Quantitative Precipitation Forecast
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133
FOUS30 KWBC 160015
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
714 PM EST WED NOV 15 2017

...VALID 01Z THU NOV 16 2017 - 12Z THU NOV 16 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
105 W S47 75 WSW AST 15 SW AST 25 W SPB 15 NW MMV 10 NW CVO
30 NNW RBG 15 WNW SXT 45 ESE CEC 15 NW O54 O54 15 ENE O54
30 ENE RDD 40 ENE CIC 35 NNE BLU TRK 55 SSE TVL 25 W MMH
30 WSW BIH 35 SSW MMH 45 W MMH 40 ENE MOD 20 ENE SCK 10 SE SAC
10 SSW MYV 15 SSW CIC 10 SSE RBL 10 NNE RBL RDD 25 W RDD
35 S O54 40 NE UKI 15 NW SUU 10 N CCR 10 ESE LVK 15 E RHV
15 NE WVI WVI 35 W WVI 65 SW SFO.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
50 W ONP 25 NW ONP 15 NNE ONP 25 SSE ONP 15 ENE OTH 30 W SXT
30 NNE CEC 30 NNW CEC 60 NW CEC 75 SW OTH.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 SSW UKI 20 WNW UKI 20 SE O87 15 NNE O87 35 NE O87 35 SSW O54
45 WSW RBL 25 E UKI 20 NNW APC 10 NNW CCR 10 NNE SJC 10 SSW NUQ
15 SW SFO 15 NNW SFO 25 S STS 15 W STS 20 SSW UKI.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
BAB OVE CIC 10 NNE CIC 20 NE CIC 25 NE CIC 40 NNW BLU
15 NNE BLU 15 WSW TVL 45 SSW TVL 45 ESE MHR 30 ESE MHR
10 SSW AUN BAB.


16/01Z UPDATE...

WITH SATL AND SFC DATA SHOWING WEAK SFC LOW ABOUT TO MOVE INTO THE
NRN CA COAST AT 00Z..THE MAIN BAROCLINIC BAND WILL CONTINUE TO
SINK SWD TOWARD CNTL CA.  THIS WILL LESSEN ANY HEAVY RAIN IMPACTS
FARTHER NORTH ALONG THE OREGON COAST AND FOR THAT REASON...HAVE
REMOVED THE MARGINAL AND SLIGHT AREAS ACROSS THAT REGION FOR THE
REMAINDER OF THE OVERNIGHT PD.  ELSEWHERE...HAVE MAINTAINED SLIGHT
RISK AREA FARTHER SOUTH ALONG THE CA COAST INTO THE BAY
AREA..EASTWARD INTO THE NORTHERN SIERRA..WHERE MDTLY STG MSTR FLUX
AHEAD OF SLOW SWD MOVING FRONTAL BAND COULD LEAD TO LOCAL HOURLY
RATES OF .50"+.  WHERE THOSE HEAVIER AMOUNTS OCCUR OVER BURN SCAR
AREAS...FLASH FLOODING/DEBRIS FLOWS MAY BE PSBL.  SULLIVAN



20Z UPDATE...

WHILE NO MAJOR CHANGE HAS BEEN MADE TO EXCESSIVE RAIN
REASONING...WE HAVE ADJUSTED THE SLIGHT RISK AREA TO COVER MUCH OF
THE SFO BAY REGION THROUGH THE OVERNIGHT PD AS EXPECT NEARLY E/W
HEAVY RAIN AXIS TO MOVE VERY SLOWLY INTO THAT AREA OVERNIGHT WITH
POTNL FOR HOURLY RAIN RATES APPROACHING OR LOCALLY EXCEEDING .50"
AN HOUR GIVEN AXIS OF 1.25"+ PWS ACCOMPANYING THE ASSOCD COLD
FRONT.   SULLIVAN/BURKE


...UPSLOPE OF THE NORTHERN SIERRA AND COASTAL RANGES IN NORTHERN
CALIFORNIA AND SOUTHWEST OREGON...

AT 15Z WE MADE A VERY MINOR CHANGE TO EXTEND MARGINAL RISK FARTHER
SOUTH IN THE SIERRAS...ACCOUNTING FOR SOME OF THE HI-RES MODEL
SOLUTIONS AND THE LIKELIHOOD THAT HEAVIER RAIN RATES WILL SLIP
SOUTHWARD BY LATE WEDNESDAY NIGHT AS THE SYNOPTIC TROUGH CONTINUES
TO PROGRESS. THE OUTLOOK OTHERWISE REMAINS UNCHANGED. THE
CONNECTION TO SUB-TROPICAL MOISTURE IS ESPECIALLY PRONOUNCED OVER
CALIFORNIA...BUT A LENGTHY PERIOD OF ENHANCED SOUTHWESTERLY
ONSHORE FLOW CONTINUES TO SUPPORT THE NOTION OF A SLIGHT RISK FOR
SOUTHWEST OREGON AS WELL. IN THE NORTHERN SIERRAS...24-HOUR
FORECAST TOTALS THROUGH 16/12Z...OF 5 TO 8 INCHES LIQUID...WOULD
MEET THE 5-YEAR AVERAGE RECURRENCE INTERVAL PER THE WPC
EXPERIMENTAL EXTREME PRECIPITATION MONITOR.

PREVIOUS DISCUSSION...A STRONG TROUGH WILL MAINTAIN DEEP LAYERED
CYCLONIC FLOW TO ITS SOUTHEAST...FROM THE PACIFIC NORTHWEST
SOUTHWARD INTO NORTHERN CALIFORNIA. THE STRONGEST ONSHORE FLOW AND
HEAVIEST PRECIPITATION FOCUSED INTO THE OLYMPIC PENINSULA EARLY
THIS MORNING WILL BE SHIFTING SOUTHWARD INTO COASTAL OREGON AND
FARTHER SOUTH BY EARLY AFTERNOON INTO NORTHERN CALIFORNIA AND THE
NORTHERN SIERRA.  RAINFALL RATES WEDNESDAY MORNING INTO EARLY
AFTERNOON WILL CONTINUE IN THE .25-50"+ RANGE INTO THE MID TO
LOWER COAST OF OREGON WITH A SLIGHT RISK AREA MAINTAINED. PLEASE
SEE WPC MESOSCALE PRECIPITATION DISCUSSIONS FOR ADDITIONAL
DETAILS.

PW VALUES THAT HAVE BEEN NEAR SEASONAL NORMS WITH THE HEAVY PRECIP
OVER THE PACIFIC NORTHWEST WILL BE INCREASING TO MUCH ABOVE
AVERAGE DURING THE DAY 1 PERIOD ACROSS NORTHERN CALIFORNIA INTO
THE NORTHERN SIERRA AS THIS TROF TAPS A POOL OF HIGH PW VALUES
EXTENDING INTO THE SUB TROPICS ALONG 130-140W. SUBSEQUENTLY THE
MOISTURE FLUX VALUES THAT HAVE BEEN SLIGHTLY ABOVE AVERAGE INTO
THE COASTAL PAC NW WILL INCREASE TO MUCH ABOVE AVERAGE...3 TO 4
STANDARD DEVIATIONS ABOVE THE MEAN INTO NORTHERN
CALIFORNIA/NORTHERN SIERRA DURING WEDNESDAY. THERE IS A STRONG
MODEL SIGNAL FOR VERY HEAVY PRECIPITATION AMOUNTS INTO THE
NORTHERN SIERRA GIVEN THE MUCH ABOVE AVERAGE PW VALUES/MOISTURE
FLUX WITH CONSENSUS FOR 5 TO 8 INCH AREAL AVERAGE AMOUNTS. TOTALS
ALONG THE NORTHWEST CALIFORNIA COAST RANGE INTO FAR SOUTHWEST
OREGON WILL NOT BE AS HEAVY...BUT AT LEAST 2 TO 4 INCH AMOUNTS ARE
EXPECTED. A SLIGHT RISK AREA WAS ADDED FOR THE NORTHERN CALIFORNIA
COAST RANGE TO COVER MANY OF THE BURN AREAS TO THE NORTH OF SAN
FRANCISCO. RAINFALL RATES 0.25 TO 0.50 INCHES PER HOUR POSSIBLE
ACROSS THESE AREAS. FARTHER INLAND---NO SIGNIFICANT CHANGES MADE
TO THE PREVIOUS EXCESSIVE RAINFALL POTENTIAL OUTLOOK. HOURLY RATES
LIKELY WILL BE GREATER HERE...IN THE 0.50 TO 0.75 INCH RANGE IN
THE UPSLOPE REGION OF THE NORTHERN SIERRA. PER SATELLITE-DERIVED
INFORMATION FROM NESDIS AND A FEW HI-RES MODEL QPFS...ISOLATED
INCH PER HOUR AMOUNTS ARE POSSIBLE LATER TONIGHT DURING THE PERIOD
OF GREATEST DEEP LAYER FORCING AND STRONGEST UPSLOPE. SNOW LEVELS
OVER THE NORTHERN SIERRA WILL REMAIN HIGH FOR MOST OF THE DAY 1
PERIOD...6000 TO 7000 FEET...WITH MOST OF THE PRECIPITATION
FALLING AS RAIN...ENHANCING THE RUNOFF POTENTIAL. PLEASE SEE WPC
MESOSCALE PRECIPITATION DISCUSSION #0932 WHICH WILL BE ISSUED
SHORTLY AND VALID UNTIL APPROXIMATELY 2100 UTC.

ORAVEC/BURKE

$$





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