MESOSCALE PRECIPITATION DISCUSSION 0678
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
346 AM EST SUN DEC 13 2015
 
AREAS AFFECTED...NWRN CA...EXTREME SWRN OR 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 130845Z - 131800Z
 
SUMMARY...HEAVY RAINS ASSOCIATED WITH AN ATMOSPHERIC RIVER EVENT
WILL BEGIN TO SHIFT SOUTHWARD MORE QUICKLY OVER THE NEXT 6-9
HOURS. RAIN RATES MAY INCREASE TO 0.50 IN/HR AS UPPER-LEVEL
DYNAMICS BECOME MORE FAVORABLE THIS MORNING. RAINFALL TOTALS WILL
BE LIMITED BY DURATION ONCE THE PLUME BEGINS TO SHIFT SOUTHWARD
LATER THIS MORNING. AN ADDITIONAL 1-3 INCHES ARE POSSIBLE ACROSS
THE FAVORED TERRAIN OF NWRN CA AND EXTREME SWRN OREGON BEFORE THE
HEAVIEST PRECIPITATION SHIFTS SOUTH OF THE OUTLINED AREA AFTER 18Z.

DISCUSSION...A PLUME OF MOISTURE CONTINUES TO STREAM INTO THE
NORTHWESTERN U.S. EARLY THIS MORNING IN ASSOCIATION WITH A STRONG
PACIFIC LOW PRESSURE SYSTEM THAT HAS MOVED INTO BRITISH COLUMBIA.
FAVORABLE ONSHORE FLOW OF 40-50 KT IS FORECAST TO CONTINUE THROUGH
THE NEXT SEVERAL HOURS. AN AMPLIFYING SHORTWAVE IS LOCATED TO THE
WEST OF THE REGION EARLY THIS MORNING AND WILL APPROACH THE
NORTHWEST DURING THE 12-18Z TIME FRAME. THIS SHORTWAVE ALONG WITH
A FAVORABLY POSITIONED UPPER-LEVEL JET STREAK WILL ENHANCE
DIVERGENCE ALOFT OVER THE OUTLINED AREA...AND ALLOW FOR RAINFALL
RATES TO INCREASE. WHILE RAINFALL RATES HAD BEEN LOWER OVER THE
PAST SEVERAL HOURS...IN THE 0.10-0.25 IN/HR RANGE...THEY APPEAR TO
BE PICKING UP AGAIN BASED ON A RECENT REPORT OF 0.50 INCH IN ONE
HOUR NEAR CRESCENT CITY. THIS TREND SHOULD CONTINUE OVER THE NEXT
SEVERAL HOURS WITH RAINFALL RATES INCREASING TO 0.50 IN/HR OR
SLIGHTLY HIGHER OVER MUCH OF THE FAVORED TERRAIN OF NWRN CA AND
EXTREME SWRN OR. AN ADDITIONAL 1-3 INCHES OF RAIN IS POSSIBLE
THROUGH 18Z.

THE PLUME OF MOISTURE AND PRECIPITATION WILL BEGIN TO SHIFT SOUTH
MORE RAPIDLY AS THE UPPER SHORTWAVE MOVES INLAND THIS
AFTERNOON...BRINGING AN END TO WIDESPREAD HIGHER RAIN RATES OVER
THE AREA.

RYAN

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

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

LAT...LON   42192405 41792335 40602297 39592302 39122375 
            39412418 40302455 41502472