MESOSCALE PRECIPITATION DISCUSSION 0022
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
356 AM EST FRI JAN 29 2016
 
AREAS AFFECTED...NORTHERN CA AND FAR SOUTHWEST OR 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 290855Z - 291800Z
 
SUMMARY..AN APPROACHING ATMOSPHERIC RIVER WILL BRING INCREASING
RAINFALL RATES UP TO 0.50 INCHES AN HOUR TO PORTIONS OF THE CA AND
FAR SOUTHWEST OR COASTAL RANGES...WITH TOTALS THROUGH 18Z UP TO
2-3 INCHES.

DISCUSSION...WATER VAPOR IMAGERY SHOWS A STRONG SHORTWAVE TRACKING
NORTHEAST TOWARDS BRITISH COLUMBIA. THIS SYSTEM IS DRAGGING A
FRONT EASTWARD TOWARDS THE PACIFIC NORTHWEST. AS THE WAVE SHEARS
OFF TO THE NORTHEAST THE FRONT WILL SLOWLY SAG SOUTHEAST...WITH
LOW LEVEL FLOW BECOMING ALIGNED WITH THE FRONTAL BOUNDARY FROM
WEST TO EAST. LOOKING AT 850 MB FLOW BECOMING MORE SOUTHWEST TO
NORTHEAST AROUND 30-40 KTS...AND 700 MB FLOW AS STRONG AS 50 KTS.
700 MB FLOW OF THIS MAGNITUDE IS AROUND THE 98TH PERCENTILE FOR
THIS TIME OF YEAR...SUGGESTING STRONG UPSLOPE FLOW IS LIKELY.
SATELLITE IMAGERY ALSO SHOWS A WELL DEFINED ATMOSPHERIC RIVER
ALIGNING WITH THIS LOW LEVEL FLOW...WITH PRECIPITABLE WATER VALUES
AS HIGH AS 1.6" OFF THE NORTHERN CA COAST PER BLENDED TPW
PRODUCT...WHICH WILL BE AROUND 3 STANDARD DEVIATIONS ABOVE NORMAL
AS IT MOVES INTO NORTHERN CA. THE COMBINATION OF THIS STRONG LOW
LEVEL FLOW AND HIGH PRECIPITABLE WATER WILL RESULT IN INTEGRATED
WATER VAPOR TRANSPORT APPROACHING THE 99TH PERCENTILE AS THE
ATMOSPHERIC RIVER MOVES ASHORE.

THE LATEST AMSU DERIVED RAINFALL RATES GENERALLY SHOW AMOUNTS
BELOW 0.25" AN HOUR OFFSHORE...SO NOT TOO IMPRESSIVE. ALTHOUGH
RECENT COOLING CLOUD TOPS SEEN ON IR IMAGERY MAY BE A SIGN OF
INCREASING PRECIPITATION INTENSITY SINCE THE MOST RECENT AMSU PASS
WHICH WAS AT 05-06Z. THE BEST SYNOPTIC FORCING FROM THE WAVE
SHOULD STAY NORTH OF THIS AREA...AND UPPER JET SUPPORT REALLY DOES
NOT ARRIVE UNTIL AFTER 18Z. SO THESE TWO FACTORS COULD HELP KEEP
RAINFALL TOTALS FROM GETTING TOO HIGH. ALSO THE SYSTEM WILL
EVENTUALLY BE PROGRESSIVE...WITH THE SHORTWAVE ENERGY HELPING PUSH
THE MOISTURE PLUME SOUTH...WHICH SHOULD FOCUS THE HEAVIER RAINFALL
SOUTH INTO PORTIONS OF THE SIERRA NEVADA AFTER 19Z OR SO. 

STILL GOOD AGREEMENT AMONG THE HIGH RES GUIDANCE FOR RAINFALL
RATES INCREASING ABOVE 0.25"...AND LOCALLY AS HIGH AS 0.5" AN HOUR
BY AROUND 12Z AS THE ONSHORE UPSLOPE FLOW INCREASES AND TURNS MORE
WESTERLY AS THE ATMOSPHERIC RIVER MOVES ASHORE. BASED ON LATEST
SATELLITE TRENDS...THINK THE HIGH RES GUIDANCE MAY BE A FEW HOURS
TOO SLOW BRINGING SOME OF THESE HIGHER RATES ASHORE...WITH LATEST
RADAR IMAGERY ALSO SHOWING THE EVENT ABOUT TO BEGIN BY 09Z.

CHENARD

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

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

LAT...LON   42952448 42542379 42042346 41732299 41522259 
            41542197 41352178 40822166 40152222 39032243 
            38722319 38852332 39302381 40382439 41812433