Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 210150
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
850 PM EST SAT DEC 20 2014

...VALID 03Z SUN DEC 21 2014 - 00Z MON DEC 22 2014...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 E MFR 20 E RBG 30 SE EUG 25 ESE SLE TTD 35 NNW CZK 45 N CZK
40 NNE CZK 15 NNW DLS 25 SSW DLS 25 NW RDM 25 SSW RDM 60 SSW RDM
60 N LMT 20 NW LMT 30 WNW LMT 15 E MFR.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 WNW TDO 15 SW KLS HIO 15 WNW SLE 10 SW EUG 25 NW SXT
45 ESE CEC 30 WNW O54 10 SSE ACV 30 NNW CEC 30 SSW OTH
30 SSW ONP 15 S AST 15 SSE HQM 30 N HQM 20 E UIL 10 SW CLM
30 SSE CLM 10 S SHN 15 WNW TDO.


...WA TO NORTHERN CA COASTAL RANGES AND THE OR/SOUTHERN WA
CASCADES...

THE WV LOOP SHOWED THE STRONG JET STREAK (NEAR 130 KNOTS) PLOWING
INTO WESTERN WA EARLY THIS EVENING. AHEAD OF THE JET STREAK AND
STRONG SHORT WAVES ASSOCIATED WITH IT...A 40 TO 50 KNOT LOW LEVEL
WESTERLY FLOW FUNNELS 1.25 INCH PRECIPITABLE WATER AIR (WHICH IS
BETWEEN TWO AND THREE STANDARD DEVIATIONS ABOVE NORMAL) INTO
WA/OR/NORTHERN CA. THE MOST SIGNIFICANT LIFT WITH THE JET STREAK
APPEARS TO OCCUR BEFORE 21/12Z...WITH THE MOST INTENSE VERTICAL
MOTION BEFORE 21/06Z ACROSS WESTERN WA/OR. THE COMBINATION OF LIFT

AND MOISTURE BECOMES FOCUSED ON THE TERRAIN... WITH A STRONG
UPSLOPE COMPONENT ON BOTH THE COASTAL RANGES AND THE CASCADES.

BOTH THE LATEST THE LATEST RAP AND HRRR SOLUTIONS SUGGEST MARGINAL
INSTABILITY CROSSES THE PACIFIC NORTHWEST (PARTICULARLY OR) BEFORE

21/06Z...IN CONJUNCTION WITH THE BEST SYNOPTIC SCALE LIFT. 00Z
SOUNDINGS AT KUIL AND KSLE SHOWED SURAFCE BASED CAPE VALUES LESS
THAN 100
J/KG...IN LINE WITH WHAT RAP SPUNDINGS IN WA/OR HAD BEEN
ADVERTIZING.  RAP MODEL SOUNDINGS SUGGEST THAT MOST OF THE
INSTABILITY IS ELEVATED... SO IT IS NOT CLEAR WHETHER THE MARGINAL
INSTABILITY WILL HAVE AN IMPACT ON QPF AMOUNTS ACROSS WA/OR
THROUGH 21/06Z. THUS...THE HEAVY RAIN POTENTIAL LOOKS AS THOUGH IT
WILL BE MORE OF A FLOOD VERSUS FLASH FLOOD SCENARIO...WITH THE
SYNOPTIC SCALE LIFT AND UPSLOPE FLOW PRODUCING THE BEST QPF IN THE
TERRAIN.

THE BEST THREAT FOR FLASH FLOODING OCCURS THROUGH ABOUT
21/06Z...AS THERE IS A MULTI MODEL SIGNAL (LED BY THE LATEST SUITE
OF HIGH RESOLUTION GUIDANCE...INCLUDING THE HRRR...RAP AND 5 KM
NAM-ARW) THAT THE BEST LIFT AND MOIST IS PLACED OVER OR. QPF
AMOUNTS WERE INCREASED ACROSS THE OR COASTAL RANGE...AS WELL AS
THE CENTRAL OR CASCADES. THE LATEST SSEO MEAN NEIGHBORHOOD
PROBABILITIES INDICATE THE POTENTIAL FOR FLASH FLOODING...MAINLY
ALONG THE OR AND SOUTHERN WA CASCADES. THUS...THE SLIGHT RISK
AREAS REMAIN IN PLACE...AS THE SSEO MEAN PROBABILITIES ARE PEGGED
OVER 70 PERCENT.  HOWEVER...THE LACK OF DEEP INSTABILITY SUGGESTS
THAT THIS EVENT MAY BE MORE OF A FLOOD EVENT...WHERE THE HEAVY
RAIN AND ANY SNOW MELT CAUSE RUNOFF ISSUES.

HAYES

$$





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