MESOSCALE PRECIPITATION DISCUSSION 0648
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1254 AM EST TUE NOV 17 2015
 
AREAS AFFECTED...WESTERN WA/NORTHWEST OR 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 170554Z - 171504Z
 
SUMMARY...AN ATMOSPHERIC RIVER AIMED AT THE PACIFIC NORTHWEST
COULD PRODUCE HOURLY RAIN RATES TO 0.75" AND LOCAL AMOUNTS IN THE
3-5" RANGE THROUGH 15Z.

DISCUSSION...MODERATE ONSHORE FLOW, OF 45 KTS, HAS BEEN ELEVATING
THE DEGREE OF MOISTURE INTO THE REGION, WITH GPS-BASED
PRECIPITABLE WATER VALUES UP TO 1.1".  A RECENT SURGE OF RAINFALL
BROUGHT HOURLY RATES UP TO 0.6" INTO PORTIONS OF THE OLYMPIC
PENINSULA BETWEEN 03-04Z, PER MESOWEST OBSERVATIONS.  AREA
SOUNDINGS FROM SALEM OR AND QUILLAYUTE WA SHOW 1-3 ISOTHERMAL/
INVERSION LAYERS, WHICH HAD BEEN LIMITING RAINFALL INTENSITY.

LOW-LEVEL INFLOW IS EXPECTED TO INCREASE TO ~60 KTS AROUND 08Z AND
13Z PER RAP FORECASTS AND SATELLITE EXTRAPOLATION.  BOTH APPEAR TO
HAVE SATELLITE FEATURES ASSOCIATED WITH THEM, WITH THE 08Z SURGE
APPEARING TO BE ASSOCIATED WITH AN AREA OF COOLER CLOUD TOPS NEAR
49N 128W AND THE 13Z SURGE WITH A BROADER AREA OF COOLER CLOUDS
TOPS OUT NEAR 49N 140W.  MICROWAVE- DERIVED RAIN RATES FROM 0030Z
SHOW AMOUNTS OF UP TO 0.32"/HOUR, WHICH CAN USUALLY BE DOUBLED
WHERE LOW-LEVEL INFLOW BEST INTERSECTS SIGNIFICANT TOPOGRAPHY.
RECENT HISTORY AND MAXIMUM PRECIPITABLE WATER VALUES IN THE PLUME
ALSO SUGGEST HOURLY RAIN RATES UP TO 0.65-0.75" ARE POSSIBLE WITH
EACH DISTURBANCE AS PRECIPITABLE WATER VALUES CLIMB TOWARDS 1.25".
 WHILE HIGHER RAIN RATES HAVE GENERALLY BEEN SEEN WITHIN THE
COASTAL RANGES, THE 18Z SSEO NEIGHBORHOOD PROBABILITIES SUGGEST
THAT RAINFALL RATES/AMOUNTS SHOULD INCREASE IN THE CASCADES AROUND
08Z.  THE CAM GUIDANCE SUPPORTS LOCAL AMOUNTS IN THE 3-5" RANGE
THROUGH 15Z.

THIS INFORMATION WAS COORDINATED WITH SAB/ NESDIS.

ROTH

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

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

LAT...LON   49332128 48622048 47542042 46382128 44602158 
            44032184 44212239 45802275 45272315 44532392 
            46342417 48412490 48532476 48322337 48012295 
            46852291 46792231 47632197 48922261