MESOSCALE PRECIPITATION DISCUSSION 0902
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
305 AM EDT SUN OCT 22 2017

AREAS AFFECTED...WESTERN WA...WESTERN OR

CONCERNING...HEAVY RAINFALL

VALID 220700Z - 221900Z

SUMMARY...HEAVY RAINFALL WILL CONTINUE THROUGH THE MID TO LATE
MORNING HOURS ON SUNDAY IN ASSOCIATION WITH THE LATEST ATMOSPHERIC
RIVER. CONDITIONS SHOULD GRADUALLY IMPROVE TOWARD MIDDAY.

DISCUSSION...STRONG ATMOSPHERIC RIVER CONDITIONS CONTINUE ACROSS
THE PACIFIC NORTHWEST AS A STRONG UPPER LEVEL JET MAX DRIVEN BY A
SHORTWAVE TROUGH CROSSES THE REGION AND ALLOWS FOR A PERSISTENT
LONG-FETCH AXIS OF DEEP PACIFIC MOISTURE TO ADVANCE INLAND ALONG
AND AHEAD OF A COLD FRONT. THE LATEST 850/700 MB MOISTURE FLUX
ANOMALIES ARE OVER 4 SIGMAS ABOVE NORMAL AIMING INTO THE COASTAL
RANGES OF NORTHWEST OR AND ADJACENT AREAS OF SOUTHWEST WA. THIS IS
RESULTING IN ENHANCED RAINFALL RATES WHICH IS BEING ALSO GREATLY
AIDED BY STRONG OROGRAPHIC ASCENT. IN FACT...OBSERVED RAINFALL
RATES OVER THE LAST HOUR CONTINUE TO BE ON THE ORDER OF 0.50 TO
0.70 INCHES/HR...WITH SOME OF THE HEAVIEST RATES NOTED ACTUALLY IN
THE FOOTHILLS OF THE SOUTHERN WA CASCADES AND THE COASTAL RANGE
WEST OF THE WILLAMETTE VALLEY.

OVER THE NEXT SEVERAL HOURS...THE COLD FRONT WILL BE GRADUALLY
SETTLING DOWN THE COAST AND SHOULD BEGIN TO TRANSPORT THE AXIS OF
DEEPEST MOISTURE AND STRONGER SURFACE FORCING FARTHER SOUTH MORE
INTO WESTERN OR. THIS WILL ALLOW THE HEAVIEST RAINS TO BEGIN
SHIFTING OUT OF WESTERN WA...ALTHOUGH THERE WILL CONTINUE TO BE
POST-FRONTAL SHOWERS GIVEN SOME STEEPENING OF THE MID-LEVEL LAPSE
RATES AND PERSISTENT ONSHORE FLOW. THE STRONGEST ONSHORE FLOW
OVERALL WHICH IS ON THE ORDER OF 40 TO 50 KTS WILL CONTINUE ALONG
AND JUST AHEAD OF THE COLD FRONT AND WILL AGAIN HELP FOCUS STRONG
MOISTURE TRANSPORT INTO THE COASTAL RANGES.

ADDITIONAL RAINFALL AMOUNTS OF AS MUCH AS 2 TO 4 INCHES WILL BE
EXPECTED THROUGH 18Z TODAY AS THE FRONT MAKES PROGRESS FARTHER
SOUTH. THE HEAVIEST TOTALS SHOULD BE OVER THE FOOTHILLS OF THE
SOUTHERN WA AND CENTRAL/NORTHERN OR CASCADES...AND ALSO THE
WESTERN OR COASTAL RANGE. HOWEVER...BY MID TO LATE MORNING THE
HIRES MODEL CONSENSUS BEGINS TO STRONGLY SUPPORT A WEAKENING OF
THE ONSHORE FLOW...AND THE OVERALL LARGE SCALE FORCING FOR HEAVIER
RAINS SHOULD BEGIN TO SLACKEN AS THE SUPPORTING UPPER LEVEL
SHORTWAVE AND JET CORE SHIFTS WELL OFF TO THE EAST. THIS WILL LEAD
THE WAY FOR THIS ATMOSPHERIC RIVER EVENT TO COME TO AN END.

NEVERTHELESS...SEVERAL INCHES OF ADDITIONAL RAIN ARE EXPECTED
THROUGH MIDDAY BEFORE CONDITIONS BEGIN TO SUBSTANTIALLY
IMPROVE...AND THIS WILL RESULT IN ADDITIONAL RUNOFF CONCERNS AND
ESPECIALLY AROUND BURN SCAR AREAS WHERE THE POTENTIAL FOR
MUDSLIDES AND DEBRIS FLOWS WILL BE POSSIBLE.

ORRISON

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

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

LAT...LON   48792192 48672157 48352124 47572119 46882114
            46072114 45352142 44572158 43832172 43252201
            42762261 42782359 42992414 43782436 45082413
            45832416 46792424 47662454 48022433 48072397
            48022324 47382300 47372246 47772239 48172217
            48672210