Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 202348
QPFERD

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

...VALID 00Z 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 150 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. THIS IN
NOT READILY EVIDENT IN THE LATEST IR LOOP. 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.

AS MENTIONED EARLIER...THERE WAS NOT MUCH IN THE WAY OF CHANGE TO
THE PREVIOUS QPF FORECAST FOR THE PACIFIC NORTHWEST (AND EVEN LESS

FOR THE NORTHERN ROCKIES). MOST OF THE CHANGE WAS MADE IN THE
FIRST 6 HOURS...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 UPDATED AMOUNTS STILL POINT TO AN
AXIS
OF 5 TO 7 INCHES (WITH LOCAL AMOUNTS POSSIBLE TOUCHING 10 INCHES)
OF QPF THROUGH 22/00Z IN THIS AREA. 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 RUNOFF FROM THE HEAVY RAIN
AND ANY SNOW MELT CAUSE RUNOFF ISSUES.

HAYES

$$





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