MESOSCALE PRECIPITATION DISCUSSION 0008
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
419 PM EST TUE JAN 12 2016
 
AREAS AFFECTED...NORTHWEST CA...WESTERN OR 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 122115Z - 130600Z
 
SUMMARY...LOCALLY HEAVY RAINS ARE EXPECTED THROUGH THE EVENING
HOURS AS A FRONTAL ZONE ARRIVES IN OFF THE PACIFIC OCEAN
OVERNIGHT. 

DISCUSSION...WV SATELLITE IMAGERY IS SHOWING A STRONG
VORTMAX/SHORTWAVE TROUGH AMPLIFYING AROUND THE BASE OF A LARGER
SCALE TROUGH THAT IS SEEN EXTENDING SOUTH FROM THE GULF OF AK.
THIS ENERGY IS HELPING TO FOCUS MULTIPLE WAVES OF LOW PRESSURE
ALONG A FRONTAL ZONE THAT IS CURRENTLY APPROACHING THE WEST COAST.

A BROAD AREA OF STRONG WARM ADVECTION RAINS HAVE ALREADY BEEN
OVERSPREADING MUCH OF THE PACIFIC NORTHWEST TODAY...AND AS THE
FRONT APPROACHES THE COASTAL RANGES TONIGHT FROM WESTERN OR
SOUTHWARD DOWN THROUGH NORTHWEST CA...THERE WILL BE AN INCREASE IN
SOUTH/SOUTHWESTERLY LOW LEVEL FLOW AND A STRONGER TRANSPORT OF
MOISTURE INTO THE FAVORED HIGHER TERRAIN. THIS WILL DRIVE AN
INCREASE IN RAINFALL RATES THROUGH THE EVENING HOURS.

THE 12Z GFS AND 12Z ECMWF SUGGEST 850 MB WINDS INCREASING TO OVER
60 KTS JUST OFFSHORE THE COASTAL RANGES OF SOUTHWEST OR AND
NORTHWEST CA BY 00Z. THIS WILL BE YIELDING MOISTURE FLUX ANOMALIES
OF 4 TO 5 STANDARD DEVIATIONS ABOVE NORMAL THROUGH THE EVENING
WITHIN AN AXIS OF 1 TO 1.25 INCH PWATS. DESPITE THE VERY HIGH
MOISTURE FLUX ANOMALIES...THE EXPECTED LOW LEVEL FLOW WILL BE
TENDING TO BE PARALLEL TO THE TERRAIN GIVEN THE ORIENTATION OF THE
APPROACHING FRONT...AND THIS WILL TEND TO MITIGATE THE OROGRAPHIC
FORCING THAT WOULD OTHERWISE GREATLY ENHANCE THE RAINFALL RATES.

NEVERTHELESS...THE HIRES GUIDANCE SHOWS A LIKELIHOOD OF RAINFALL
RATES APPROACHING A 0.50 INCH/HOUR FOR THE COASTAL RANGES AND
THESE RATES WILL LIKELY BE EXCEEDED AT LEAST PERIODICALLY FOR THE
FAVORED SOUTHWEST FACING TERRAIN OF SOUTHWEST OR AND NORTHWEST CA
WHERE THERE IS EXPECTED TO BE A BIT STRONGER ONSHORE FLOW.

MULTIPLE RUNS OF THE HRRR ALONG WITH THE 12Z ARW AND 12Z NMMB
SUGGEST ADDITIONAL RAINFALL AMOUNTS OF 1 TO 2 INCHES IN GENERAL
FOR THE COASTAL RANGES THROUGH 06Z...BUT HEAVIER AMOUNTS OF
LOCALLY 3 TO 4 INCHES IS ANTICIPATED IN PARTICULAR FOR THE
SOUTHWEST OR COASTAL RANGES. ADDITIONAL HEAVY RAINFALL WILL BE
EXPECTED AFTER THIS PERIOD FOR SOUTHWEST OR AND ESPECIALLY DOWN
INTO NORTHERN CA AS THE AFOREMENTIONED FRONT DROPS SOUTHEAST
ACROSS THE AREA.

ORRISON

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

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

LAT...LON   45892391 45832329 44742324 43672328 42822319 
            41792310 40732292 39432259 39102325 39332383 
            39902425 40442443 41392426 42362454 42972451 
            44102424 45222407