MESOSCALE PRECIPITATION DISCUSSION 0629
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
706 AM EDT SAT OCT 31 2015
 
AREAS AFFECTED...MOUNTAINS OF WESTERN WA/ NORTHWEST OR 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 311105Z - 311705Z
 
SUMMARY...INCREASING RAIN RATES WITHIN AN ATMOSPHERIC RIVER COULD
CAUSE DEBRIS FLOWS IN THE MOUNTAINS.

DISCUSSION...AN ATMOSPHERIC RIVER STRETCHING FROM NORTHWEST OF HI
INTO WA HAS BEEN EXHIBITING INCREASING RAIN RATES OVER THE PAST
FEW HOURS.  MESONET OBSERVATIONS AND DUAL POL RADAR IMAGERY REVEAL
LOCALIZED 0.50"+ RAIN RATES ACROSS THE TOPOGRAPHY OF WESTERN WA. 
VAD WIND PROFILES SHOW 850 HPA INFLOW OF 30-40 KTS ACROSS THE
REGION.  PRECIPITABLE WATER VALUES OF 1.1-1.4" LIE ACROSS THE
REGION, OVER TWO SIGMAS ABOVE THE NORM FOR THE END OF OCTOBER. 
SPC MESOANALYSES SHOW AN AREA OF WEAK INSTABILITY APPROACHING
WESTERN WA, WHERE MUCAPES ARE 100-300 J/KG.

RAP GUIDANCE INDICATES THAT LOW LEVEL INFLOW INCREASES TOWARDS 60
KTS OVER THE NEXT FEW HOURS, WHICH ALONG WITH THE INVADING
INSTABILITY SHOULD FURTHER INCREASE RAIN RATES.  THIS IS IMPLIED
BY THE THREE HOUR NEIGHBORHOOD PROBABILITIES OF EXCEEDING FLASH
FLOOD GUIDANCE, WHICH MAXIMIZE ALONG THE EASTERN CASCADES BY 15Z. 
THE CAM GUIDANCE THIS MORNING IS MORE EMPHATIC THAN THIS TIME
YESTERDAY, SHOWING LOCAL AMOUNTS IN THE 4" RANGE DURING THE NEXT
SIX HOURS.  PRECIPITABLE WATER VALUES ARE FORECAST TO PEAK AROUND
1.5" WITHIN THE NEXT 2-3 HOURS, BEFORE SLOWLY FADING THEREAFTER. 
WITH INSTABILITY APPROACHING THE REGION, HOURLY RAIN RATES UP TO
1" ARE POSSIBLE, WHICH COULD CAUSE DEBRIS FLOWS.  FOR THE MOST
PART, THIS REMAINS A LONG DURATION RAIN EVENT. ACROSS WA, THE
LONGER DURATION RAIN EVENT SHOULD BE FADING LATER THIS AFTERNOON
AS THE LOW-LEVEL INFLOW/ ATMOSPHERIC RIVER SHIFTS SOUTHWARD INTO
OR WITH TIME.

ROTH

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

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

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            45312192 46052272 47302198 47822240 47522287 
            46332303 45762330 45422354 45332377 45592410 
            46092409 47302452 48282481 48472445 48312295 
            49062300