HYDROMETEOROLOGICAL DISCUSSION NATIONAL WEATHER SERVICE / CALIFORNIA NEVADA RFC / SACRAMENTO CA 930 AM PST SAT DEC 18 2010 ...CONTINUED HEAVY PRECIP MOST AREAS THROUGH THE WEEKEND... .24 HOUR PRECIPITATION TOTALS (ENDING DEC 18 AT 400 AM PST)... AN INITIAL WARM FRONT TRAVERSED THE REGION EARLY IN THE PERIOD...FOLLOWED BY THE APPROACH OF A STRONG COLD FRONT WITH A TAP INTO SUBTROPICAL MOISTURE. WINDS VEERING FROM SOUTH IN THE LOW-LEVELS TO WEST IN THE MID-LEVELS ADDED TO THE MIX WITH EXCELLENT OROGRAPHIC ENHANCEMENT TO THE PRECIP. GREATEST TOTALS WERE OBSERVED OVER THE LENGTH OF THE SIERRA WITH A RANGE OF 2.00- TO 4.00-INCHES (GREATEST OVER THE SOUTHERN HALF OF THE MOUNTAIN CHAIN). THERE WERE EVEN A FEW LOCATIONS AROUND AND JUST SOUTH OF YOSEMITE NATIONAL PARK THAT REACHED CLOSE TO 5.00-INCHES. THE OTHER LOCATION THAT DID QUITE WELL FOR PRECIP PRODUCTION WAS THE COASTAL MOUNTAINS...PRIMARILY BETWEEN THE SANTA CRUZ MOUNTAINS TO THE NORTH AND THE SAN GABRIEL MOUNTAINS TO THE SOUTH. AMOUNTS GENERALLY RANGED FROM 1.00- TO 3.00-INCHES WITH A FEW POCKETS REACHING NEAR 3.50-INCHES. FOR THE NORTH COAST AND SHASTA DRAINAGE AMOUNTS WERE NOT AS IMPRESSIVE AS THE AREA WAS A BIT DISCONNECTED FROM THE BEST MOISTURE SOURCE AIMED AT THE CENTRAL CA COAST. TOTALS WERE GENERALLY FROM 1.00- TO 2.00-INCHES. INLAND ACROSS THE CENTRAL VALLEY...AMOUNTS WERE GREATEST BETWEEN SACRAMENTO AND JUST SOUTH OF FRESNO WITH 0.50- TO 1.00-INCH (LOCAL TO 1.50-INCHES). ELSEWHERE WITHIN THE VALLEY TOTALS WERE PRIMARILY ON THE ORDER OF 0.25- TO 0.50-INCH. FOR THE STATE OF NEVADA...WIDESPREAD PRECIP WAS ALSO OBSERVED. GREATEST TOTALS WERE OVER THE EXTREME WESTERN PORTION OF THE STATE AROUND THE RENO AREA. HIGHER TERRAIN LOCATIONS RANGED FROM 0.50- TO 1.00-INCH WITH A FEW LOCATIONS REACHING CLOSER TO 1.50-INCHES. ELSEWHERE OVER THE STATE...LOWER ELEVATIONS WERE ON THE ORDER OF 0.10- TO 0.25-INCH...WHILE HIGHER TERRAIN SAW 0.25- TO 0.50-INCH (LOCALLY TO 0.75-INCH). .METEOROLOGICAL CONDITIONS... CURRENT SETUP ACROSS THE CENTRAL AND EASTERN PACIFIC IS FAIRLY SIMILAR TO YESTERDAY. UPR RIDGE ALONG THE INT/L DATELINE CONTINUES TO HOLD STRONG...BECOMING SLIGHTLY POSITIVELY TILTED OVER THE PAST 24 HOURS. WEAK UPR LOW CONTINUES TO MEANDER SLOWLY TOWARD THE SOUTH...NOW NEAR 22N/178W. THIS CIRCULATION CONTINUES TO DRAW MOISTURE NORTHWARD FROM JUST WEST OF HAWAII TO 30N. A DEEP POLAR UPR LOW IS SINKING SOUTH...CURRENTLY SPINNING WEST OF THE OREGON COAST NEAR 34N/135W. WITH THE CONSOLIDATED WESTERLY FLOW AT THE BASE OF THIS MASSIVE UPR LOW...MOISTURE FROM THE CENTRAL PACIFIC IS BEING PULLED EASTWARD AND CURRENTLY INTERACTING WITH THE WEST COAST. AMSU TPW PASSES FROM OVERNIGHT SHOW THE CONNECTION OF HIGHER MOISTURE CONTENT AIR STREAMING FROM THE CENTRAL CA COAST BACK TOWARD NW OF HAWAII. PEAK VALUES WITHIN THIS PLUME OF MOISTURE ARE ON THE ORDER OF 1.50- TO 1.60-INCHES. 18/12Z KVBG RAOB INDICATES A SATURATED COLUMN WITH A RECORDED PW OF 1.23-INCHES (ABOUT 250-PERCENT OF NORMAL). WINDS SHOW A CLASSIC WAA PATTERN AHEAD OF THE COLD FRONT NOW TRAVERSING THE REGION. STRONG S/WV TROF PIVOTING AROUND THE OFFSHORE UPR LOW CLIPPED THE NORTH COAST OVERNIGHT AND IS NOW LOCATED ALONG THE PACNW COAST...WHERE THE STRONG DYNAMICS AND LOWERED STABILITY IS GENERATING PLENTY OF SHOWERS AND T-STORMS. ASSOCIATED SFC LOW IS ALSO WELL TO THE NORTH...MATURING JUST TO THE SW OF VANCOUVER ISLAND. COLD FRONT IS CURRENTLY MOVING INLAND ACROSS THE SIERRA AND DOWN TOWARD THE CENTRAL CA COAST. THIS IS CURRENTLY WHERE THE GREATEST PRECIP IS FALLING. THROUGH THE AFTERNOON...EXPECT PRECIP TO DECREASE IN INTENSITY ACROSS NORTHERN SECTIONS AS THE COLD FRONT CONTINUES TO MOVE INLAND AND TO THE SOUTH. HOWEVER...PRECIP IS EXPECTED TO CONTINUE IN EARNEST ACROSS THE SOUTHERN SIERRA AND THE TRANSVERSE RANGES OF SOUTHERN CA AS THE MOISTURE PLUME CONTINUES AIMED AT THIS AREA WITH STRONG ONSHORE FLOW. HOWEVER...THE DECREASE ACROSS THE NORTH WILL BE SHORT-LIVED AS MID-LEVEL VORT ENERGY MOVES ALONG THE CA/ORE BORDER DURING THE EVENING HOURS. FOR THE OVERNIGHT INTO SUNDAY...THE REGION WILL BEGIN TO SEE THE IMPACTS FROM THE NEXT STRONG S/WV TROF ROTATING TOWARD THE NORTH COAST. THE ASSOCIATED SFC LOW WILL ONCE AGAIN REMAIN WELL TO THE NORTH OFF THE PACNW COAST. THE COLD FRONT WILL MOVE TOWARD THE REGION AND REACH THE COAST SUNDAY MORNING...BEFORE SHIFTING INLAND. THE TREND OF THE GFS HAS BEEN TO DELAY THE MAXIMUM INTENSITY OF THE PRECIP BY ABOUT 6 HOURS AS COMPARED TO YESTERDAY/S THINKING. THE BEST MOISTURE WILL ONCE AGAIN BE AIMED AT CENTRAL CA BEFORE DROPPING SOUTH TOWARD SOUTHERN CA. WITH THE TAP TO THE HIGHER MOISTURE CONTENT AIR STREAMING EAST ALONG 30N CONTINUING...MAXIMUM PW VALUES ARE ONCE AGAIN NEAR 1.50-INCHES. AHEAD OF THE COLD FRONT...SOUTHERLY WINDS WILL INCREASE AS THE MSLP GRADIENT ONCE AGAIN TIGHTENS. THIS WILL ONLY ADD TO THE MIX WHERE UPSLOPE CONDITIONS WILL AID IN PRECIP PRODUCTION FOR THE TRANSVERSE RANGES OF SOUTHERN CA AND THE SHASTA DRAINAGE UP ACROSS NORTHERN CA. IN THE MID-LEVELS...700-MB WINDS PEAK DURING THE LATER MORNING HOURS SUNDAY WITH SW WINDS NEAR 60-KT ACROSS THE NORTHERN SIERRA AND 40-KT FOR THE SOUTHERN SIERRA. HOWEVER...EVEN THOUGH THE WINDS ARE SLIGHTLY LESS IN MAGNITUDE FOR THE SOUTHERN SIERRA...THE AMPLE MOISTURE WILL GENERATE HEAVY PRECIP AMOUNTS. AHEAD OF THE COLD FRONT...WAA WILL ONCE AGAIN BUMP SNOW LEVELS UPWARD. THE MOST CRITICAL AREA REMAINS THE EAST SIDE SIERRA BASINS SOUTH OF LAKE TAHOE AND THE MERCED RIVER ON THE WEST SLOPE. PER COORDINATION WITH THE RENO WFO...BUMPED THESE VALUES UP A BIT (500-FEET OR SO). AS THE COLD FRONT PRESSES INLAND AND TO THE SOUTH...PRECIP WILL BEGIN TO LET UP AGAIN ACROSS NORTHERN SECTIONS LATER ON SUNDAY. HOWEVER...PRECIP WILL NOT COMPLETELY SHUT DOWN AS ONSHORE CYCLONIC FLOW CONTINUES. AVAILABLE MOISTURE WILL BE ON THE DECREASE BEHIND THE COLD FRONT...ON THE ORDER OF 0.25- TO 0.50-INCH PW VALUES. INTO DAY 3...THE MODELS ARE STILL NOT IN THE BEST AGREEMENT. HOWEVER...IT IS LOOKING AS IF THE UPR LOW OFF THE PACNW WILL DIG SOUTH AND DISPLACE THE GREATEST MOISTURE TOWARD SOUTHERN CA. UPR JET WILL PUSH SOUTH AND SLICE THE REGION IN HALF...MOVING ACROSS THE CENTRAL CA COAST INLAND TOWARD CENTRAL NEV. THIS IS BEGINNING TO INDICATE THE BEST PRECIP ON DAY 3 ACROSS SOUTHERN CA...ESPECIALLY THE SAN GABRIEL AND SAN BERNARDINO MOUNTAINS. THE GFS-BASED OROGRAPHIC AID HAS BEEN HINTING AT THIS FOR THE PAST 24 HOURS NOW...AND HEAVY PRECIP TOTALS WERE INCLUDED IN THE MORNING ISSUANCE OF THE QPF ACROSS THIS AREA. WILL DEFINITELY NEED TO KEEP A CLOSE EYE ON THIS AREA FOR THE POSSIBILITY OF INTENSE PRECIP. DAY 1 QPF (SAT 18/12Z TO SUN 19/12Z): HEAVY PRECIP CONTINUES OVER MUCH OF THE AREA. GREATEST AMOUNTS ARE ALONG THE LENGTH OF THE SIERRA AND THE TRANSVERSE MOUNTAINS BETWEEN THE SANTA YNEZ AND SAN GABRIEL MOUNTAINS WITH 2.00- TO 4.00-INCHES (LOCAL 5.00-INCHES). ELSEWHERE OVER THE HIGHER TERRAIN OF CA...EXPECT 0.75- TO 1.50-INCHES (LOCALLY TO 2.00-INCHES CENTRAL COAST AND SMITH RIVER BASIN). LOWER ELEVATIONS OVER CA...EXPECT 0.50- TO 1.00-INCH. ACROSS NEVADA...TOTALS WILL RANGE FROM 0.10- TO 0.25-INCH (LOCALLY 0.50-INCH). HIGHER TOTALS ARE LIKELY CLOSER TO THE EAST SLOPES OF THE SIERRA WITH AMOUNTS REACHING OR JUST SURPASSING 1.00-INCH. DAY 2 QPF (SUN 19/12Z TO MON 20/12Z): HEAVY PRECIP CONTINUES OVER THE REGION. GREATEST AMOUNTS ONCE AGAIN ALONG THE LENGTH OF THE SIERRA AND THE HIGHER TERRAIN OF SOUTHERN CA WITH 2.00- TO 5.00-INCHES. ELSEWHERE OVER THE HIGHER TERRAIN OF CA...EXPECT 1.00- TO 2.50-INCHES. LOWER ELEVATIONS OF CA WILL RANGE FROM 0.50- TO 1.25-INCHES. FOR NEVADA...TOTALS WILL BE FROM 0.25- TO 0.75-INCH WITH LOCAL AMOUNTS REACHING 1.00-INCH. CLOSER TO THE EAST SLOPES OF THE SIERRA TOTALS WILL RANGE FROM 1.00- TO 2.00-INCHES. DAY 3 QPF (MON 20/12Z TO TUE 21/12Z): WIDESPREAD PRECIP FOR A THIRD DAY WITH GREATEST AMOUNT OVER SOUTHERN CA HIGHER TERRAIN WITH 2.00- TO 5.00-INCHES (LOCAL 6.00-INCHES OVER THE SAN GABRIEL MOUNTAINS). FOR THE REST OF THE HIGHER TERRAIN OF CA...EXPECT 0.75- TO 1.50-INCHES (LOCAL 2.00-INCHES FOR THE SOUTHERN SIERRA). LOWER ELEVATIONS OF CA WILL BE FROM 0.25- TO 0.75-INCH...EXCEPT FROM 0.75- TO ALMOST 2.00-INCHES FOR SOUTHERN CA. ACROSS NEVADA...BEST AMOUNTS WILL BE SOUTH WITH 0.25- TO 1.00-INCH. LIGHTER AMOUNTS NORTH CAN BE EXPECTED. .HYDROLOGICAL CONDITIONS... THE WELL ADVERTISED START OF AN EXTENDED PERIOD OF MODERATE TO HEAVY PRECIPITATION FOR CALIFORNIA AND NEVADA BEGAN YESTERDAY. FREEZING LEVELS ARE NOT HIGH ENOUGH TO GENERATE MUCH IN THE WAY OF HIGHER ELEVATION RUNOFF. BELOW THE HIGHER ELEVATIONS..MANY RIVERS AND STREAMS ARE EXPERIENCING SHARP RISES THIS MORNING. THESE CONDITIONS WILL CONTINUE THROUGHOUT MOST OF CALIFORNIA FOR THE NEXT 3 OR 4 DAYS. MONITOR LEVELS HAVE BEEN EXCEEDED OR ARE EXPECTED TO BE EXCEEDED ON NUMBEROUS RIVERS AND SOME FLOOD STAGES ARE A POSSIBILITY. MORE INFORMATION ON THE CNRFC WEBSITE AT HTTP://WWW.CNRFC.NOAA.GOV DRK/AM