MESOSCALE PRECIPITATION DISCUSSION 0675
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
343 PM EST SAT DEC 12 2015
 
AREAS AFFECTED...WESTERN OR...NORTHWEST CA 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 122035Z - 130900Z
 
SUMMARY...HEAVY RAINS ASSOCIATED WITH A NEW ATMOSPHERIC RIVER
EVENT WILL OVERSPREAD WESTERN OR THIS AFTERNOON...AND THEN SHIFT
SOUTH OVERNIGHT INTO NORTHWEST CA.

DISCUSSION...THE LATEST WV SATELLITE IMAGERY SHOWS ANOTHER
WELL-DEFINED FRONTAL SYSTEM MOVING ACROSS THE EASTERN PACIFIC WITH
A RAPIDLY DEEPENING LOW CENTER OFFSHORE BRITISH COLUMBIA...AND
THIS WILL BE IMPACTING THE PACIFIC NORTHWEST THIS EVENING AND
OVERNIGHT. AN ANOMALOUS SURGE OF MOISTURE ALONG AND JUST AHEAD OF
THE FRONTAL ZONE WILL AIM INTO THE WESTERN OR AND NORTHWEST CA
COASTAL RANGES...WITH INITIAL IMPACTS FOCUSED FARTHER NORTH FOR
COASTAL OR AND THEN GRADUALLY SHIFTING SOUTH THROUGH THE EVENING
INTO NORTHWEST CA AS THE FRONTAL ZONE ADVANCES EAST-SOUTHEAST AND
INLAND. THIS WILL BE FACILITATED BY THE EASTWARD ADVANCE OF A 130
TO 150 KT 250 MB JET STREAK WHICH WILL BE AMPLIFYING A LARGE SCALE
UPPER TROUGH JUST OFFSHORE THE WEST COAST BY SUN MORNING.

UPSTREAM PWATS ALONG THE FRONT ARE RUNNING BETWEEN 1 TO 1.2 INCHES
WITH SOME OF THESE VALUES RUNNING ABOUT 200% OF NORMAL FOR THE
MIDDLE OF DECEMBER. THE STRONG LOW AND MID LEVEL JET ENERGY IS
FOSTERING 850 MB/700 MB MOISTURE FLUX ANOMALIES OF 2 TO 3 STANDARD
DEVIATIONS ABOVE NORMAL. THE NOSE OF THIS WILL COME INTO SOUTHWEST
OR AND NORTHWEST CA AFTER 21Z AS A WARM FRONT ARRIVES AND THIS
WILL FOSTER A NOTABLE INCREASE IN RAINFALL RATES. AN 1850Z AMSU
PASS SHOWED RAIN RATES ALREADY OF 0.25 TO 0.50 INCHES/HR ALONG THE
WARM FRONT APPROACHING THE OR COASTLINE. THE EXPECTATION IS THAT
TOWARD 00Z AND THEREAFTER THE HEAVIEST RAIN RATES WILL FOCUS OVER
SOUTHWEST OR AND NORTHWEST CA...WITH RATES OVER THE IMMEDIATE WEST
AND SOUTHWEST FACING SLOPES OF THE COASTAL RANGES REACHING AS MUCH
AS 0.50 TO 0.75 INCHES/HR.

THE HIRES MODEL GUIDANCE INCLUDING THE LAST COUPLE RUNS OF THE
HRRR AND ALSO THE 12Z ARW AND 12Z NMMB SUGGEST A FAIRLY LARGE AREA
OF 1 TO 3 INCH RAINFALL AMOUNTS WITHIN THE THREAT AREA...BUT
ADDITIONAL AMOUNTS WILL BE AS HIGH AS 4 TO 6 INCHES THROUGH 09Z
WITH ISOLATED HEAVIER AMOUNTS FOR FAR SOUTHWEST OR AND NORTHWEST
CA WHERE OROGRAPHIC FORCING WILL BE ENHANCED THE MOIST AND
COINCIDING WITH THE STRONGEST MOISTURE TRANSPORT. WILL CONTINUE TO
MONITOR.

ORRISON

ATTN...WFO...EKA...MFR...PQR...

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

LAT...LON   44932352 44182339 43182351 42442362 41402354 
            41092372 41112410 41712442 42652456 43602432 
            44882401