HYDROMETEOROLOGICAL DISCUSSION NATIONAL WEATHER SERVICE / CALIFORNIA NEVADA RFC / SACRAMENTO CA 930 AM PST FRI DEC 17 2010 ...WIDESPREAD HEAVY PRECIPITATION THROUGH THE WEEKEND... ...GREATEST AMOUNTS WILL BE FOCUSED ON THE SOUTHERN SIERRA... .24 HOUR PRECIPITATION TOTALS (ENDING DEC 17 AT 400 AM PST)... AN OLD FRONTAL BOUNDARY AND WEAK UPPER-LEVEL DISTURBANCE BROUGHT LIGHT PRECIPITATION TO SRN CA EARLY IN THE PERIOD. LIGHT PRECIPITATION AHEAD OF A WARM FRONT SPREAD ACROSS THE CENTRAL COAST INTO THE SIERRAS LATE IN THE PERIOD. 24-HOUR TOTALS WERE LESS THAN 0.1" IN THE RUSSIAN/NAPA BASINS AND 0.2" OR LESS SWD TO PT CONCEPTION AND ALSO THROUGH THE TRANSVERSE RANGES AND DOWN TO THE SAN DIEGO COUNTY COAST. THE ONLY EXCEPTION WAS LOCAL AMOUNTS NEAR 0.3" IN THE SAN BERNADINO MTNS. GENERALLY 0.1" OR LESS WAS ALSO REPORTED IN THE SAN JOAQUIN VALLEY AS WELL AS THE WEST SIDE OF THE SIERRAS FROM AROUND I-80 SWD. DESERT AREAS OF SE CA AND SRN NV SAW LIGHT RAINFALL AMOUNTS AS WELL. .METEOROLOGICAL CONDITIONS... INITIAL SHOT OF PRECIP IS CURRENTLY MOVING ACROSS THE SOUTHERN TWO-THIRDS OF THE REGION FROM THE I-80 CORRIDOR SOUTHWARD TO NEAR THE US/MEXICO BORDER. THIS IS ASSOCIATED WITH A WEAK SFC LOW MOVING NNE TO A LOCATION NOW JUST WEST OF THE SF BAY AREA...AND A WARM FRONT MOVING ONSHORE ALONG THE CENTRAL CA COAST. AVAILABLE MOISTURE ASSOCIATED WITH THIS FEATURE IS ON THE ORDER OF 1.00- TO 1.25-INCHES...ARCHING ALONG THE BOUNDARY. THE 17/12Z KVBG RAOB INDICATED 1.01-INCHES OF PW WITH SATURATION IN THE COLUMN UP TO APPROX 650-MB. WIND PROFILE SHOWS A CLASSIC WAA PATTERN WITH STRONG VEERING WITH HEIGHT. THE MORE AMPLE MOISTURE SOURCE...WHICH WILL BRING THE REGION WIDESPREAD MODERATE TO HEAVY PRECIP IS STILL ADVECTING ENE OVER THE CENTRAL AND EASTERN PACIFIC. CURRENT SETUP SHOWS A HIGHLY AMPLIFIED UPR RIDGE ALIGNED BETWEEN 170W AND THE INT/L DATELINE...WHILE A WEAK UPR LOW IS DRIFTING SOUTH NEAR 28N/178W. THIS WEAK CIRCULATION IS DRAWING SUBTROPICAL MOISTURE NORTHWARD FROM JUST WEST OF HAWAII. TO THE NORTHEAST...A DEEP POLAR UPR LOW IS SLOWLY DROPPING SOUTH OUT OF THE GULF OF ALASKA NEAR 47N/145W. CONSOLIDATED WESTERLY FLOW CENTERED ON 30N IS ALLOWING THE HIGHER PW AIR TO ADVECT EASTWARD TOWARD THE WEST COAST. THE NOSE OF THIS HIGHER MOISTURE CONTENT AIR IS NOW WITHIN 140W...AND EXPECTED TO REACH THE CENTRAL CA COAST THIS EVENING INTO THE OVERNIGHT. AMSU TPW PASSES FROM OVERNIGHT INDICATE MAXIMUM VALUES WITHIN THIS PLUME JUST SHY OF THE 1.75-INCH MARK. A S/WV TROF ROTATING AROUND THE PARENT DEEP POLAR UPR LOW WILL BRUSH THE NW CORNER OF THE STATE TONIGHT...AS A SFC TROF/WAVE REACHES COASTAL SECTIONS AROUND 18/06Z. THE NOSE OF A NEARLY 160-KT UPR JET WILL MOVE OVER THE NORTH COAST AT THIS TIME AS WELL. THIS COINCIDES WITH THE ARRIVAL OF THE HIGHER PW AIR...WHICH WILL BRING THE FIRST ROUND OF MODERATE TO HEAVY PRECIP. MODELS HAVE BEEN VERY CONSISTENT WITH HITTING THE ENTIRE LENGTH OF THE SIERRA WITH HEAVY PRECIP OVERNIGHT. ALONG WITH THE AMPLE MOISTURE AND FAVORABLE DYNAMICS...MID-LEVEL WINDS RAMP UP FROM THE SW BETWEEN 45- AND 65-KT AT 700-MB FOR IDEAL OROGRAPHIC ENHANCEMENT. AT THE SFC...AN INCREASING SOUTHERLY MSLP GRADIENT WILL ALLOW FOR WINDS TO RAMP UP OVER THE SACRAMENTO VALLEY AND CONVERGE OVER THE SHASTA DRAINAGE. WINDS AT 925-MB WILL PEAK OVER THE CENTRAL AND SOUTHERN SACRAMENTO VALLEY JUST OVER 50-KT AT 18/12Z. ALTHOUGH AVAILABLE MOISTURE IS NOT AS IMPRESSIVE AS POINTS SOUTH...WILL SEE A 6-HOUR PERIOD OF INCREASED PRECIP IN THIS FLOW PATTERN. INTO SATURDAY...IT APPEARS AS IF PRECIP WILL RAMP DOWN ACROSS THE NWRN PORTION OF THE REGION...ALONG THE COAST NORTH OF THE GOLDEN GATE. HOWEVER...CLOSER TO THE AMPLE MOISTURE SOURCE AND STRONG ONSHORE FLOW...PRECIP WILL CONTINUE UNABATED. HEAVIEST LOCATIONS LOOK TO BE OVER THE SOUTHERN SIERRA BACK TOWARD THE CENTRAL CA COAST DOWN TO THE TRANSVERSE MOUNTAINS OF SOUTHERN CA. THE REGION WILL BE WAITING FOR THE NEXT S/WV TROF TO ROTATE AROUND THE POLAR UPR LOW AND REACH THE NORTH COAST SUNDAY MORNING. IT SHOULD BE NOTED THAT THERE ARE SLIGHT TIMING DIFFERENCES BETWEEN THE EC AND THE GFS...WITH THE LATTER BEING ABOUT 6 HOURS FASTER. FOR NOW...CONTINUED TO LEAN TOWARD THE FASTER AND WETTER SOLUTION OF THE GFS. AS THE S/WV TROF MOVES TOWARD THE COAST...THE UPR JET WILL BEGIN TO SAG TOWARD THE SOUTH. PRECIP WILL ONCE AGAIN BE ON THE INCREASE AS THE ENTIRE LENGTH OF THE SIERRA WILL EXPERIENCE A PERIOD OF HEAVY PRECIP...PRIMARILY SUNDAY MORNING INTO THE EARLY AFTERNOON. AS THE S/WV TROF PUSHES INLAND...THE UPR JET WILL CONTINUE TO DROP TOWARD THE SOUTH AND THE AXIS OF HIGHEST MOISTURE CONTENT AIR WILL BE AIMED AT THE SOUTHERN CA COAST. EXPECT WIDESPREAD PRECIP TO INCREASE OVER THE HIGHER TERRAIN AS AMPLE MOISTURE IS LIFTED BY THE STRONG ONSHORE FLOW. AN INCREASING SOUTHERLY MSLP GRADIENT WILL AID IN PRECIP PRODUCTION ALONG THE TRANSVERSE RANGES FROM SANTA BARBARA AND SAN BERNARDINO COUNTIES. THE LATEST (17/00Z) GFS-BASED OROGRAPHIC AID ACROSS THE TRANSVERSE MOUNTAINS IS RATHER WET FOR THE 24-HOUR PERIOD ENDING (20/12Z). THE FOLLOWING TABLE SUMMARIZES THESE VALUES ALONG WITH THE MOUNTAINS OF ORANGE AND SAN DIEGO COUNTIES... .SANTA YNEZ MOUNTAINS....... 4.75-INCHES .TOPA TOPA MOUNTAINS........ 5.05-INCHES .SAN GABRIEL MOUNTAINS...... 5.70-INCHES .NEAR MOUNT WILSON.......... 7.25-INCHES .SAN BERNARDINO MOUNTAINS... 3.65-INCHES .SANTA ANA MOUNTAINS........ 4.40-INCHES .PALOMAR MOUNTAIN........... 4.60-INCHES LATEST MORNING ISSUANCE OF THE QPF DID BUMP NUMBERS ABOVE WHAT RAW MODEL OUTPUT WAS INDICATING...BUT NOT AS HEAVY AS THE GFS-BASED OROGRAPHIC AID. WILL NEED TO SEE HOW THE 17/12Z RUN ENDS UP TO SEE IF ANOTHER BUMP UP OF THE NUMBERS IS NECESSARY. SNOW LEVELS CONTINUE TO BE A CONCERN WITH THE MODELS SHOWING SOME FLUCTUATIONS OVER THE PAST 24 HOURS. FOR THE CARSON AND WALKER BASINS ON THE EAST SLOPES OF THE SIERRA...SNOW LEVELS AT THE PEAK OF EACH EVENT RAMP UP TO NEAR 7000- TO 7500-FEET. SIMILAR VALUES ARE EXPECTED OVER THE MERCED RIVER ON THE WEST SLOPE OF THE SIERRA. THESE VALUES WERE DERIVED FROM THE 700-MB TEMP OF MINUS 2- TO MINUS 3-DEG C AT THE PEAK OF THE EVENT. WILL CONTINUE TO REFINE AS THE EVENT DRAWS CLOSER AS THE GRADIENT FROM THE COOLER AIR TO THE NORTH AND WARMER AIR TO THE SOUTH IS QUITE TIGHT. ANY SLIGHT SHIFT IN THE PLACEMENT OF THE UPR JET OVER THE AREA COULD ALTER THESE LEVELS. BEYOND THE 3-DAY QPF PERIOD...THERE ARE STILL ENOUGH DISCREPANCIES TO NOT HAVE MUCH MORE THAN A LOW CONFIDENCE WITH THE NEXT SYSTEM EXPECTED FOR THE LATER MONDAY INTO TUESDAY SYSTEM. JUST GLANCING AT THE 17/12Z GFS ARRIVING ON-STATION...THE MAIN PRECIP BAND IS EVEN FARTHER TO THE SOUTH...AFFECTING MAINLY THE SOUTHERN HALF OF THE REGION AND IN PARTICULAR SOUTHERN CA WITH THE HEAVIEST PRECIP AMOUNTS. THE EC SEEMS TO BE TRENDING A BIT FARTHER SOUTH AS WELL OVER THE PAST SEVERAL RUNS. DAY 1 QPF (FRI 17/12Z TO SAT 18/12Z): WIDESPREAD PRECIP WILL SPREAD ACROSS THE REGION. GREATEST TOTALS WILL FALL OVER THE LENGTH OF THE SIERRA WITH 1.50- TO 3.00-INCHES. ELSEWHERE OVER THE HIGHER TERRAIN OF NORTHERN AND CENTRAL CA EXPECT TOTALS BETWEEN 1.00- TO 2.00-INCHES. LOWER ELEVATIONS FOR NORTHERN AND CENTRAL CA WILL PRIMARILY RANGE FROM 0.50- TO 1.00-INCH. FROM PT CONCEPTION SOUTH AMOUNTS OF 0.75- TO 1.50-INCHES ARE LIKELY OVER THE HIGHER TERRAIN WITH 0.25- TO 0.75-INCH LOWER ELEVATIONS. FOR THE STATE OF NEVADA...EXPECT AMOUNTS PRIMARILY BETWEEN 0.10- AND 0.25-INCH. HOWEVER...CLOSER TO THE SIERRA NEAR THE CA BORDER TOTALS WILL RANGE FROM 0.50- TO LOCALLY 1.00-INCH. DAY 2 QPF (SAT 18/12Z TO SUN 19/12Z): WIDESPREAD PRECIP WILL CONTINUE ACROSS THE REGION. GREATEST AMOUNTS AGAIN OVER THE LENGTH OF THE SIERRA WITH 2.00- TO 4.00-INCHES (LOCAL TO 5.00-INCHES SOUTHERN AREAS). FROM THE CENTRAL CA COAST SOUTH TO THE SAN GABRIEL MOUNTAINS WILL SEE 1.00- TO 3.00-INCHES. FOR THE REST OF THE HIGHER TERRAIN OF NORTHERN CA EXPECT 1.00- TO 2.00-INCHES. LOWER ELEVATIONS WILL RUN BETWEEN 0.50- AND 1.00-INCH. FOR THE STATE OF NEVADA...EXPECT AMOUNTS FROM 0.10- TO 0.25-INCH. HOWEVER...CLOSER TO THE SIERRA NEAR THE CA BORDER EXPECT AMOUNTS TO RANGE FROM 0.50- TO LOCALLY 1.00-INCH. DAY 3 QPF (SUN 19/12Z TO MON 20/12Z): AGAIN...PRECIP WILL BE WIDESPREAD OVER THE AREA. GREATEST AMOUNTS WILL BE FOCUSED ON THE LENGTH OF THE SIERRA WITH 1.50-INCHES NORTH TO 4.00-INCHES SOUTH (LOCAL TO 5.00-INCHES SOUTHERN AREAS). ALSO...FOR THE HIGHER TERRAIN OF SOUTHERN CA...PRECIP WILL PICK UP QUITE A BIT WITH AMOUNTS RANGING FROM 2.00- TO 5.00-INCHES. FOR THE REST OF THE HIGHER TERRAIN OF NORTHERN AND CENTRAL CA EXPECT 0.75- TO 2.00-INCHES. LOWER ELEVATIONS WILL RANGE FROM 0.25- TO 0.75-INCH. FOR THE STATE OF NEVADA...EXPECT AMOUNTS IN THE RANGE OF 0.25- TO 0.50-INCH. HOWEVER...CLOSER TO THE SIERRA NEAR THE CA BORDER EXPECT AMOUNTS TO RANGE FROM 0.50- TO LOCALLY 1.00-INCH. .HYDROLOGICAL CONDITIONS... THE WET WEATHER FORECAST TO CONTINUE INTO NEXT WEEK IS EXPECTED TO PRODUCE RISING RIVER CONDITIONS ACROSS MOST OF CALIFORNIA. SOME WEIR FLOW IS EXPECTED ON THE SACRAMENTO RIVER AND MONITOR STAGES ARE EXPECTED ON THE COSUMNES AND LOWER TUOLUMNE RIVERS. SNOW LEVELS ARE NOT EXPECTED TO BE HIGH ENOUGH TO GENERATE SIGNIFICANT MOUNTAIN STREAM FLOWS. AT THIS TIME THERE ARE NO FLOOD STAGES EXPECTED THROUGH THE WEEKEND. ALTHOUGH PRECIPITATION CONTINUES AT ABOVE NORMAL LEVELS FOR THE SEASON..THE ACCUMULATION AND RUNOFF HAS BEEN ORDERLY AND MANAGEABLE. MORE INFORMATION ON THE CNRFC WEBSITE AT HTTP://WWW.CNRFC.NOAA.GOV KL/DRK/AM