MESOSCALE PRECIPITATION DISCUSSION 0040
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
153 AM EST SAT MAR 05 2016
 
AREAS AFFECTED...NORTHWEST CA...SOUTHWEST OR 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 050645Z - 051800Z
 
SUMMARY...A PERIOD OF HEAVY RAIN IS EXPECTED TO ARRIVE ALONG WITH
A FRONT OVER THE NEXT FEW HOURS. A STRONGER UPSTREAM SYSTEM WITH
EVEN HEAVIER RAIN RATES IS EXPECTED TO ARRIVE BY AROUND 18Z.

DISCUSSION...AN AMPLIFYING SHORTWAVE TROUGH AND DEEPENING SURFACE
LOW IS LIFTING NORTH-NORTHEAST UP OFF THE WEST COAST WITH A
TRAILING COLD FRONT THAT EXTENDS WELL SOUTH TO OFFSHORE NORTHWEST
CA AND THEN WEST ACROSS THE EAST-CENTRAL PACIFIC OCEAN. A
RELATIVELY DEEP MOISTURE PLUME IS POOLED UP ALONG THE FRONT WITH A
RIBBON OF 1 TO 1.2 INCH PWATS AS PER BLENDED TPW IMAGERY AND OVER
THE NEXT FEW HOURS THIS FRONTAL ZONE AND POOL OF MOISTURE WILL BE
OVERSPREADING THE COASTAL RANGES OF SOUTHWEST OR/NORTHWEST CA.

THE LATEST KBHX VWP DATA SHOWS A SOUTHWESTERLY LOW LEVEL JET
ALREADY APPROACHING 50 KTS...AND THIS WILL ENHANCE THE MOISTURE
TRANSPORT INTO THE TERRAIN AS THE FRONT ARRIVES AND SHOULD THUS
ENHANCE RAINFALL RATES. THE FLOW WILL NOT BE IDEAL FOR MAXIMIZING
OROGRAPHIC ASCENT...BUT NEVERTHELESS SUFFICIENT ONSHORE/UPSLOPE
FLOW AND SURFACE FRONTAL CONVERGENCE IS EXPECTED FOR LOCALLY HEAVY
RATES THAT MAY APPROACH OF LOCALLY EXCEED A 0.50"/HR. LATEST AMSU
PASSES HAVE BEEN SHOWING RATES OFFSHORE OF 0.20" TO 0.40"/HR...BUT
EXPECT THESE RATES TO INCREASE GIVEN THE INFLUENCE OF HIGHER
TERRAIN. 

THE LATEST HRRR SUGGESTS RAINS BEGINNING TO ARRIVE IN EARNEST BY
AROUND 08Z...WITH RATES LOCALLY EXCEEDING A 0.50"/HR IN THE 09Z TO
12Z TIMEFRAME AS THE FRONT PUSHES INLAND. OVERALL THE HEAVIEST
RAINFALL AMOUNTS ARE EXPECTED TO BE OVER NORTHWEST CA IN VICINITY
OF THE COASTAL RANGES AND THEN FARTHER INLAND OVER THE
SHASTA/SISKIYOU RANGE...WHERE TOTALS THROUGH 15Z MAY APPROACH 2
INCHES. AFTER 12Z...THERE SHOULD BE A GRADUAL SUBSIDING OF THE
RAINS AS THE FRONT PUSHES INLAND AND LOSES ITS IDENTITY.

ADDITIONAL HEAVY RAINS ARE EXPECTED ACROSS THIS REGION THOUGH
BEGINNING BY AROUND 18Z AS AN UPSTREAM AMPLIFYING SHORTWAVE TROUGH
NEAR 40N AND 145W ARRIVES AND RESULTS IN A STRONGER RESURGENCE OF
MOISTURE TRANSPORT/ONSHORE FLOW AND FORCING. ALREADY AMSU DATA IS
SHOWING POCKETS OF RAIN RATES REACHING 0.50" TO 0.60"/HR WITH THIS
NEXT SYSTEM. WILL CONTINUE TO MONITOR THIS NEXT SHORTWAVE THROUGH
THE MORNING AND WITH THE EXPECTATION THAT THIS SYSTEM WILL BEGIN
TO EVENTUALLY DRIVE SOME RUNOFF/FLOODING CONCERNS.

ORRISON

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

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

LAT...LON   43412345 42752264 42152204 41572152 40952157 
            40412208 40142262 39662308 39602378 40452443 
            41842443 43182432