Quantitative Precipitation Forecast
Issued by NWS

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000
FOUS30 KWBC 201406
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
905 AM EST MON NOV 20 2017

...VALID 15Z MON NOV 20 2017 - 12Z TUE NOV 21 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
70 WNW CEC 50 SW OTH 30 SSE OTH 20 NW SXT 15 SSW SXT 30 SW MFR
20 SSE MFR 15 ESE MFR 20 NE MFR 45 NNE MFR 45 ESE EUG 40 ESE SLE
40 SSE TTD 35 NNW RDM 65 SSW RDM 45 NNE LMT LMT 35 S LMT
30 E MHS 15 S MHS 15 ENE O54 30 W RBL 40 NE UKI 15 W UKI
65 SSW O87 85 SW O87.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
50 NW CEC 40 WNW SXT 35 NW O54 35 ENE O87 O87 40 W O87.


1500 UTC UPDATE

NO CHANGES MADE TO THE PREVIOUS EXCESSIVE RAINFALL POTENTIAL
OUTLOOK.

ORAVEC

INITIAL DISCUSSION

...PACIFIC NORTHWEST/NORTHERN CA...

THE FOCUS FOR HEAVY RAINFALL DURING DAY 1 DROPS SOUTH AFTER
20/18Z...WHERE A 40 TO 50 KNOT LOW LEVEL SOUTH SOUTHWEST FLOW
TRANSPORTS 1.25 INCH PRECIPITABLE WATER AIR (WHICH IS BETWEEN TWO
AND THREE STANDARD DEVIATIONS ABOVE THE MEAN) ACROSS FAR SOUTHWEST
OR INTO NORTHWEST CA. THE SOURCE FOR THIS MOISTURE PLUME CAN BE
TRACED BACK TO SOUTH OF HI...AND THE LOW LEVEL FLOW FOCUSES THIS
MOISTURE ON THE HIGHER TERRAIN. MODEL SOUNDINGS FROM THE 00Z GFS
SHOWED MARGINAL INSTABILITY IN THE COLUMN EMBEDDED IN THE PACIFIC
MOISTURE PLUME (WITH MUCAPE BETWEEN 100 AND 250 J/KG)...WHICH
AUGMENTS THE STRONG UPSLOPE FLOW. THE LOW LEVEL SOUTHWEST FLOW
MOVES NORTH INTO OR AND EVENTUALLY WA STATE BY THE END OF THE
PERIOD.

THERE WAS A MULTI MODEL SIGNAL FOR 4.00 TO 5.25 INCHES OF QPF
ACROSS FAR SOUTHWEST OR AND NORTHWEST CA...WHICH REPRESENTS A
SOUTHWARD DISPLACEMENT FROM THE PREVIOUS MODEL SOLUTIONS. THE BEST
LIFT AND HIGHEST RAINFALL RATES ARE EXPECTED BETWEEN 20/18Z AND
21/06Z...DURING WHICH TIME FLOODING IS POSSIBLE...ESPECIALLY IN
BURN SCAR AREAS...POSSIBLY RESULTING IN ROCK SLIDES. A SLIGHT RISK
WAS PLACED OVER NORTHWEST CA AND FAR SOUTHWEST OR ON DAY 1 AS A
RESULT.

A MARGINAL RISK WAS EXTENDED NORTHWARD INTO THE SOUTHERN AND
CENTRAL OR CASCADES...AS THE SOUTHWEST FLOW IN THE LOWER LEVELS
RAISES SNOW LEVELS. ALMOST ALL OF THE 2.50 TO 3.50 INCHES OF QPF
ARE EXPECTED TO FALL IN THE FORM OF RAIN...AND RAINFALL RATES NEAR
0.50 INCHES COULD RESULT IN ROCK SLIDES AND LOCALIZED FLOODING
OVER BURN SCARS.

HAYES
$$





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