIERRA NEVADA BETWEEN
19-23Z.  SKIES ARE CLEARING IN THE WAKE OF EARLY MORNING
CONVECTION MOVING UP THE CENTRAL CA COAST, WHICH SHOULD ALLOW
MLCAPES TO RISE FURTHER DURING THE DAY TODAY.  CONSIDERING THE
AVAILABLE MOISTURE, HOURLY RAIN RATES UP TO 1.5" ARE POSSIBLE. 
THE MESOSCALE QPF GUIDANCE INDICATES LOCAL AMOUNTS OF 2-3". 
HOWEVER, GUIDANCE AMOUNTS WERE UNDERDONE IN AREAS NEAR THE
AZ/CA/NV BORDER YESTERDAY.  DIRECTIONAL WIND SHEAR WITH HEIGHT
EXISTS WHICH IS EXPECTED TO CAUSE A NORTHWARD MOVEMENT TO
ORGANIZED THUNDERSTORMS AT 10-15 KTS; LESS ORGANIZED ACTIVITY
SHOULD MOVE NORTHEAST AT 10-15 KTS WITH THE MEAN WIND.  THIS
SHOULD ALLOW CONVECTION WHICH FORMS NEAR THE COASTAL RANGE TO
DRIFT AND/OR DIRECT OUTFLOW BOUNDARIES INTO THE SAN JOAQUIN
VALLEY.  THUNDERSTORMS CAUSED BY INSTABILITY, ANOMALOUS MOISTURE,
AND CONVERGENCE AT 850/700 HPA NEAR THE SIERRA NEVADA COULD ALSO
BECOME PROBLEMATIC.  CELL TRAINING IS EXPECTED TO BE THE MAIN
FLASH FLOOD THREAT.  OVERALL, THE FLOW PATTERN APPEARS TO BE A
WEAKER VERSION OF THE PATTERN PRESENT DURING TROPICAL CYCLONE
OLIVIA'S APPROACH TO THIS REGION IN LATE SEPTEMBER 1982, WHICH
CAUSED LOCAL AMOUNTS OF 7" IN THE SIERRA NEVADA.  FLASH FLOODING,
WITH ASSOCIATED DEBRIS FLOWS, IS POSSIBLE.

ROTH

ATTN...WFO...HNX...LOX...MTR...PSR...REV...SGX...STO...VEF...

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

LAT...LON   38671988 38661912 37931806 37001722 36681735 
            35951691 34821628 33951542 33451548 33671609 
            33171605 32671595 32491634 32481682 32791697 
            33581759 34251887 34471962 34521978 34722019 
            35382057 35672056 36312087 36792044 35681959 
            35671901 35811905 36151896 36901936 37681990 
            38311995