MESOSCALE PRECIPITATION DISCUSSION 0674
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
404 AM EST SAT DEC 12 2015
 
AREAS AFFECTED...SISKIYOUS...OREGON COASTAL RANGE AND OREGON
CASCADES 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 120903Z - 122100Z
 
SUMMARY...A MOISTURE PLUME ACCOMPANIED BY A SURFACE FRONTAL SYSTEM
WILL BEGIN TO MOVE ONSHORE THIS AFTERNOON -- BRINGING THE
POTENTIAL FOR HEAVY RAINFALL TO THE SISKIYOUS...OREGON COASTAL
RANGE...AND THE SOUTHERN CASCADES. CURRENT TIMING ALONG WITH MODEL
DATA SUGGEST RAINFALL RATES SHOULD BEGIN TO INCREASE AFTER 21Z.
LATEST AMSU DATA SUGGEST CURRENT RAINFALL RATES IN THIS PLUME ARE
GENERALLY A QUARTER INCH OR LESS PER HOUR. IMPROVED
MID/UPPER-LEVEL FORCING ALONG WITH OROGRAPHIC EFFECTS SHOULD
RESULT IN RAINAFLL RATES INCREASING TO NEAR 0.50 IN/HR OVER THESE
AREAS AFTER 21Z.

DISCUSSION...CURRENT WATER VAPOR/IR SATELLITE IMAGERY SHOW A
WELL-DEFINED LOW PRESSURE SYSTEM OVER THE NORTHEASTERN PACIFIC
OCEAN EARLY THIS MORNING. BLENDED TPW DATA INDICATE PWAT VALUES IN
MOISTURE PLUME AHEAD OF THIS SYSTEM ARE GENERALLY IN THE 1.25-1.50
INCH RANGE. INSTABILITY IS LIMITED AND RATHER SHALLOW AS INDICATED
BY RAP/GFS DATA AND A LACK OF LIGHTNING AT THE CURRENT TIME.

WARM FRONTAL PRECIPITATION WILL ARRIVE ACROSS THE NORTHWESTERN
U.S. THIS MORNING...WITH RAINFALL RATES GENERALLY LESS THAN 0.25
IN/HR. THE MAIN EVENT WILL BEGIN IN THE 21Z-00Z TIME FRAME AS THE
PLUME OF DEEP MOISTURE ARRIVES AT THE COAST AND LOW-LEVEL ONSHORE
FLOW INCREASES TO NEAR 50 KNOTS. AT THE SAME TIME...MID/UPPER-LVL
FORCING WILL IMPROVE AS HEIGHTS FALL AND A MID/UPPER-LVL LOW
CLOSES OFF WEST OF BRITISH COLUMBIA. AS A RESULT...BROAD
UPPER-LEVEL DIVERGENCE WILL INCREASE ACROSS THE NORTHWESTERN U.S.
WHILE CURRENT AMSU INDICATE RAINFALL RATES IN THIS MOISTURE PLUME
ARE GENERALLY 0.25 IN/HR OR LESS...THESE SHOULD INCREASE TO NEAR
0.50 IN/HR AS THE PLUME REACHES THE COAST THIS AFTERNOON DUE TO
OROGRAPHIC EFFECTS AND THE IMPROVING MID/UPPER-LEVEL DYNAMICS. 1-2
INCHES OF RAINFALL ARE POSSIBLE THROUGH 00Z...WITH ADDITIONAL
HEAVIER AMOUNTS EXPECTED AFTER 00Z.

RYAN

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

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

LAT...LON   45512310 43792217 42202254 41412319 40632381 
            41112484 43472542 45222440