MESOSCALE PRECIPITATION DISCUSSION 0620
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
713 AM EDT FRI OCT 30 2015
 
AREAS AFFECTED...TERRAIN OF SOUTHWEST WA & NORTHWEST OR 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 301112Z - 301712Z
 
SUMMARY...AN ATMOSPHERIC RIVER IS AIMING TOWARDS SOUTHWEST
WA/NORTHWEST OR EARLY THIS MORNING.  HOURLY RAIN RATES UP TO 1"
ARE CONSIDERED POSSIBLE, WHICH COULD LEAD TO DEBRIS FLOWS.

DISCUSSION...INFRARED AND WATER VAPOR SATELLITE IMAGERY SHOW AN
ATMOSPHERIC RIVER AIMING TOWARDS SOUTHWEST WA/NORTHWEST OR. 
PRECIPITABLE WATER VALUES OF 1.1-1.2" LIE ACROSS THE REGION,
AROUND TWO SIGMAS ABOVE THE MEAN FOR THE REGION FOR LATE OCTOBER. 
INFLOW AT 850 HPA HAS RAMPED UP TO 30 KTS PER THE PORTLAND OR VAD
WIND PROFILE.  MUCAPES OF 100-300 J/KG LIE OFFSHORE, PER THE SPC
MESOANALYSIS.  THE HIGHEST RAIN RATES NOTED SO FAR PER
MESONET/AIRPORT OBSERVATIONS ARE AROUND 0.2" AN HOUR.

INFLOW AT 850 IS EXPECTED TO STRENGTHEN FURTHER INTO THE 50-60 KT
RANGE (JUST EXCEEDING THE MEAN 850-400 WIND) WITHIN THE NEXT FEW
HOURS AND PERSIST PAST NOON PDT, WHICH SHOULD INCREASE MOISTURE
TRANSPORT INTO THE REGION, WITH PRECIPITABLE WATER VALUES
INCREASING INTO THE 1.25-1.5" RANGE WITHIN THE COASTAL RANGES AND
1" IN THE CASCADES.  THE MUCAPE OFFSHORE IS ALSO ANTICIPATED TO
MAKE LANDFALL IN THE NEXT COUPLE HOURS, WHICH WOULD INCREASE
HOURLY RAIN RATES.  RULES OF THUMB REGARDING HEAVY RAINFALL INTO
THE TOPOGRAPHY OF THE WEST WITH THIS AMOUNT OF MOISTURE AND
LOW-LEVEL INFLOW INDICATE 0.5" AN HOUR RATES ARE ANTICIPATED EVEN
WITHOUT ANY INSTABILITY.  THE CAM GUIDANCE INDICATES LOCAL AMOUNTS
OF ~3" IS POSSIBLE OVER THE NEXT SIX HOURS, WHICH WITHIN A SIX
HOUR PERIOD HAS A RECURRENCE INTERVAL OF CLOSE TO 100 YEARS WITHIN
THE CASCADES.  CONSIDERING THE INSTABILITY MOVING INTO THE REGION,
BELIEVE HOURLY RAIN RATES UP TO 1" ARE POSSIBLE OVER THE NEXT
SEVERAL HOURS, WHICH COULD CREATE LOCAL ISSUES SUCH AS DEBRIS
FLOWS.  WHILE THIS IS EXPECTED TO BE A LONGER DURATION RAIN EVENT,
FLASH FLOODING IS POSSIBLE.

ROTH

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

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

LAT...LON   48082348 47922302 47422332 47112307 46742318 
            46362299 46082293 46632260 47152230 47932163 
            47802084 46412106 45642125 44752155 44242209 
            44392255 45262246 45442244 45832266 45612318 
            45492322 44262350 44292405 44942405 45952390 
            46502392 47462396 47982413