MESOSCALE PRECIPITATION DISCUSSION 0741
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
844 AM EDT SUN OCT 16 2016
 
AREAS AFFECTED...PORTIONS OF CENTRAL CA 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 161243Z - 170013Z
 
SUMMARY...AN INCOMING JET STREAK IS EXPECTED TO LEAD TO INCREASING
LOW-LEVEL INFLOW AND CONVERGENCE INTO A VAGUE POLAR FRONT SEGMENT
TODAY.  HOURLY RAIN RATES REACHING 0.50" ARE POSSIBLE, WHICH COULD
LEAD TO ISSUES IN RUGGED TERRAIN, BURN SCARS, AND WHERE SOILS ARE
SATURATING.

DISCUSSION...BOUNDARY LAYER MOISTURE CONVERGENCE WITHIN A VAGUE
SEGMENT OF THE POLAR FRONT HAS BEEN FOSTERING HOURLY RAIN RATES UP
TO 0.3" AS OF LATE PER MESONET ESTIMATES IN THE COASTAL RANGES AND
SIERRA NEVADA OF CENTRAL CA. FREEZING LEVEL FORECASTS FROM THE 06Z
NAM/06Z GFS AND AVAILABLE SURFACE OBSERVATIONS INDICATE THAT
TEMPERATURES IN THE SIERRA NEVADA ARE ABOVE FREEZING AT THE
MOMENT, WHICH SHOULD PERSIST THROUGH 00Z EXCEPT PERHAPS THE
HIGHEST PEAKS.   PRECIPITABLE WATER VALUES ARE CLOSE TO 1" IN THE
COASTAL RANGE, 1.25" IN THE CENTRAL VALLEY, AND ROUGHLY 0.5" IN
THE SIERRA NEVADA PER GPS ESTIMATES.  THESE VALUES ARE 3+ SIGMAS
ABOVE THE MEAN FOR MID-OCTOBER.  INSTABILITY IS MINIMAL IN THIS
AREA, IN THE 50-100 J/KG RANGE MUCAPE-WISE PER SPC MESOANALYSES
AND 03Z SREF MEAN FORECAST FIELDS.

INFRARED AND WATER VAPOR IMAGERY SHOWS AN UPSTREAM UPPER LEVEL JET
STREAK (150+ KTS) AND RELATED SHORTWAVE NEAR 137W LONGITUDE RACING
FOR THE REGION.  THIS SYSTEM SHOULD ACT TO NEARLY DOUBLE LOW-LEVEL
INFLOW WITH TIME, UP TO 40-45 KTS AT 850 HPA BY 20Z.  RAP MASS
FIELDS INDICATE THAT BOUNDARY LAYER MOISTURE CONVERGENCE IN THE
AREA RISES BETWEEN NOW AND 18Z.  THE 06Z GFS FORECAST SHOWS
INCREASING FRONTOGENESIS IN THE 850-700 HPA LAYER IN THE SIERRA
NEVADA BETWEEN 15-21Z.  PRECIPITABLE WATER VALUES COULD BROACH
1.5" ACROSS THE COASTAL RANGES AND CENTRAL VALLEY AFTER 14Z.   THE
MESOSCALE GUIDANCE HAS A STRONG SIGNAL FOR LOCAL AMOUNTS IN THE
2-4" RANGE ALONG THE COASTAL RANGES AND PORTIONS OF THE SIERRA
NEVADA THROUGH 00Z.  WHILE ARW HOURLY PRECIPITATION FORECASTS KEEP
AMOUNTS GENERALLY UNDER 0.5" AN HOUR, 0.5" AN HOUR RAIN RATES ARE
CONSIDERED POSSIBLE LOCALLY DUE TO THE DEGREE OF LOW-LEVEL INFLOW
AND MOISTURE AVAILABLE; IN REGIMES RELATIVELY DEVOID OF
INSTABILITY, ACHIEVING AMOUNTS 1/3 OF THE AVAILABLE PW PER HOUR
ARE POSSIBLE.  THIS SORT OF RAINFALL COULD LEAD TO ISSUES NEAR
BURN SCARS AND COULD ALSO LEAD TO DEBRIS FLOWS IN/NEAR RUGGED
TERRAIN.

ROTH

ATTN...WFO...HNX...LOX...MTR...REV...STO...

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

LAT...LON   39592083 39312034 38231936 37491891 36701885 
            37472022 37902043 38262083 38492119 38142184 
            37762182 36992133 36702152 36572152 35752099 
            35752133 36562203 36842197 36952225 37352255 
            37892271 38352235 39252170