MESOSCALE PRECIPITATION DISCUSSION 0801
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
242 PM EST MON DEC 19 2016
 
AREAS AFFECTED...SRN WA / CNTRL TO NRN OR 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 191941Z - 200740Z
 
SUMMARY...A SURGE IN LOW LEVEL MOISTURE FLUX INTO THE SRN WA/NRN
OR COASTAL RANGES DOWNSTREAM INTO THE CASCADES STARTING BETWEEN
03-06Z IS EXPECTED TO LEAD TO A SURGE IN RAINFALL RATES IN EXCESS
OF 0.5 IN/HR.  

DISCUSSION...GOES WEST WATER VAPOR IMAGERY AT 19Z SHOWED THE
PLACEMENT OF A POTENT MID-LEVEL SHORTWAVE CENTERED NEAR 50N
150W...WITH AMSU 89 GHZ IMAGERY CONFIRMING THE PRESENCE OF A
REINFORCING COLD FRONT JUST AHEAD OF THE SHORTWAVE AT THE SURFACE.
SATELLITE IMAGERY ALSO SHOWED A PRIMARY COLD FRONT CROSSING THE
OLYMPIC PENINSULA WITH A FRONT EXTENDING WEST TOWARD A LOW NEAR
46N 143W. SOUTH OF THE PRIMARY FRONT...ORIENTED ROUGHLY EAST/WEST
THROUGH THE ERN PACIFIC...WAS A PLUME OF ANOMALOUS PRECIPITABLE
WATER VALUES...BETWEEN 1-2 STANDARD DEVIATIONS ABOVE THE MEAN. THE
CIRA LAYERED PW SHOWED A BREAK BETWEEN THIS PLUME OF MOISTURE AND
A RICHER SUBTROPICAL PLUME CURRENTLY OVER HAWAII...CHARACTERIZED
BY TOTAL PRECIPITABLE WATER VALUES OF 1.5 TO NEARLY 2.0 INCHES. IN
ADDITION...GOES HD WINDS CONFIRMED THE PRESENCE OF A 150+ JET
STREAK CENTERED NEAR 45N 147W...WHICH WAS IDENTIFIED SLIGHTLY
LARGER THAN THE 06Z AND 12Z GFS. WHILE THE MOISTURE PLUME OVER
HAWAII IS LIKELY TOO FAR AWAY TO GET CAUGHT UP IN ADVANCE OF THE
APPROACHING SHORTWAVE...OTHER FACTORS MAY CONTRIBUTE TO A PERIOD
OF HEAVY RAIN OVERNIGHT. 

THE SHORTWAVE IS PROGGED TO BE JUST WEST OF VANCOUVER ISLAND BY
12Z...WITH THE JET STREAK WEAKENING IN ITS APPROACH TOWARD THE
PACIFIC NORTHWEST COAST BETWEEN 00Z AND 12Z. DESPITE
WEAKENING...THE EXIT REGION OF THE JET STREAK IS EXPECTED TO
PROVIDE ENHANCEMENT TO LIFT INTO SRN WA TOWARD 06Z. 850 MB AND 700
MB WINDS OF 60-70 KT ARE EXPECTED TO BE JUST SOUTH OF DUE WEST
UNDER THE WARM ADVECTION PATTERN ALONG THE COASTS OF WASHINGTON
AND OREGON TONIGHT. THIS ORIENTATION SHOULD LIMIT RAINFALL RATES
INTO THE WILLAMETTE VALLEY BUT ENHANCE INTO THE COASTAL RANGES AND
CASCADES GIVEN THE NEARLY PERPENDICULAR ORIENTATION. 

FORECAST PRECIPITABLE WATER VALUES OF 1.2 TO 1.3 INCHES ALONG THE
COAST AND THE EXPECTED SURGE OF LOW LEVEL WINDS ARE EXPECTED TO
SHARPLY ENHANCE RAINFALL RATES FOR A FEW HOURS...BEGINNING IN THE
03-06Z TIME FRAME ABOVE 0.50 IN/HR. A CONSENSUS OF THE 12Z HI-RES
GUIDANCE SUPPORTS THIS SCENARIO WITH 24 HR TOTALS OF 3-5 INCHES
FROM SRN PORTIONS OF WA INTO CNTRL AND NRN OR THROUGH 12Z.

OTTO

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

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

LAT...LON   46942200 46912184 46722161 46482147 46252136 
            45942145 45582164 45242170 44722179 44232204 
            44182244 44362276 44522291 44762280 45052265 
            45332261 45632267 45802270 46012279 46072297 
            46012318 45772325 45452332 45012346 44772355 
            44442380 44522428 45872411 46422382 46442343 
            46742245