MESOSCALE PRECIPITATION DISCUSSION 0038
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
100 AM EST TUE MAR 01 2016
 
AREAS AFFECTED...TERRAIN OF NORTHWEST WA 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 010600Z - 011800Z
 
SUMMARY...AN ATMOSPHERIC RIVER AIMING FOR NORTHWEST WA COULD LEAD
TO 0.50" AN HOUR RAIN RATES BETWEEN NOW AND 09Z AND AGAIN BETWEEN
12Z AND 18Z ACROSS THE OLYMPIC PENINSULA AND CASCADES OF NORTHWEST
WA.

DISCUSSION...AN ATMOSPHERIC RIVER, IN THE FORM OF A PINEAPPLE
CONNECTION, EXTENDS FROM HAWAI'I TOWARDS THE PACIFIC NORTHWEST. 
THE FIRST SKIRMISH OF RAIN IS IMPACTING WESTERN WA AT THIS TIME,
WITH RADAR ESTIMATES OF 0.50" AN HOUR RATES JUST OFFSHORE.  RAIN
RATES THUS FAR HAVE BEEN IN THE 0.1" AN HOUR RANGE PER MESONET
OBSERVATIONS.  PRECIPITABLE WATER VALUES (PWS) HAVE INCREASED TO
~0.75" IN THE OLYMPIC PENINSULA AND ~0.9" ALONG THE OR COAST. 
AMSU ESTIMATES OF PWS OFFSHORE CURRENTLY ARE 1.33". 

PER A BRIEFING FROM SAB/NESDIS, THE FIRST PERIOD OF MODERATE TO
HEAVY RAIN CAUSED BY AN INVADING WARM FRONT SHOULD CONTINUE UNTIL
09Z.  IN ITS WAKE, FREEZING LEVELS RISE ACROSS THE WA CASCADES AS
SOUTH-SOUTHWEST INFLOW OF 60+ KTS AND INCREASINGLY UNIDIRECTIONAL
FLOW WITH HEIGHT BRINGS PRECIPITABLE WATER VALUES TO ~1" PER
RECENT RAP RUNS, 2-3 SIGMAS ABOVE THE MEAN FOR THE BEGINNING OF
MARCH.  

A SHORTWAVE AND ITS ASSOCIATED OCCLUDED CYCLONE NEAR 140W ARE
EXPECTED TO BRING A SECOND SKIRMISH OF HEAVY RAINS, MAINLY DURING
THE 12-18Z PERIOD.  AMSU-BASED SATELLITE HOURLY RAIN ESTIMATES
INCREASED BETWEEN THE 19Z AND 23Z PASSES, WITH THE 01Z PASS ALSO
SHOWING SPOTS OF 0.5" AN HOUR.  THE MESOSCALE GUIDANCE THAT MOST
SUPPORTS THESE RAIN RATES ARE THE 02Z RAP AND 00Z ARW RUNS, WHICH
SHOW LOCAL AMOUNTS OF ~2" BETWEEN 12-18Z.

ROTH

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

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

LAT...LON   49032208 49032207 49022161 49032090 49032026 
            48881982 47912037 47392102 47602168 48592209 
            48592293 47832297 47272338 47312397 48152461 
            48542358 48812259 48912237