MESOSCALE PRECIPITATION DISCUSSION 0735
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
730 PM EDT THU OCT 13 2016
 
AREAS AFFECTED...SOUTHWEST WA...WESTERN OR...NORTHERN CA 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 132330Z - 141000Z
 
SUMMARY...HEAVY RAINS ASSOCIATED WITH STRONG ATMOSPHERIC RIVER
CONDITIONS ARE EXPECTED TONIGHT...WITH THE HEAVIEST AMOUNTS OVER
SOUTHWEST OR AND NORTHWEST CA.

DISCUSSION...A POWERFUL 250 MB JET CORE OF 130 TO 150 KTS
CONTINUES TO AMPLIFY ACROSS THE EASTERN PACIFIC OCEAN SOUTH OF THE
GULF OF AK WHERE A DEEP UPPER TROUGH CONTINUES TO ALLOW
SIGNIFICANT AMOUNTS OF ENERGY AND VERY STRONG MOISTURE TRANSPORT
TO ENCROACH ON THE PACIFIC NORTHWEST. ONE SHORTWAVE CENTERED NEAR
46N 133W IS RAPIDLY AMPLIFYING AS SEEN IN GOES-WEST WV IMAGERY AND
IS RESULTING IN THE RAPID DEEPENING OF SURFACE LOW PRESSURE OF THE
PACIFIC NORTHWEST COAST. LOW PRESSURE IS EXPECTED TO CONTINUE TO
DEEPEN OVERNIGHT AS ADDITIONAL SHORTWAVE ENERGY NEAR 45N 139W
ADVANCES EAST AND HELPS TO CONSOLIDATE A DEEP LOW CENTER THAT WILL
ADVANCE NORTHEAST TOWARD BETWEEN 06Z AND 12Z TONIGHT. THIS IN TURN
WILL DRIVE A COLD FRONT INLAND ACROSS THE PACIFIC NORTHWEST AND
SETTLING DOWN THROUGH NORTHERN CA BY 12Z.

IN THE MEANTIME...WHILE SURFACE LOW PRESSURE CONTINUES TO
DEEPEN...THE LOW LEVEL SOUTHWESTERLY FLOW AHEAD OF A COLD FRONT
WILL INCREASE...AND THE LATEST RAP FORECASTS INDICATE A 850 MB
SOUTHWESTERLY JET INCREASING TO 70 TO 80 KTS WHILE AIMING INTO THE
SOUTHWEST OR AND FAR NORTHWEST CA COASTAL RANGES BY 03Z. THIS WILL
FOSTER 850/700 MB MOISTURE FLUX ANOMALIES WELL IN EXCESS OF 5
STANDARD DEVIATIONS ABOVE NORMAL. MEANWHILE...AN IMPRESSIVE CORE
OF HIGH PWATS...REACHING AS HIGH 1.5 INCHES WILL BE NOSED RIGHT
INTO THE SOUTHWEST FACING SLOPES OF THESE COASTAL RANGES...WITH
PWATS AS HIGH AS 1.75 INCHES REACHING WITHIN 100 MILES OF THE
COAST. THESE VALUES WILL BE OVER 3 STANDARD DEVIATIONS ABOVE
NORMAL AND EXCEEDING THE 95TH CLIMATOLOGICAL PERCENTILE. THE
ANOMALOUS PWAT VALUES ARE BEING INFLUENCED BY A WELL-DEFINED
CHANNEL OF 500/300 MB LAYER EMANATING FROM FORMER TYPHOON SONGDA
OVER THE WESTERN PACIFIC...WITH THE CIRA-LPW PRODUCT DEPICTING
THIS MOISTURE SHEARING AND STREAMING RAPIDLY DOWNSTREAM TO THE
PACIFIC NORTHWEST.

THESE ANOMALOUS MOISTURE FIELDS/TRANSPORT AND STRONG FORCING ALONG
AND AHEAD THE COLD FRONT WILL RESULT IN HEAVY RAINFALL
OVERNIGHT...WITH PARTICULARLY EFFICIENT RAINFALL PROCESSES
COUPLING WITH STRONG OROGRAPHICS TO RESULT IN ENHANCED RAINFALL
RATES ACROSS SOUTHWEST OR AND NORTHWEST CA. IN FACT...RAINFALL
RATES ARE LIKELY TO REACH AS HIGH AS 1 INCH/HR THIS EVENING AS
PEAK LOW LEVEL JET FORCING/MOISTURE FLUX ARRIVES. THE LATEST
ARW/NMMB AND NSSL-WRF ALONG WITH THE LATEST HRRR GUIDANCE SUPPORTS
AS MUCH AS 4 TO 6 INCHES OF ADDITIONAL RAIN BETWEEN 00Z AND
12Z...WITH ISOLATED HEAVIER AMOUNTS PERHAPS CLOSE TO 8 INCHES OVER
THE SOUTHWEST FACING PEAKS.

LESSER AMOUNTS OF 2 TO 4 INCHES ARE EXPECTED ELSEWHERE...AND THIS
WILL INCLUDE THE REMAINDER OF COASTAL OR/SOUTHWEST WA AND THE WEST
SLOPES OF THE CASCADES. ALSO THIS WILL INCLUDE PORTIONS OF THE
NORTHERN SIERRA-NEVADA LATE TONIGHT INCLUDING THE SHASTA/SISKIYOU
RANGES AS THE COLD FRONT ADVANCES SOUTHEAST OVER THE TERRAIN.

ORRISON

ATTN...WFO...EKA...MFR...MTR...PDT...PQR...REV...SEW...STO...

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

LAT...LON   46722276 46342179 45502143 44242156 42492173 
            40982136 39962092 39672130 39802181 40432225 
            40112281 39022243 38442276 38392298 38502339 
            40072433 41842464 43292469 44382458 45892419 
            46382378