905  
AWUS01 KWNH 130456  
FFGMPD  
CAZ000-ORZ000-WAZ000-131200-  
 
MESOSCALE PRECIPITATION DISCUSSION 0009  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1156 PM EST TUE JAN 12 2021  
 
AREAS AFFECTED...PACIFIC NORTHWEST  
 
CONCERNING...HEAVY RAINFALL  
 
VALID 130500Z - 131200Z  
 
SUMMARY... ONE FINAL ROUND OF HEAVY RAINFALL IS IN STORE FOR THE  
PACIFIC NORTHWEST BEFORE A COLD FRONT PASSES THROUGH THE REGION  
LATER TONIGHT. SOAKING RAINFALL ATOP OVERLY SATURATED GROUND MAY  
LEAD TO ADDITIONAL FLOODING AND EXACERBATE ONGOING AREAS OF  
FLOODING.  
 
DISCUSSION... SURFACE ANALYSIS AROUND 04Z INDICATED A SURFACE LOW  
NEAR VANCOUVER ISLAND WAS TRACKING NORTHEAST WHILE ITS ASSOCIATED  
COLD FRONT WAS RACING TOWARDS THE WESTERN WASHINGTON COAST. A  
500MB SHORTWAVE TROUGH IS INDUCING PVA ALOFT ALONG WITH LINGERING  
850-700MB WAA AHEAD OF THE COLD FRONT. IN ADDITION, THERE IS A  
STRONG 850MB JET EXPECTED TO PEAK AS HIGH AS 80 KNOTS OVER WESTERN  
OF OREGON BETWEEN 05-07Z THAT WOULD HELP TO PROMOTE HEAVIER  
RAINFALL RATES VIA STRONG UPSLOPE FLOW. THERE ALSO REMAINS NO  
SHORTAGE OF RICH PACIFIC MOISTURE AS PWS REMAIN 2.5-3 STDS ABOVE  
NORMAL THROUGHOUT THE REGION. IN FACT, VIA THE 00Z SLE SOUNDING,  
THE MEASURED PW VALUE WAS 1.19" IS MORE THAN DOUBLE THE AVERAGE PW  
VALUE FOR THIS TIME OF YEAR.  
 
THESE INGREDIENTS CONTINUE TO SUPPORT A SOAKING RAINFALL FOR MUCH  
OF THE WATER-LOGGED AREAS OF WESTERN WASHINGTON, WESTERN OREGON,  
AND FAR NORTHWESTERN CALIFORNIA. RAINFALL RATES OF 0.4-0.75" PER  
HOUR ARE LIKELY WITH THE HIGHEST RATES TO BE OBSERVED ALONG THE  
WINDWARD SIDES OF THE COASTAL RANGE AND CASCADES OF WESTERN OREGON  
AND SOUTHWEST WASHINGTON. THESE RAINFALL RATES COULD TRIGGER  
ADDITIONAL AREAS OF URBAN AND RIVER FLOODING. THERE IS ALSO  
ONGOING CONCERNS FOR MUDSLIDES GIVEN THE OVERLY SATURATED AND  
UNSTABLE SOIL.  
 
THE COLD FRONT SWINGS THROUGH THE OLYMPICS AND PUGET SOUND FIRST  
SOMETIME BETWEEN 06-08Z. ONCE THE COLD FRONT HAS PASSED THROUGH,  
DRIER WESTERLIES ALOFT SHOULD QUICKLY REDUCE RAINFALL RATES,  
ALTHOUGH LINGERING UPSLOPE SHOWERS ARE STILL LIKELY TO OCCUR ALONG  
THE CASCADES INTO THE EARLY MORNING HOURS. COLD AIR ADVECTION  
ALOFT WILL FORCE SNOW LEVELS TO CRASH LATER TONIGHT AND FORCE  
PRECIPITATION AT LOWER ELEVATIONS TO CHANGE OVER TO SNOW. THE COLD  
FRONT THEN CROSSES THROUGH WESTERN OREGON BETWEEN 08-10Z, LEADING  
TO THE SAME RESULTS THE NEIGHBORS FARTHER NORTH WITNESS. BY 12Z,  
THE COLD FRONT IS LIKELY TO HAVE REACHED FAR NORTHWESTERN CA AND  
DRIER CONDITIONS RETURN AS HIGH PRESSURE BUILDS IN FROM THE WEST.  
 
MULLINAX  
   
..PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT
 
 
ATTN...WFO...EKA...MFR...PDT...PQR...SEW...  
 
ATTN...RFC...CNRFC...NWRFC...NWC...  
 
LAT...LON 49312233 48812158 47652137 45192173 43122299  
42092316 41102408 42322472 44472458 46062432  
47482476 48462490 48582453 48572401 48782345  
49152300  
 
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