MESOSCALE PRECIPITATION DISCUSSION 0631
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
103 PM EDT SAT OCT 31 2015
 
AREAS AFFECTED...MOUNTAINS OF WESTERN WA AND NORTHWEST OR 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 311701Z - 312201Z
 
SUMMARY...A SHORTWAVE MOVING TOWARDS THE COAST OF WA IN A COUPLE
HOURS WILL BEGIN TO SHIFT THE ATMOSPHERIC RIVER SOUTHWARD INTO OR.
 UNTIL THEN, HOURLY RAIN RATES UP TO 1" COULD LEAD TO DEBRIS FLOWS.

DISCUSSION...AN UPSTREAM SHORTWAVE/BAROCLINIC LEAF IS MOVING
QUICKLY TOWARDS THE COAST OF THE PACIFIC NORTHWEST, NEARING THE
128TH MERIDIAN AT THIS TIME AND IS SLATED TO MOVE ASHORE IN A
COUPLE HOURS.  MESONET OBSERVATIONS ALOFT WITH DUAL POL RADAR
IMAGERY HAVE SHOWN HOURLY RAIN RATES OF 0.4-0.7" AS OF LATE, WITH
SOME OF THE HIGHER RATES IN NORTHWEST WA CASCADES.  REFLECTIVITY
DATA ALONG WITH IR SATELLITE IMAGERY INDICATE SOME CONVECTIVE
ELEMENTS APPEAR TO BE BECOMING INCREASINGLY INVOLVED WITHIN THE
STRATIFORM RAIN SHIELD.  INFLOW AT 850 HPA OF 25-50 KTS CONTINUES
TO STREAM MOISTURE INTO THE REGION, WITH MORNING RAOBS INDICATING
PRECIPITABLE WATER VALUES 2.5-3 SIGMAS ABOVE THE MEAN FOR THE END
OF OCTOBER.  SPC MESOANALYSES INDICATE 100-300 J/KG MUCAPE NEAR
THE REGION, WHICH ARE PROBABLY CONTRIBUTING TO THE ELEVATED
CONVECTION.

RECENT RADAR IMAGERY HAS INDICATED SOME EROSION OF RAINFALL ACROSS
THE NORTHERN PORTIONS OF THE OLYMPIC PENINSULA.  ONCE THE
BAROCLINIC LEAF/SHORTWAVE MOVES INLAND IN A COUPLE HOURS, THE
ONSHORE FLOW SHOULD BEGIN TREKKING SOUTHWARD AT A MODEST RATE DOWN
INTO OR.  CAM GUIDANCE CONTINUES TO ADVERTISE LOCAL AMOUNTS OF 3"
THROUGH 22Z, PARTICULARLY ACROSS THE MOUNTAINS OF SOUTHWESTERN WA.
 WITH HOURLY RATES UP TO 1", THIS RAINFALL COULD FALL WITHIN A FEW
HOURS.  ANY RAINFALL RELATED ISSUES SHOULD BEGIN TO DECREASE IN 3+
HOURS AS THE INFLOW BAND SHIFTS SOUTHWARD AND PRECIPITABLE WATER
VALUES BEGIN TO DECLINE.  UNTIL THEN, DEBRIS FLOWS ARE POSSIBLE,
PARTICULARLY NEAR ANY RECENT BURN SCARS.

ROTH

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

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

LAT...LON   49192255 49182117 48022066 46592162 45272176 
            44942199 44492215 44552250 45142253 45842262 
            45732311 45122336 44592358 44472387 44962399 
            46152411 46742437 47812443 47982412 47842347 
            47402319 46682314 46412286 46522256 47482208 
            48452221 
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