Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 101409
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
908 AM EST SAT DEC 10 2016

...VALID 15Z SAT DEC 10 2016 - 12Z SUN DEC 11 2016...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 SSE TVL 35 NW MMH 25 WSW MMH 35 NNE FAT 35 NE MCE 35 NE MOD
25 ESE MHR 15 NE MHR 20 E CIC 35 NE CIC 55 N BLU 15 S TRK
40 SSE TVL.


...NORTHERN AND CENTRAL SIERRA-NEVADA...

15Z UPDATE...
NO CHANGE TO PREVIOUS DISCUSSION OR REASONING.  RADAR IMAGERY AS
OF 14Z SHOWED A NEAR PERSISTENT BAND OF MDT TO LOCALLY HEAVY RAIN
ACRS PORTION PORTIONS OF CA NEAR A STATIONARY BNDRY WHERE BLENDED
TPWS INDICATED PW VALUES OF BETWEEN 1.25 AND 1.5 INCHES WITHIN A
WELL DEFINED ATMOSPHERIC RIVER WHICH EXTENDED WELL SW INTO HAWAII.
 RAINFALL RATES WITHIN THIS BAND HAVE BEEN AS HIGH AS .50 TO .70"
PER HR.  THE LARGER IMPACTS WILL REMAIN ACRS THE NRN SIERRA
THROUGH THE AFTERNOON, WITH BAND EXPECTED TO EVENTUALLY SHIFT SWD
AND GRADUALLY WEAKEN WITH PASSAGE OF UPSTREAM S/WV ENERGY LATER
THIS AFTN/THIS EVENING.  THE LONG DURATION OF RAIN COULD LEAD TO
ADDITIONAL RAINFALL AMOUNTS IN EXCESS OF 5 INCHES WHICH GIVEN THE
VERY HI SNOW LEVELS AND RELATIVELY MILD AIR...THE COMBINATION OF
SNOW MELT AND HEAVY RAIN COULD LEAD TO SOME ISOLD RUNOFF PROBLEMS.
SULLIVAN   PREVIOUS DISCUSSION FOLLOWS.

A ROBUST ATMOSPHERIC RIVER WILL BE IMPACTING THE FOOTHILLS AND
HIGHER TERRAIN OF THE NORTHERN AND CENTRAL SIERRA-NEVADA THROUGH
SAT WITH VERY HEAVY RAIN AS A STRONG DEEP LAYER WESTERLY FETCH OF
PACIFIC MOISTURE AND ENHANCED FORCING CROSSES THE REGION. A
CONSENSUS OF THE 00Z HIRES ARW/NNMB AND NSSL-WRF SHOW ENHANCED LOW
TO MID LEVEL ONSHORE FLOW ARRIVING THIS MORNING OUT AHEAD OF A
SHORTWAVE IMPULSE AND AN ASSOCIATED 130 TO 150 KT 250 MB JET
STREAK. THIS WILL DRIVE STRONG LARGE SCALE ASCENT...AND
ADDITIONALLY THE ENERGY WILL BE DRIVING A WEAK AREA OF LOW
PRESSURE ALONG A FRONTAL ZONE THAT WILL CROSS NORTHERN CA BETWEEN
12Z AND 18Z. THE LARGELY ORTHOGONAL COMPONENT OF THE LOW TO MID
LEVEL FLOW RELATIVE TO THE TERRAIN ALONG WITH THE FRONT...COUPLED
WITH 850/700 MB MOISTURE FLUX ANOMALIES OF 3 TO 5 STANDARD
DEVIATIONS ABOVE NORMAL...AND PRECIPITABLE WATER VALUES OF OVER 3
STANDARD DEVIATIONS ABOVE NORMAL WILL FOSTER HEAVY RAINFALL RATES
FOR THE WEST-FACING SLOPES OF THE NORTHERN AND CENTRAL
SIERRA-NEVADA. RAINFALL RATES ARE LIKELY TO REACH UP BETWEEN A
0.50 TO 1 INCH/HR DURING THE HEIGHT OF THE ATMOSPHERIC RIVER
CONDITIONS THIS MORNING...AND THEN GRADUALLY BEGIN TO TAPER DOWN
THIS AFTERNOON. CONDITIONS WILL STEADILY IMPROVE SAT NIGHT WITH
HEAVY RAINS TAPERING OFF BY EARLY SUN. RAINFALL TOTALS OF AS MUCH
AS 5 TO 8 INCHES WILL BE POSSIBLE...WITH THE HEAVIEST AMOUNTS OVER
THE NORTHERN SIERRA-NEVADA. GIVEN THE HEAVY RAINFALL RATES DURING
THE HEIGHT OF THE EVENT AND THE OVERALL STORM TOTALS...SOME RUNOFF
PROBLEMS/CONCERNS ARE POSSIBLE.

ORRISON

$$





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