Hydrometeorological Discussion
Issued by NWS California-Nevada RFC

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HYDROMETEOROLOGICAL DISCUSSION NATIONAL WEATHER SERVICE / CALIFORNIA
NEVADA RFC / SACRAMENTO CA 930 AM PDT WED OCT 29 2014

...WIDESPREAD PRECIP EVENT ON TRACK FOR FRIDAY INTO SATURDAY...

.24 HOUR PRECIPITATION TOTALS (ENDING OCT 29 AT 500 AM PDT)...

A WEAK FRONTAL BOUNDARY MOVED INTO THE NW CORNER OF CA AND SRN
OREGON AND REMAINED STATIONARY THROUGH THE PERIOD.  PRECIPITATION IN
THESE AREAS WAS LIGHT, WITH MOST LOCATIONS REMAINING BELOW 0.1" AND
REMAINING MAINLY NORTH OF CAPE MENDOCINO.  LOCATIONS WITH HIGHER
AMOUNTS INCLUDED 0.1-0.4" IN THE SRN OREGON CASCADES, 0.2-0.5" IN
THE SMITH BASIN, AND UP TO 0.3" IN THE LOWER KLAMATH BASIN.


.METEOROLOGICAL CONDITIONS (WED AM - TUE AM)...

WEAK ANTI-CYCLONIC FLOW ALOFT IS SITUATED OVER THE WEST COAST THIS
MORNING AS AN UPR RIDGE AXIS IS ALIGNED JUST INLAND. A VERTICALLY
STACKED POLAR SYSTEM IS SPINNING OVER THE GULF OF ALASKA NEAR
53N/150W WITH THE ASSOCIATED COLD FRONT EXTENDING PRIMARILY ALONG
140W BEFORE ANGLING BACK TO THE SOUTHWEST SOUTH OF 40N. PLENTY OF
COLD AIR IS WRAPPING INTO THE SYSTEM BEHIND THE FRONTAL
BOUNDARY...EVIDENT BY THE CELLULAR CLOUDINESS ON THE LATEST IR
IMAGERY. ALSO...A WEAK STATIONARY BOUNDARY IS ALIGNED ALONG THE
CA/ORE BORDER AND THEN WEST-SOUTHWEST OFFSHORE. THIS IS THE FOCUSING
MECHANISM FOR LIGHT PRECIP TODAY PRIMARILY OVER FAR NW CALIFORNIA
AND THE CREST OF THE SOUTHERN OREGON CASCADES. 88-D MOSAIC DOES
INDEED SHOW SCATTERED ECHOES STREAMING ONSHORE FROM WSW TO ENE
PRIMARILY NEAR AND NORTH OF ARCATA. SINCE 29/12Z...PRECIP GAGES IN
THIS AREA SHOW AMOUNTS TO BE LIGHT...GENERALLY A FEW HUNDREDTHS.

OVER THE NEXT 24 HOURS...LOOK FOR AN AMPLIFICATION OF THE OVERALL
PATTERN TO OCCUR AS AN APPROX 155-KT UPR JET ON THE BACKSIDE OF THE
UPR LOW OVER THE GULF OF ALASKA DIGS THE SYSTEM SOUTHWARD ACROSS THE
NORTHEAST PACIFIC. THIS WILL ALSO RESULT IN THE DOWNSTREAM UPR RIDGE
TO PUMP UP OVER THE INTERMOUNTAIN WEST. ALONG THE COAST...THE
RESULTING FLOW WILL BECOME SWLY WITH THE STATIONARY BOUNDARY LIFTING
NORTHWARD AND BECOMING MORE SSW-TO-NNE ORIENTED.

THE MOISTURE PLUME THAT WILL BECOME ENTRAINED INTO THE SYSTEM
CURRENTLY STRETCHES FROM JUST WEST OF THE HAWAIIAN ISLANDS TO THE
NE...REACHING THE NORTH COAST. PEAK VALUES WITHIN THE PLUME ARE NEAR
1.50-INCHES WITH A DECENT CHANNEL OF AT LEAST 1.00-INCH PW VALUES
ACCORDING TO THE LATEST BLENDED TPW IMAGERY. AS THE FLOW BECOMES
MORE AMPLIFIED TODAY...ALSO EXPECT THE MOISTURE PLUME TO TEMPORARILY
LIFT NORTHWARD ACROSS THE PACNW.

FOR THE MAJORITY OF THURSDAY...THE SYSTEM WILL REMAIN OFFSHORE WITH
ONLY SOME LIGHT PRECIP EXPECTED ALONG THE NORTH COAST FROM CAPE
MENDOCINO NORTHWARD AND THEN INLAND TO THE CREST OF THE SOUTHERN
OREGON CASCADES. THE FRONTAL BAND WILL REALLY BEGIN TO AFFECT
COASTAL SECTIONS FROM THE SF BAY AREA NORTHWARD DURING THE OVERNIGHT
PERIOD FROM THURSDAY INTO FRIDAY. THE HEAVIEST TOTALS WILL BE
FOCUSED ON THE EEL RIVER BASIN NORTHWARD TO THE SMITH RIVER BASIN.
ONLY LIGHT AMOUNTS WILL FALL OVER THE RUSSIAN RIVER BASIN AND INLAND
OVER THE CREST OF THE SOUTHERN OREGON CASCADES.

AS THE SUN RISES ON FRIDAY...THE COLD FRONT WILL BEGIN TO PRESS
SOUTH AND INLAND. GIVEN THE ORIENTATION OF THE UPR TROF ALIGNED
ALONG 130W AND ITS DEEPENING NATURE ALONG THE COAST...THE FLOW AT
THE LOW- AND MID-LEVELS DEVELOPS A DECENT SOUTHERLY COMPONENT THAT
WILL ALLOW FOR GOOD CONVERGENCE ACROSS THE SHASTA LAKE DRAINAGE. THE
HI-RES 29/06Z NAM IS GENERATING A LLJ AT THE NORTH END OF THE
SACRAMENTO VALLEY WITH VALUES AROUND OR JUST EXCEEDING 30-KT AT THE
925-MB LEVEL. AS A RESULT...FEEL THE EXPECTED PRECIP AMOUNTS NEEDED
TO BE INCREASED FOR THE MORNING AND AFTERNOON PERIOD ACROSS THE
SHASTA LAKE DRAINAGE.

FOR THE AFTERNOON...THE COLD FRONT WILL MOVE ACROSS NORTHERN CA AND
BEGIN TO ENTER WESTERN NEV...TRAILING SSW ACROSS THE SACRAMENTO AREA
AND SF BAY AREA. THE MOISTURE PLUME LOOKS TO HANG TOGETHER DECENTLY
WITH 0.75-INCH PW VALUES ADVECTING INLAND TOWARD THE NORTHERN
SIERRA...1.00-INCH PW VALUES ACROSS THE SOUTHERN SACRAMENTO AND
NORTHERN SAN JOAQUIN VALLEY AND JUST OVER THE 1.00-INCH PW MARK
ALONG THE COAST. WITH INITIAL SOUTHERLY FLOW AT THE MID-LEVELS
TURNING MORE TOWARD THE SOUTH-SOUTHWEST...OROGRAPHICS WILL BECOME
SLIGHTLY MORE FAVORABLE ALONG THE WEST FACING SLOPE OF THE SIERRA
FROM ABOUT I-80 NORTHWARD.

BY THE EVENING...THE SYSTEM IS SHOWING A TENDENCY TO SPLIT WITH AN
UPR LOW JUST WEST OF THE SF NORTH BAY AND A ROBUST MID-LEVEL VORT
MAX AT THE BASE OF THE SYSTEM SWINGING TOWARD PT CONCEPTION. THE
MODELS HAVE COME INTO SLIGHTLY BETTER AGREEMENT IN TERMS OF HOW FAR
THE SYSTEM WILL DIG TO THE SOUTH. AS THIS MID-LEVEL VORT MAX REACHES
THE COAST...THE 29/00Z EC REALLY LIGHTS UP THE PRECIP AMOUNTS FROM
NEAR LAKE TAHOE SOUTH ACROSS THE SAN JOAQUIN VALLEY AND THE CENTRAL
CA COAST. THE 29/06Z GFS IS SLIGHTLY HEAVIER FROM LAKE TAHOE SOUTH
ACROSS THE SAN JOAQUIN VALLEY...BUT DRIER ALONG THE CENTRAL CA
COAST. GIVEN THE FAVORABLE DYNAMICS...TENDED TO LEAN TOWARD THE EC
ALONG THE CENTRAL CA COAST.

OVERNIGHT...AS THE MID-LEVEL VORT MAX ROTATES INLAND THE HEAVIEST
PRECIP SHIFTS OVER THE SOUTHERN SIERRA WITH LIGHTER AMOUNTS OVER
MUCH OF SOUTHERN CA. THE MODELS HAVE BEEN WAFFLING WITH EXPECTED
PRECIP AMOUNTS ACROSS SOUTHERN CA DUE TO HOW FAR SOUTH THE SYSTEM
WILL DIG...BUT ITS STARTING TO APPEAR THAT PRECIP WILL MAKE ITS WAY
SOUTH OF PT CONCEPTION AND ALL THE WAY TO THE US/MEXICO
BORDER...HEAVIEST OVER THE HIGHER TERRAIN. AT THIS TIME...WIDESPREAD
PRECIP WILL ALSO BEGIN TO SHIFT OVER WESTERN NEVADA AHEAD OF THE UPR
TROF AXIS.

WITH THE BROAD UPR TROF SITUATED ACROSS CA AND NEV...ALONG WITH THE
PRIMARY CIRCULATION SITUATED OVER WESTERN NEV LATER SATURDAY
MORNING...LOOK FOR PRECIP BE FOCUSED OVER THE NORTHERN TWO-THIRDS OF
NEVADA. HOWEVER...WITH CYCLONIC FLOW ALOFT CONTINUING ACROSS
CA...LOOK FOR PRECIP TO CONTINUE OVER MUCH OF CA AS WELL WITH THE
FOCUS BEING OVER WEST FACING SLOPES.

THROUGH THE REST OF SATURDAY...THE PRIMARY CIRCULATION WILL LIFT OFF
TOWARD THE NE AND OUT OF THE AREA...BUT AN ELONGATED UPR TROF WILL
REMAIN IN PLACE ACROSS CA AND NEV. LOOK FOR SHOWERS TO END SLOWLY
FROM WEST TO EAST FOR THE LATER AFTERNOON INTO THE EVENING HOURS.

THE ELONGATED UPR TROF NOW APPEARS TO FORM AN UPR LOW SOUTH OF THE
ARIZ/MEXICO BORDER ACCORDING TO THE LATEST MODELS. A POSITIVELY
TILTED UPR RIDGE WILL SITUATE ITSELF FROM OFFSHORE ACROSS CA AND NEV
FOR DRY CONDITIONS EARLY NEXT WEEK.


.HYDROLOGICAL CONDITIONS...

PRECIPITATION FORECAST FOR LATE THURSDAY INTO SATURDAY IS EXPECTED
TO PRODUCE SMALL TO MINOR RISES TO RIVERS IN THE COASTAL...AND
NORTHERN CENTRAL VALLEY REGIONS OF CALIFORNIA.

ALL RIVERS AND STREAMS IN THE CNRFC FORECAST AREA ARE CURRENTLY
BELOW...AND ARE EXPECTED TO REMAIN BELOW MONITOR LEVELS THROUGH THE
WEEKEND.


MORE INFORMATION ON THE CNRFC WEBSITE AT HTTP://WWW.CNRFC.NOAA.GOV

KL/DRK/AT

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