Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 190001
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
801 PM EDT WED OCT 18 2017

...VALID 01Z THU OCT 19 2017 - 12Z THU OCT 19 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
45 W UIL 45 NNW UIL 20 W CWSP 10 ESE CLM 25 NNW PWT 15 NW BFI
BFI TIW 10 WNW GRF 10 ESE OLM 20 SE OLM 25 SSE GRF 25 SE TCM
15 ESE TCM 20 SE SEA RNT 20 ESE PAE 10 ESE AWO 10 SE BLI CYXX
CWZA CYHE 15 WSW CWPR 30 SSE CWPR 25 N KS52 KS52 20 S KS52
35 N EAT 30 NNW EAT 35 NW EAT 30 WNW EAT 25 W EAT SMP 20 SW SMP
20 SSW SMP 25 S SMP 30 WNW YKM 35 W YKM 45 WSW YKM 35 NNW DLS
10 ENE CZK CZK 10 NE TTD 15 NNE PDX KLS 15 WNW KLS 15 WSW KLS
25 WSW KLS 20 NW HIO 15 WNW MMV 20 SSE S47 25 WSW MMV 25 W SLE
25 NW CVO 20 NE ONP 30 N ONP 40 WNW S47.


0100 UTC UPDATE

NO CHANGES MADE TO THE PREVIOUS EXCESSIVE RAINFALL POTENTIAL
OUTLOOK.  SEE MPD#0891 VALID UNTIL 0530Z FOR ADDTIONAL DETAIL.

SULLIVAN

INITIAL DISCUSSION

...PACIFIC NORTHWEST...

FLAT SHORTWAVE UPPER RIDGE AXIS WILL PUSH EAST OF THE NORTHERN
ROCKIES LATER TODAY...GIVING WAY TO AN AMPLIFYING UPPER TROUGH
THAT WILL APPROACH THE PAC NW COAST ON THU. INCREASING UPPER LEVEL
DIFFLUENCE AND DEEP-LAYER MOIST ISENTROPIC ASCENT WITHIN THE WARM
CONVEYOR BELT (WCB) AHEAD OF THE AMPLIFYING TROUGH WILL DIRECT AN
ATMOSPHERIC RIVER/ELONGATED PLUME OF DEEP-LAYER MOISTURE INTO
WESTERN WA AND EVENTUALLY WESTERN OR WEDNESDAY AND WEDNESDAY
NIGHT. SOUTHWESTERLY 850 MB FLOW INCREASING TO ~50 KTS WILL LEAD
TO ROBUST LOW-LEVEL MOISTURE TRANSPORT INTO THE COASTAL AND
CASCADE RANGES...WITH 850 MB MOISTURE FLUX ANOMALIES EXPECTED TO
TOP OUT BETWEEN 4-5 STANDARD DEVIATIONS ABOVE NORMAL ACROSS
WESTERN WA AND FAR NORTHWEST OR PER THE SREF AND GEFS.

IN ADDITION TO THE FAVORABLE DYNAMICAL SETUP AND MOISTURE PROFILE
AHEAD OF THE TROUGH...STRENGTHENING RIGHT ENTRANCE REGION FORCING
VIA THE BUCKLING UPPER JET STREAK THAT WILL DROP SOUTH OF B.C. AND
INTO THE PAC NW LATER TODAY/TONIGHT WILL ENHANCE THE DEEP-LAYER
ASCENT AND LOW LEVEL FRONTOGENESIS. THIS WILL LIKELY COMPOUND PCPN
TOTALS OVER THE FAVORED HIGHER TERRAIN (WESTERN SLOPES) OF THE
COASTAL-CASCADE RANGES WED-WED NIGHT AS THE GUIDANCE OF LATE HAS
CONSISTENTLY SHOWN. TO THAT END...CONSIDERING THE PROLIFIC
TOTALS...CHANGES TO THE QPF FROM CONTINUITY (PREVIOUS WPC
FORECAST) WERE NEGLIGIBLE...WHICH YIELDED 5KM AREAL-AVERAGE TOTALS
OF 6-8 INCHES ACROSS THE OLYMPICS AND 3-5 INCHES OVER THE NORTHERN
WA CASCADES. LESSER AMOUNTS (1-3") WERE NOTED ACROSS THE SOUTHERN
WA COASTAL-CASCADE RANGES ALONG WITH THE NORTHERN OR COASTAL MTNS.
THE LACK (ABSENCE) OF DEEP-LAYER INSTABILITY WILL INHIBIT
SHORT-TERM RAINFALL RATES AND THUS LIMIT THE RISK OF FLASH
FLOODING...HOWEVER...THE VOLUME OF RAINFALL EXPECTED COULD VERY
WELL LEAD TO SOME LONGER DURATION RUNOFF ISSUES LATER IN THE
PERIOD (LATE WED-WED NIGHT) AS THE SOIL CONTINUES TO SATURATE.

HURLEY

$$





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