MESOSCALE PRECIPITATION DISCUSSION 0802
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
107 AM EST TUE DEC 20 2016
 
AREAS AFFECTED...COASTAL AND CASCADE RANGES OF SOUTHERN
WA...NORTHERN AND CENTRAL OR 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 200615Z - 201515Z
 
SUMMARY...NEARLY ORTHOGONAL LOW TO MID-LEVEL FLOW WITH MODERATELY
DEEP MOISTURE/FLUX TO AFFECT COASTAL AND DOWNSTREAM CASCADE RANGES
THROUGH THE EARLY MORNING HOURS WITH RATES UP TO .5"/HR PROBABLE.

DISCUSSION...WV LOOP DENOTES SHEARED NW TO SE UPPER LEVEL TROF
WITH WESTERLY DRIER AIR PRESSING THE MID-LEVEL TROF AXIS EASTWARD
INTO NW WA ATTM...WITH WAA REGIME CURRENTLY AFFECTING SW WA/NW OR
WITH LIGHT TO MODERATE SHOWERS WHILE SIMULTANEOUSLY SUPPORTING
FLATTENING OF THE MOISTURE PLUME AND SLIGHTLY VEERING ONSHORE
FLOW.  IN THE NEAR TERM OVER THE NEXT 3-6 HRS WILL INCREASE LOW
LEVEL JET MORE ORTHOGONAL TO THE COAST WITH INCREASING FLOW FROM
40 KTS CURRENTLY AT 850 TOWARD 60-65 KTS BY 09Z NEAR THE MOUTH OF
THE COLUMBIA RIVER.  BLENDED TPW AND RECENT AMSU/SSMIS PASSES
SUPPORT VALUES UP TO 1.25" WITH CIRA LAYERED BLENDED ANALYSIS
SHOWING FAIRLY GOOD OVERLAYING SUPPORTING DEEPER CLOUD
CONDENSATION/RAINFALL GENERATION REGION FROM SFC TO 6H EAST OF THE
COAST RANGE BECOMING 850-6H CROSSING TOWARD THE CASCADES.  GIVEN
STRENGTH OF WIND/MOISTURE AVAILABILITY AND ANGLE TO THE
TERRAIN...EXPECTATION OF RATES OF .5"/HR ARE POSSIBLE FOR A HOUR
OR SO AS THE MOISTURE AXIS SLIDES SOUTHWARD SLOWLY INTO CENTRAL
OREGON 9-12Z BEFORE HEADING INTO THE SOUTHERN COASTAL RANGE AFTER
12Z.  
IT IS AFTER ABOUT 10Z THAT THE UPPER LEVEL JET BEGINS TO WANE AND
THE PLUME MOVES WELL SOUTH OF THE JET AXIS/RIGHT EXIT REGION. 
ALONG WITH BEST SHORTWAVE FORCING LIFTING NW AWAY FROM THE BEST
MOISTURE FLOW BECOMES INCREASINGLY WEAKER REDUCING MAIN DRIVING
FORCE FOR REMAINING ASCENT TOWARD 30-40KT AT 850MB LIKELY LIMITING
RATES TO .25-.3" AT MOST.  

OVERALL THE HI-RES GUIDANCE HAD BEEN RUNNING A BIT FAST IN
BRINGING IN THE WAA REGIME BUT WITH NEW 00Z CYCLES AND RECENT HRRR
RUNS SEEMS TO BE BETTER ALIGNED IN TIMING AND RAINFALL PRODUCTION
WITH MAX ADDITIONAL TOTALS OF 2-4" POSSIBLE FOR MID SLOPES OF THE
CASCADES INCLUDING MT HOOD/MT BACHELOR AND THE SISTERS BELOW THE
FREEZING LEVELS WITH LESS THAN .5" EXPECTED IN THE WILLAMETTE
VALLEY GIVEN THE 1.5-3" RAINED OUT ALONG THE COASTAL RANGES.

GALLINA

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

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

LAT...LON   47032139 46692118 45782153 45102149 44102162 
            42972211 43572282 43962280 44392274 45142255 
            45762238 46092273 45992302 45372334 44952340 
            44402346 44192346 43612414 44402408 45042402 
            45602396 45992400 46362400 46612408 46802398 
            46822321 46492319 46342290 46452244 46612220 
            46942189