Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 191432
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
932 AM EST SUN NOV 19 2017

...VALID 15Z SUN NOV 19 2017 - 12Z MON NOV 20 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 NW UIL 35 WNW CWSP CWSP 15 SSE CLM 10 NNW SHN 25 WSW OLM
20 WNW TDO 10 WSW TDO TDO 15 ENE TDO 20 ENE TDO 30 ENE TDO
35 ENE TDO 40 E TDO 45 N CZK 30 N CZK 20 NNE CZK 10 E CZK
20 SE CZK 30 SSE CZK 40 S CZK 35 SE UAO 10 S TTD PDX 10 ENE SPB
10 SSE KLS 10 NNW SPB 15 W SPB 10 W HIO MMV 10 NW SLE 15 SW SLE
CVO 20 WSW CVO 25 S ONP 55 W ONP.


...PACIFIC NORTHWEST...

IN THE 15Z UPDATE A MARGINAL RISK OF EXCESSIVE RAINFALL REMAINS
FOR THE AFTERNOON AND OVERNIGHT PERIOD IN WESTERN WASHINGTON AND
OREGON. WE EXPANDED THE RISK AREA SOUTHWARD JUST VERY
MINIMALLY...ABOUT 30 MILES...ALONG BOTH THE COASTAL RANGES AND THE
CASCADES...OWING TO THE SOUTHWARD EXTENT OF DEEP FORCING AND
ONSHORE FLOW IN OUR PREFERRED GFS/CMC SOLUTION...ALTHOUGH WE DID
NOT TAKE THE QPF DETAILS FROM THOSE TWO MODELS. IT APPEARED
SEVERAL OF THE HI-RES MODELS MET OUR EXPECTATIONS...AND THOSE SUCH
AS THE NSSL WRF BRING RAIN RATES APPROACHING A HALF INCH PER HOUR
DOWN TO ABOUT HIGHWAY 20 IN THE CASCADES AND TO SEAROSE BEACH
ALONG THE COAST BY 12Z MONDAY.

PREVIOUS DISCUSSION...AS A POSITIVELY TILTED LONG WAVE TROUGH
APPROACHES THE PACIFIC NORTHWEST DURING DAY 1...SHORT WAVE ENERGY
EJECTED FROM THE LONG WAVE TROUGH INTERACTS WITH 1.25 INCH
PRECIPITABLE WATER AIR TRANSPORTED TO THE WA/OR COAST ON THE NOSE
OF A 50/60 KNOT LOW LEVEL SOUTHWEST FLOW. THERE IS A MULTI MODEL
SIGNAL FOR THE BEST LOW LEVEL MOISTURE CONVERGENCE TO OCCUR IN THE
20/00Z TO 20/06Z TIME FRAME (WHEN THE MOISTURE FLUX IS EXPECTED TO
PEAK BETWEEN THREE AND FOUR STANDARD DEVIATIONS ABOVE THE MEAN).
THE STRONG UPSLOPE FLOW...COUPLED WITH SYNOPTIC SCALE ASCENT IN
THE LEFT FRONT QUADRANT OF A 110 KNOT JET STREAK APPROACHING THE
OR COAST...SHOULD BE SUFFICIENT TO PRODUCE AXES OF 2.50 TO 4.00
INCHES OF QPF IN THE FAVORED UPSLOPE REGIONS...WITH THE HIGHEST
AMOUNTS ACROSS THE HIGHER TERRAIN OF THE WA OLYMPIC RANGE (WHICH
IS FAVORED IN THE LOW LEVEL SOUTHWEST FLOW).

HOURLY RAINFALL RATES IN EXCESS OF 0.50 INCHES COULD BEGIN ACROSS
PORTIONS OF WA/OR AS EARLY AS 20/00Z...BUT GIVEN THE TIMING OF THE
BEST LOW LEVEL MOISTURE AND UPSLOPE FLOW...THE BEST THREAT FOR
HIGHER RAINFALL RATES SHOULD OCCUR BETWEEN 20/00Z AND 20/06Z
(COINCIDING WITH THE BEST LOW LEVEL MOISTURE FLUX). THE MOISTURE
PLUME DROPS SOUTH AS THE LONG WAVE TROUGH REACHES THE COAST AFTER
20/06Z...WITH THE HIGHEST RAINFALL RATES MOVING INTO FAVORED
UPSLOPE AREAS OF OR.

AFTER COLLABORATING WITH WFO SEW...A MARGINAL RISK AREA WAS
EXTENDED FROM THE OLYMPIC RANGE IN WA INTO THE OR COASTAL
RANGE...AS WELL AS THE SOUTHERN WA/NORTHERN OR CASCADES FOR DAY 1.

HAYES/BURKE
$$




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