HYDROMETEOROLOGICAL DISCUSSION
NATIONAL WEATHER SERVICE / CALIFORNIA NEVADA RFC / SACRAMENTO CA
915 AM PST THU DEC 16 2010

...LIGHT PRECIP ACROSS SOUTHERN CA TODAY...
...WIDESPREAD PRECIP EVENT EXPECTED TOMORROW INTO THE WEEKEND...

.24 HOUR PRECIPITATION TOTALS (ENDING DEC 16 AT 400 AM PST)...

WESTERLY FLOW ALOFT COMBINED WITH A STREAM OF PACIFIC MOISTURE THRU 
THE SRN HALF OF CA LED TO MOSTLY LIGHT PRECIPITATION INTO THE SRN 
HALF OF THE SIERRA NEVADA. AMOUNTS THERE WERE GENERALLY AROUND A 
THIRD OF AN INCH OR LESS..MOST OF WHICH FELL DURING THE DAY THURSDAY 
WHILE TAPERING OFF TOWARDS AFTN/EVE. PRECIPITATION SPREAD INTO SRN 
CA OVERNIGHT WITH ACCUMULATIONS OF 0.1-0.25 INCHES INTO THE SAN 
GABRIEL AND SAN BERNARDINO MTNS WHILE JUST A FEW HUNDREDTHS FELL 
ALONG THE COASTAL MTNS TO THE WEST AND SOUTH.  THERE WERE A FEW 
LIGHT SHOWERS FAR NW CA WHERE TOTALS WERE GENERALLY UNDER A TENTH OF 
AN INCH. THERE WERE ALSO A FEW EARLY MORNING SNOW SHOWERS NORTHEAST 
NEVADA WITH JUST A FEW HUNDREDTHS OF AN INCH ACCUMULATIONS. 

.METEOROLOGICAL CONDITIONS...

LATEST WV/IR IMAGERY INDICATES A WEAK S/WV TROF MOVING TOWARD THE 
COAST NEAR THE US/MEXICO BORDER THIS MORNING...FOCUSING PRECIP ALONG 
A FRONTAL BOUNDARY ALIGNED FROM NORTHERN ARIZ BACK TOWARD THE 
EXTREME SOUTHERN CA COAST. KNKX 16/12Z RAOB INDICATES AVAILABLE 
MOISTURE JUST SHY OF 1.00-INCH...WHICH IS APPROX 200-PERCENT OF 
NORMAL. FLOW IS RATHER UNIFORM THROUGH THE COLUMN...ONSHORE FROM THE 
WEST. 88-D DATA INDICATES SOME ENHANCEMENT TO THE PRECIP OVER THE 
PAST SEVERAL HOURS WITH THE HEAVIEST SHOWERS DIVING ESE FROM THE 
ORANGE COUNTY COAST INLAND TOWARD WESTERN RIVERSIDE COUNTY.

ATTENTION NEXT TURNS OFFSHORE ACROSS THE CENTRAL AND EASTERN
PACIFIC. CURRENTLY...A HIGHLY AMPLIFIED UPR RIDGE IS ALIGNED ALONG
THE INT/L DATELINE WITH A WEAK UPR LOW SPINNING UNDERNEATH THIS
FEATURE AROUND 32N/177W. THIS HAS FORMED A REX BLOCK TYPE PATTERN
OVER THE CENTRAL PACIFIC. THE WEAK UPR LOW IS DRAWING SUBTROPICAL 
MOISTURE NORTHWARD JUST WEST OF HAWAII WITH OVERNIGHT AMSU TPW 
PASSES INDICATING VALUES HOVERING AROUND 1.75-INCH. THE OTHER 
FEATURE OF INTEREST IS THE DEEP UPR LOW SPINNING OVER THE GULF OF 
ALASKA WITH A FOCUSED WESTERLY FLOW AT THE BASE OF THIS SYSTEM 
BETWEEN 30N AND 40N. THIS ZONAL FLOW IS BEGINNING TO PULL THE 
SUBTROPICAL MOISTURE ENE TOWARD THE CALIFORNIA COAST WITH THE NOSE 
OF THE GREATEST MOISTURE CONTENT AIR REACHING 140W. 

THE MOST RECENT MODEL RUNS ARE IN A BIT BETTER AGREEMENT IN TERMS OF 
PRECIP INITIATION ALONG THE CENTRAL CA COAST SOUTH TO NEAR PT 
CONCEPTION THIS EVENING...AHEAD OF A WEAK SFC LOW SPINNING FROM NEAR 
ITS CURRENT LOCATION OF 33N/130W NE TOWARD THE COAST. LIGHT PRECIP 
WILL SPREAD INLAND TO THE SOUTHERN SIERRA AND SOUTH TOWARD THE 
US/MEXICO BORDER. 

THROUGH THE FIRST HALF OF FRIDAY...LIGHT PRECIP WILL SPREAD ACROSS 
MUCH OF THE REGION AS WAA/ISENTROPIC LIFT BEGINS TO RAMP UP WITH 
DIFFLUENT FLOW ALOFT. EXPECT THE MAIN ACT TO BEGIN DURING THE LATTER 
HALF OF FRIDAY AS THE UPR JET NOSES TOWARD THE SF BAY AREA INLAND 
TOWARD THE CENTRAL SIERRA...AND THE PLUME OF DEEPER MOISTURE REACHES 
THE COAST JUST SOUTH OF THE SF BAY AREA. SFC TROF/WAVE ALONG THE 
FRONTAL BOUNDARY WILL BE THE FOCUSING FEATURE FOR THE HEAVIEST 
PRECIP. AT THE LOW-LEVELS...WINDS WILL RAMP UP FROM THE SOUTH AND 
CONVERGE OVER THE SHASTA DRAINAGE. 925-MB WINDS ON THE LATEST GFS 
PEAK NEAR 50-KT OVER THE CENTRAL AND SOUTHERN SACRAMENTO VALLEY LATE 
FRIDAY INTO EARLY SATURDAY. THIS SHOULD DRAW MOISTURE NORTHWARD WITH 
GOOD UPSLOPE CONDITIONS OVER THE SHASTA DRAINAGE. A BIT FARTHER UP 
IN THE ATMOSPHERE...THE 850- AND 700-MB WINDS VEER TO A MORE SWLY 
DIRECTIONS...WHICH WILL BRING PLENTY OF OROGRAPHIC ENHANCEMENT TO 
THE PRECIP ALONG THE WEST SLOPE OF THE COASTAL MOUNTAINS AND LENGTH 
OF THE SIERRA. 700-MB WINDS OVER THE NORTHERN AND CENTRAL SIERRA 
WILL RANGE FROM 45- TO 65-KT...WHILE OVER THE SOUTHERN SIERRA WINDS 
ARE SLIGHTLY WEAKER. HOWEVER...THE ABUNDANCE OF MOISTURE CLOSER TO 
THE AXIS OF THE PW PLUME WILL ALLOW FOR HEAVY PRECIP AMOUNTS OVER 
THIS AREA AS WELL.

INTO SATURDAY...PRECIP WILL CONTINUE IN A MOIST ONSHORE FLOW 
PATTERN. THROUGH THE DAY...THE UPR JET LIFTS A BIT TO THE NORTH AND 
TAKES ON A BIT OF A CYCLONIC CURVATURE AHEAD OF A S/WV TROF ROUNDING 
THE BASE OF THE DEEP GULF OF ALASKA POLAR UPR LOW. THE PW PLUME WILL 
ONCE AGAIN REFOCUS ON THE CENTRAL CA COAST BEFORE SHIFTING INLAND 
TOWARD THE SOUTHERN SIERRA. PW VALUES REACHING THE COAST ARE JUST 
SHY OF THE 1.50-INCHES MARK. AS THE S/WV TROF REACHES NEAR THE NORTH 
COAST EARLY SUNDAY...EXPECT PRECIP TO RAMP UP ALONG THE LENGTH OF 
THE SIERRA...ACROSS THE CENTRAL CA COAST...AND ACROSS THE TRANSVERSE 
RANGES BETWEEN SANTA BARBARA AND LOS ANGELES COUNTIES.  

CRITICAL TO THIS SERIES OF STORMS MOVING ACROSS THE REGION WILL BE 
THE SNOW LEVEL. THE MODELS HAVE WAVERED A BIT ON THE AMOUNT OF WARM 
AIR THAT GETS DRAWN ACROSS THE CENTRAL AND SOUTHERN PORTIONS OF THE 
AREA AHEAD OF EACH SFC TROF/WAVE. AHEAD OF THE FIRST UPTICK IN 
PRECIP LATE FRIDAY INTO EARLY SATURDAY...SNOW LEVELS FOR EASTERN 
SIERRA BASINS LIKE THE CARSON AND WALKER RIVERS LOOK TO PEAK AROUND 
7000-FEET. A TOUCH HIGHER IS POSSIBLE FOR THE UPPER PORTIONS OF THE 
MERCED RIVER (BRIEFLY TOUCHING 7500-FEET). SIMILAR HEIGHTS ARE 
EXPECTED FOR THE PEAK OF THE WARM AIR ASSOCIATED WITH THE HEAVIER 
PRECIP LATER SATURDAY INTO EARLY SUNDAY. THIS APPEARS TO BE A TAD 
BIT LOWER THAN THINKING YESTERDAY.

BEYOND THE 3-DAY QPF PERIOD...AFTER CONTINUED MODERATE TO HEAVY 
PRECIP ACROSS THE REGION FOR THE FIRST HALF OF SUNDAY (THROUGH 
20/00Z)...THERE BECOMES A RATHER SIGNIFICANT DISCREPANCY BETWEEN THE 
EC AND THE GFS. THE EC KEEPS PRECIP GOING ACROSS THE REGION FOR THE 
24 HOUR PERIOD BETWEEN 20/00Z AND 21/00Z...WHILE THE GFS CONTINUES 
TO INDICATE MORE OF A BREAK (OR AT LEAST MUCH LIGHTER PRECIP). THEN 
INTO LATER MONDAY INTO EARLY TUESDAY THE EC BEGINS TO SHOW A DROP 
OFF IN PRECIP COVERAGE...WHILE THE GFS WANTS TO BRING THE NEXT WAVE 
OF PRECIP TO THE COAST BEFORE SHIFTING INLAND. DEFINITELY LOW 
CONFIDENCE BEYOND THE 20/00Z MARK.


DAY 1 QPF (THU 16/12Z TO FRI 17/12Z): LIGHT PRECIP EARLY ACROSS 
EXTREME SOUTHERN CA WILL BRING AMOUNTS NEAR OR LESS THAN 0.25-INCH. 
LIGHT PRECIP DEVELOPING LATE FROM THE SF BAY AREA SOUTH TO THE LA 
BASIN WILL BRING AMOUNTS GENERALLY NEAR OR LESS THAN 0.25-INCH.

DAY 2 QPF (FRI 17/12Z TO SAT 18/12Z): WIDESPREAD PRECIP WILL MOVE
ACROSS THE REGION. GREATEST AMOUNTS WILL BE ALIGNED OVER THE SIERRA
WITH 1.50- TO 3.00-INCHES. ELSEWHERE OVER THE HIGHER TERRAIN OF
NORTHERN AND CENTRAL CA EXPECT 0.75- TO 1.50-INCHES (LOCAL
2.00-INCHES). FOR THE HIGHER TERRAIN OF SOUTHERN CA EXPECT TOTALS TO
RANGE FROM 0.50- TO 1.00-INCH. FOR THE LOWER ELEVATIONS OF CA EXPECT
TOTALS GENERALLY FROM 0.25- TO 0.50-INCH (LOCAL 0.75-INCH). FOR THE
STATE OF NEVADA...AMOUNTS WILL RANGE FROM 0.10- TO 0.25-INCH EXCEPT
HIGHER OVER THE EXTREME WESTERN PORTION OF THE STATE NEAR THE
EASTERN SIERRA WITH 0.75- TO 1.25-INCHES POSSIBLE.

DAY 3 QPF (SAT 18/12Z TO SUN 19/12Z): WIDESPREAD PRECIP WILL
CONTINUE. HEAVIEST AMOUNTS OVER THE SIERRA WITH 1.50- TO 3.00-INCHES
(LOCAL TO 4.00-INCHES SOUTHERN SIERRA). CENTRAL CA COASTAL MOUNTAINS
SOUTH TO SANTA YNEZ AND SAN RAFAEL MOUNTAINS WILL SEE 1.00- TO
2.50-INCHES (LOCAL 3.00-INCHES). ELSEWHERE OVER THE HIGHER TERRAIN
OF CA EXPECT 0.50- TO 1.50-INCHES (LOCAL 2.00-INCHES SMITH RIVER
BASIN AND SHASTA DRAINAGE). LOWER ELEVATIONS WILL GENERALLY RANGE
FROM 0.25- TO 0.75-INCH (LOCAL 1.00-INCH CENTRAL SAN JOAQUIN
VALLEY). FOR THE STATE OF NEVADA...AMOUNTS WILL RANGE FROM 0.25- TO
0.50-INCH EXCEPT FOR THE EXTREME WESTERN PORTION OF THE STATE NEAR
THE EASTERN SIERRA WITH 0.75- TO 1.25-INCHES POSSIBLE.


.HYDROLOGICAL CONDITIONS...

CONDITIONS IN THE NORTHERN CALIFORNIA BASINS AND SIERRA BASINS ARE 
RIPE FOR SIGNIFICANT RISES WITH UPCOMING STORMS.  

RAINFALL FROM THIS PAST MONDAY AND TUESDAY HAS PRODUCED A MODERATE 
RISE ON THE SACRAMENTO RIVER.  THE CREST IS NEAR COLUSA BRIDGE THIS 
MORNING. NO WEIR OVERFLOW IS EXPECTED WITH THIS RISE.  HOWEVER 
CURRENT PROJECTIONS FOR EARLY NEXT WEEK SHOW COLUSA AND TISDALE 
WEIRS FLOWING BEGINNING ON SUNDAY.

THE SYSTEM FORECAST FOR FRIDAY THROUGH SUNDAY IS EXPECTED TO HAVE 
SOME MINOR IMPACTS. THE CURRENT FOCUS OF PRECIPITATION APPEARS TO BE 
THE CENTRAL AND SOUTHERN SIERRA WHERE RESERVOIRS WILL SEE MODERATE 
INFLOWS.  TODAYS NUMERICAL WEATHER MODELS ARE MUCH COLDER WITH THIS 
SYSTEM SO NO FLOODING IS EXPECTED AT THIS TIME. HOWEVER THE COSUMNES 
RIVER IS FORECAST TO RISE ABOVE MONITOR LEVEL ON SATURDAY AND INTO 
MONDAY.  

THERE HAS BEEN A GREAT DEAL OF VARIABILITY IN THE RAINFALL AND SNOW 
LEVELS EXPECTED WITH THIS EVENT.  WHILE TODAYS FORECAST IS MORE 
MODEST WE WILL HAVE TO KEEP MONITORING THE STORMS DEVELOPMENT.

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

ME/DRK/PF