MESOSCALE PRECIPITATION DISCUSSION 0896
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
238 PM EDT SAT OCT 21 2017

AREAS AFFECTED...WESTERN WA INTO THE NORTHERN HALF OF WESTERN OR

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 211837Z - 220600Z

SUMMARY...HEAVY RAIN WITH TOTALS OF 3-6 INCHES...LOCALLY
HIGHER...IS EXPECTED TO AFFECT NORTHWESTERN OR INTO WESTERN WA
THROUGH 06Z. THE HEAVIEST TOTALS ARE EXPECTED TO BE JUST SOUTH AND
NORTH OF THE OR/WA BORDER ALONG THE COASTAL RANGES AND CASCADE
RANGE.

DISCUSSION...A 13Z PASS SEEN ON THE 89 GHZ CHANNEL FROM AMSU
INDICATED A WEAK WAVE NEAR 47N 137W DEVELOPING ON A PACIFIC COLD
FRONT. GOES 15 WATER VAPOR IMAGERY SHOWED A DEEPENING SHORTWAVE
NEAR 46N 145W WHICH WAS ASSOCIATED WITH A POWERFUL UPPER LEVEL JET
CORE WITH 12Z GFS AND GOES HD WIND DATA INDICATING THE LEADING
EDGE OF THE 150 KT ISOTACH NEAR 46N 140W. BLENDED TPW AND GPS
OBSERVATIONS SHOWED 1.2+ INCH PRECIPITABLE WATER VALUES EXTENDING
EAST-NORTHEASTWARD ACROSS THE PACIFIC REACHING MUCH OF THE OR
COAST WITH 1.3 INCHES OBSERVED BETWEEN COOS BAY AND PORT ORFORD
FROM 15-18Z.

AS THE UPPER SHORTWAVE CONTINUES TO AMPLIFY TOWARD THE COAST OF
BRITISH COLUMBIA OVER THE NEXT 12 HOURS...THE FRONTAL WAVE IS
EXPECTED TO ADVANCE NORTH OF VANCOUVER ISLAND WHICH SHOULD ALLOW
THE AXIS OF STRONGEST MOISTURE TRANSPORT TO REACH THE WA/OR BORDER
BETWEEN 21-00Z. 850-700 MB WINDS FROM THE WSW ARE EXPECTED TO
RANGE IN THE 50-60 KT RANGE ALONG THE COAST WITH PRECIPITABLE
WATER VALUES OF 1.3 TO 1.4 INCHES ALONG THE WA/OR COASTAL
BORDER...AND 1.2-1.3 INCHES IN THE NORTHERN WILLAMETTE VALLEY.

00Z AND 12Z HI-RES GUIDANCE SHOWED GOOD RUN TO RUN CONTINUITY WITH
THE QPF VALUES INTO WESTERN WA AND NORTHERN OR THROUGH 06Z.
RAINFALL RATES OF 0.5+ IN/HR SHOULD BEGIN TO IMPACT THE NORTHERN
OREGON CASCADES AND COASTAL RANGES WITHIN THE NEXT HOUR OR
TWO...WITH THE 0.5+ IN/HR RATES FOCUSING INTO SOUTHWESTERN WA AND
THE WA CASCADES BY 00Z. BETWEEN 00Z AND 06Z...THERE ARE SEVERAL
HI-RES MODELS AND THE 12Z HREF MEAN VERSION 2 SHOWING RAINFALL
RATES EXCEEDING 0.75 IN/HR INTO THE SOUTHERN WA CASCADES. THESE
RAINFALL RATES MAY LEAD TO FLASH FLOODING ACROSS MORE SENSITIVE
BURN SCAR LOCATIONS IN THE STEEP TERRAIN FROM NORTHERN OR INTO WA.
OVERALL...3-6+ INCHES IS FORECAST ACROSS WESTERN WA INTO
NORTHWESTERN OR WITH A FOCUS WITHIN 75 MILES OF THE WA/OR BORDER
ACROSS THE CASCADES AND COASTAL RANGES.

OTTO

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

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

LAT...LON   48952185 48932116 48662066 48232029 47652029
            46762079 46302106 45682135 44892153 44002158
            43842201 44212280 44462377 45892414 48002439
            48062377 47692278 47732207 48372203