Quantitative Precipitation Forecast
Issued by NWS

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000 FOUS30 KWBC 230003 QPFERD EXCESSIVE RAINFALL DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 703 PM EST WED NOV 22 2017 ...VALID 01Z THU NOV 23 2017 - 12Z THU NOV 23 2017... ...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL... MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM 35 NNW CWZA 10 N CWZA 10 SSE CWZA 25 S CWZA 35 ENE BVS 35 ENE AWO 40 E AWO 25 NNE SMP 15 ENE SMP 15 ESE SMP 30 SSE SMP 35 W YKM 45 WSW YKM 25 NNW DLS 15 E CZK 15 W DLS 15 S CZK 15 ESE TTD TTD 20 NNE PDX 25 E KLS 25 E TDO 25 SE TCM 10 SE RNT 15 SE PAE 10 SE AWO BVS 10 S BLI 10 E ORS ORS CWZO CWDR 15 SSE CWDR 20 SW NUW 25 SSW NUW 20 W PAE 20 N PWT PWT 10 S PWT 10 W TIW 15 ESE SHN 10 WSW OLM 15 SW OLM 10 NW TDO 10 SW TDO 10 NW KLS 10 WNW SPB 15 W SPB 15 WNW HIO MMV 15 W SLE CVO 10 WSW EUG 20 SW EUG 30 NE OTH 15 NNE OTH 20 NNW OTH 40 NNW OTH 40 WSW ONP. SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM 50 NW UIL 25 N UIL 10 SSW CWSP 10 NW CLM CLM 20 SE CLM 25 NW PWT 15 N SHN HQM 40 WSW HQM. 23/01 UTC UPDATE NO CHANGES MADE TO THE PREVIOUS EXCESSIVE RAINFALL POTENTIAL OUTLOOK. LATEST HI RES CONTINUES TO SUPPORT AN UPTICK IN RAINFALL RATES ACROSS PORTIONS OF WRN WA/OR..ESPECIALLY THE OLYMPICS OVERNIGHT AS SECONDARY FRONTAL BNDRY APPROACHES FROM THE WEST ALONG WITH GRADUAL VEERING OF THE LOW LEVEL FLOW TO A MORE SW OR WSWLY DIRECTION. RECENT HRRR RUNS SUPPORT ADDITIONAL RAINFALL AMOUNTS UP TO 2 INCHES OVER PORTIONS OF THE OLYMPICS AND CASCADES. HOURLY RATES ARE NOT FCST TO GET MUCH OVER .30" AN HOUR BASED ON THE LATEST HRRR RUNS..BUT GIVEN THE ANTECEDENT VERY WET GROUNDS...THESE HIGHER RATES COULD SUPPORT THE POTENTIAL FOR LAND/MUD SLIDES AS WELL AS EXACERBATE LOCAL FLOODING ISSUES. SULLIVAN INITIAL DISCUSSION ...PACIFIC NORTHWEST... THE CURRENT SHOT OF DEEPER ATMOSPHERIC MOISTURE IS EXPECTED TO CLEAR THE PACIFIC NORTHWEST AND NORTHERN ROCKIES DURING THE FIRST PART OF DAY 1. HOWEVER...THE MOST RECENT IR IMAGES SHOWED THE NEXT SLUG MOISTURE RIDING NORTHWARD FROM NEAR 32N 133W...WHERE THE BLENDED TPW PRODUCT SHOWED PRECIPITABLE WATER VALUES BETWEEN 1.50 AND 2.00 INCHES. THE ONSHORE FLOW WILL CONTINUE TO PRODUCE UPSLOPE CONDITIONS ACROSS THE PACIFIC NORTHWEST... WITH RAINFALL RATES INCREASING AFTER 23/00Z...AS SHORT WAVE ENERGY EJECTED FROM THE LONG WAVE TROUGH APPROACHES THE PACIFIC NORTHWEST. THERE WAS A MULTI MODEL SIGNAL FOR HOURLY RAINFALL RATES TO INCREASE TO 0.50 INCHES OR GREATER IN THE 23/06Z TO 23/12Z TIME FRAME...AS A 40 KNOT LEVEL SOUTHWEST FLOW TRANSPORTS 1.00 INCH PRECIPITABLE WATER ACROSS WESTERN WA. THE INCREASED MOISTURE AND UPSLOPE FLOW IS EXPECTED TO RESULT IN LOCAL 3.00 TO 4.00 INCH QPF AMOUNTS CENTERED ON THE WA OLYMPIC RANGE (WHICH IS FAVORED IN THE SOUTHWEST FLOW REGIME). SINCE THIS AREA HAS SEEN HEAVY RAINFALL OVER THE PAST COUPLE OF DAYS...THE ADDITIONAL RAINFALL RAISES THE POTENTIAL FOR ROCK SLIDES AND LAND SLIDES IN FAVORED LOCATIONS. A SLIGHT RISK AREA WAS PLACED HERE FOR DAY 1 TO ACCOUNT FOR THE ELEVATED FLOOD THREAT. THE LOW LEVEL SOUTHWEST FLOW ALSO TARGETS THE WA CASCADES...WHERE THE COMBINATION OF UPSLOPE FLOW AND SYNOPTIC SCALE ASCENT IS EXPECTED TO PRODUCE 1.75 TO 2.25 INCHES OF QPF...WITH THE HIGHEST AMOUNTS OVER THE NORTHERN WA CASCADES. SNOW LEVELS REMAIN ELEVATED ACROSS THE WA CASCADES...SO ALMOST ALL OF THE QPF IS EXPECTED TO BE IN THE FORM OF RAINFALL. WITH THIS IN MIND...A MARGINAL RISK WAS EXTENDED HERE TO COVER THE FLOOD THREAT. HAYES $$

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