MESOSCALE PRECIPITATION DISCUSSION 0624
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
519 PM EDT FRI OCT 30 2015
 
AREAS AFFECTED...WESTERN SLOPES OF THE WASHINGTON CASCADES 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 302115Z - 310115Z
 
SUMMARY...STRONG MOIST DEEP LAYER PACIFIC MOISTURE FETCH FAVORING
LOCALLY VERY HEAVY RAINS. EXPECTING SOME INCREASED POTENTIAL FOR
RUNOFF PROBLEMS THIS EVENING.

DISCUSSION...A WELL-DEFINED ATMOSPHERIC RIVER CONTINUES TO AIM
INTO THE PACIFIC NORTHWEST...WITH AN AXIS OF OFFSHORE PWATS AS PER
GOES-SOUNDER DATA THAT IS AS HIGH AS 1.25 INCHES AND EXTENDING
WELL UPSTREAM TO THE SOUTHWEST IN ASSOCIATION WITH A STRONG UPPER
LEVEL JET AND ASSOCIATED FRONTAL ZONE.

THE STRONG DEEP LAYER LOW TO MID LEVEL WESTERLY PACIFIC FLOW IS
NEARLY PERPENDICULAR TO THE WESTERN SLOPES OF THE CASCADES RIGHT
NOW AND IS COINCIDING WITH THE LEADING EDGE OF WARM FRONT WHICH IS
STALLING OUT AND ORIENTED WEST/EAST ACROSS WESTERN WA. THE DEEPER
LAYER FLOW ALONG THIS FRONT IS FORECAST TO INCREASE SOMEWHAT IN
INTENSITY THROUGH THE EVENING HOURS AND THUS ENHANCING THE
MOISTURE TRANSPORT. H8/H7 MOISTURE FLUX ANOMALIES AS PER THE 12Z
GFS ARE PROGGED TO BE AS HIGH AS 3 TO 4 STANDARD DEVIATIONS ABOVE
NORMAL THROUGH 00Z...AND LOCAL VWP DATA SHOWS A PERSISTENT 40 TO
45 KT WESTERLY FETCH IN THE H8/H7 LAYER ACROSS WESTERN WA THE LAST
FEW HOURS.

THE RELATIVELY UNIDIRECTIONAL LOW TO MID LEVEL WESTERLY FETCH
PARALLELING THE FRONT MAY ALLOW FOR SOME LOCALLY INTENSE AND
ORGANIZED BANDS OF VERY HEAVY SHOWERS AND PERHAPS EVEN A COUPLE OF
THUNDERSTORMS OVER THE NEXT COUPLE OF HOURS. IN FACT...SURFACE
OBSERVATIONS SHOW A RATHER WELL-DEFINED LOW LEVEL CONVERGENCE ZONE
ASSOCIATED WITH THE FRONT THAT IS FACILITATING A NARROW BUT
INTENSE BAND OF VERY HEAVY RAIN ACROSS SOUTHERN SNOHOMISH COUNTY
WHERE RAINFALL RATES OF LOCALLY OVER 1 INCH/HR ARE BEING DENOTED
IN LOCAL DUAL-POL RADAR IMAGES.

THE LAST FEW RUNS OF THE HRRR SUGGEST THE ADDITIONAL POTENTIAL FOR
THESE BANDS OF INTENSE RAINS THROUGH THE EARLY EVENING HOURS. THIS
MAY BE ENOUGH TO ENCOURAGE SOME DEBRIS FLOWS AND LOCALLY INTENSE
RUNOFF GIVEN THE STRONG UPSLOPE FLOW AND ORIENTATION INTO WESTERN
SLOPES OF THE CASCADES. WILL CONTINUE TO MONITOR.

ORRISON

ATTN...WFO...PDT...PQR...SEW...

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

LAT...LON   48242142 47442127 46052172 45762215 45752250 
            46252272 47072231 48132212