MESOSCALE PRECIPITATION DISCUSSION 0668
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
305 AM EST TUE DEC 08 2015
 
AREAS AFFECTED...WASHINGTON OLYMPICS...NRN WASHINGTON CASCADES 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 080802Z - 081800Z
 
SUMMARY...RAIN RATES WILL SIGNIFICANTLY INCREASE ACROSS THE
OLYMPICS AND THE NRN WASHINGTON CASCADES AFTER ABOUT 12Z WITH THE
APPROACH OF STRONG LOW LEVEL MOISTURE TRANSPORT AHEAD OF A PACIFIC
COLD FRONT. THROUGH 18Z...2-3 INCHES ARE EXPECTED WITH MAXIMUM 10
HR RAINFALL TOTALS NEAR 4 INCHES.

DISCUSSION...07Z WATER VAPOR IMAGERY SHOWED A WELL DEFINED CYCLONE
WITH A CLOSED LOW CENTERED NEAR 53N 147W...ADVANCING NEWD THROUGH
THE GULF OF ALASKA. AT THE SURFACE...AN OCCLUDED FRONTAL SYSTEM
WAS ANALYZED WITH THE HELP OF AMSU 89 GHZ IMAGERY WHICH CLEARLY
IDENTIFIED THE FRONTAL BOUNDARIES WITH A PASS NEAR 06Z. THE SAME
AMSU PASS INDICATED THE HIGHEST AREAL AVERAGE RAIN RATES OF
0.2-0.3 IN/HR OCCURRING JUST AHEAD OF THE OCCLUDED FRONT...LIKELY
OWING TO BETTER DYNAMIC FORCING WITH THE PRESENCE OF STRONG UPPER
DIVERGENCE/DIFFLUENCE AHEAD OF A POWERFUL 150+ KT JET SHOWN BY THE
00Z GFS AND CONFIRMED WITH A RECENT GOES HIGH DENSITY WIND PLOT.
MEANWHILE...RAIN RATES AHEAD OF THE WARM FRONT WERE ONLY 0.05 TO
0.15 IN/HR. THESE RAINFALL RATES AHEAD OF THE WARM FRONT WERE ALSO
SHOWN BY THE 00Z NMMB/ARW WHICH HELPS TO INCREASE CONFIDENCE IN
THESE MODELS FORECAST EVOLUTION.

AS THE SHORTWAVE ADVANCES CLOSER TO THE COAST THROUGH THE NEXT
6-12 HOURS...A LONG FETCH OF 850-700 MB MOISTURE TRANSPORT LOCATED
JUST AHEAD OF THE PACIFIC COLD FRONT SHOULD REACH THE OLYMPIC
PENINSULA NEAR 12Z. WIND SPEEDS ARE EXPECTED TO BE NEAR 50 KTS
FROM THE WSW AND GIVEN OROGRAPHIC ENHANCEMENT...RAINFALL RATES ARE
EXPECTED TO PEAK NEAR 0.50 IN/HR IN THE OLYMPICS AND NRN
WASHINGTON CASCADES. GIVEN THESE LOCATIONS HAVE RECEIVED BETWEEN
200 AND 400 PERCENT OF NORMAL RAINFALL OVER THE PAST
WEEK...FLOODING CONCERNS WILL BE EXACERBATED. ADDITIONAL RAINFALL
TOTALS OF 2-3 INCHES WITH MAXIMUM AMOUNTS NEAR 4 INCHES ARE
EXPECTED...MAINLY IN THE WSW FACING SLOPES OF THE OLYMPICS...BASED
ON 00Z NMM/ARW GUIDANCE.

OTTO

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

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

LAT...LON   49182179 49182129 48872086 48432069 47562085 
            46692119 46612235 46782336 46942407 47382432 
            47732442 48412430 48352322 48992241