Quantitative Precipitation Forecast
Issued by NWS

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003
FOUS30 KWBC 200003
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
803 PM EDT THU OCT 19 2017

...VALID 01Z FRI OCT 20 2017 - 12Z FRI OCT 20 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 NNW CEC 40 SSW OTH 35 SSE OTH 25 SSW SXT 35 W SIY 25 N O54
20 NW O54 30 ENE ACV 10 S CEC 40 NNW CEC.


01 UTC UPDATE

A SMALL WINDOW REMAINS OVER THE NEXT FEW HOURS FOR LOCAL RAIN
RATES OF .50"+ WITHIN THE CURRENT MARGINAL RISK AREA BEFORE MAIN
AREA OF HEIGHT FALLS FROM APPROACHING S/WV TROF OVER THE ERN PAC
OFF THE OR/CA COAST HELPS PUSH FRONTAL PCPN BAND SOUTH AND EAST OF
THE SW OREGON/NW CA COASTS.  THOUGH SOME POST FRONTAL SHWER
ACTIVITY IS EXPECTED...HOURLY RATES ARE NOT EXPECTED TO BE NEARLY
AS HEAVY AND THUS OVERALL RISK OF ANY ISOLD FLOODING CONCERNS
SHOULD DIMINISH BY 04-05Z.

SULLIVAN

INITIAL DISCUSSION


...PACIFIC NORTHWEST...

A PLUME OF MOISTURE AND ASSOCIATED RAINFALL AXIS WILL SHIFT SOUTH
ACROSS THE PACIFIC NORTHWEST TODAY AS THE MID/UPPER LEVEL TROUGH
CONTINUES TO PUSH EASTWARD. TO START THE DAY WILL HAVE MOISTURE
TRANSPORT MAGNITUDES AROUND THE CLIMATOLOGICAL 99TH
PERCENTILE...PWAT VALUES AROUND THE 90TH PERCENTILE...AND LOW
LEVEL ONSHORE UPSLOPE FLOW ABOVE THE 95TH PERCENTILE. THUS WILL
CONTINUE TO BE A GOOD SETUP FOR OROGRAPHICALLY ENHANCED MODERATE
TO LOCALLY HEAVY RAINFALL. AS WE HEAD THROUGH THE DAY THESE
PARAMETERS ALL TEND TO SHOW A DOWNWARD TREND...WHICH SHOULD ALLOW
FOR RAINFALL RATES TO BE A BIT LESS IMPRESSIVE WITH TIME. THE
QUICK FORWARD MOVEMENT OF THE TOUGH AND ASSOCIATED ATMOSPHERIC
RIVER WILL ALSO HELP CAP THE UPPER MAGNITUDE OF RAINFALL AMOUNTS.
STILL LIKELY TO SEE SHOWERY PRECIPITATION CONTINUE BEHIND THE MAIN
ATMOSPHERIC RIVER GIVEN THE COLD MID LEVEL AIR UNDERNEATH THE
TROUGH AND CONTINUED ONSHORE FLOW. IN GENERAL LOOKING AT
ADDITIONAL RAINFALL AMOUNTS OF 1-3" ACROSS THE WA/OR CASCADES AND
THE COASTAL RANGE OF FAR NORTHWEST CA AND SOUTHWEST OR. LOCALIZED
3-5" AMOUNTS APPEAR PROBABLE BOTH ALONG THIS COASTAL RANGE AND THE
CASCADES FROM NORTHERN OR INTO FAR SOUTHERN WA. RAINFALL RATES
SHOULD BE MAXIMIZED ACROSS THE COASTAL RANGE LOCATIONS IN
NORTHWEST CA AND SOUTHWEST OR...WHERE AMOUNTS AROUND 0.5" IN AN
HOUR ARE POSSIBLE...MAINLY BETWEEN 18Z AND 0Z...AND WILL MAINTAIN
A DAY 1 MARGINAL RISK ACROSS THIS AREA. CAN NOT RULE OUT SOME
MINOR FLOODING ISSUES ALSO DEVELOPING ACROSS PORTIONS OF THE OR
CASCADES...ALTHOUGH GENERALLY WEAKER RAINFALL RATES HERE SHOULD
LIMIT THE OVERALL THREAT.

CHENARD
$$





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