HYDROMETEOROLOGICAL DISCUSSION NATIONAL WEATHER SERVICE / CALIFORNIA NEVADA RFC / SACRAMENTO CA 950 AM PST THU DEC 23 2010 ...MAINLY DRY CONDITIONS EXPECTED THROUGH FRI NIGHT... ...WET STORM WILL SPREAD ACROSS MUCH OF CA AND SRN OREGON SAT AND SAT NIGHT... .24 HOUR PRECIPITATION TOTALS (ENDING DEC 23 AT 400 AM PST)... HEAVIEST PRECIPITATION WAS AGAIN FOCUSED ACROSS CENTRAL/SRN CA AND THE SRN HALF OF NEVADA WEDNESDAY AS AN UPPER TROUGH APPROACHED SRN CA IN THE MORNING AND TRAVERSED THE AREA DURING THE AFTN/EVE. MOIST ONSHORE FLOW AHEAD OF THE TROUGH CONTINUED STEADY OROGRAPHIC PRECIPITATION THROUGHOUT SRN CA EARLIER IN THE DAY...CHANGING OVER TO A SHOWER/THUNDERSTORM PATTERN BY AFTERNOON AS THE COLD CORE OF THE UPPER TROUGH PASSED OVER SRN CA. PRECIPITATION BEGAN TO FINALLY DIMINISH DURING THE OVERNIGHT PERIOD AS THE UPPER LOW TRACKED EAST INTO NRN ARIZONA BY 12UTC THIS MORNING. GREATEST 24-HOUR PRECIPITATION TOTALS WERE IN THE 2-3.5 INCH RANGE OVER MUCH OF THE HIGHER SRN CA TERRAIN FROM VENTURA COUNTY TO THE U.S./MEXICO BORDER WITH LOCALLY 3.5-5 INCHES FOCUSED MOSTLY ON THE SAN GABRIEL AND SAN BERNARDINO MTNS. GENERALLY 0.5-1.5 INCHES FELL ALONG THE LOW-LYING COASTAL AREAS OR SRN CA. HEAVIER RAINS ALSO CONTINUED OVER THE SERN CA DESERT INTO SRN HALF OF NEVADA WHERE TOTALS WERE IN THE 0.5-1.5 INCH RANGE AND LOCALLY UP TO AROUND 2 INCHES OVER THE HIGHER TERRAIN. MORE MODERATE PRECIPITATION FELL ACROSS CENTRAL CA WHERE COASTAL AREAS GENERALLY RECEIVED GENERALLY 0.1-0.5 INCHES BUT LOCALLY UP TO JUST OVER AN INCH IN THE SRN PORTION OF THE SANTA LUCIA RANGE. THE CENTRAL/SRN SIERRA REPORTED 0.5-1.25 INCHES WITH 0.2-0.5 INCHES IN THE SAN JOAQUIN VALLEY. FOR MUCH OF NRN CA AND NRN NV...AMOUNTS TENDED TO BE CONSIDERABLY LIGHTER WITH GENERALLY A FEW HUNDREDTHS TO A QUARTER OF AN INCH COVERAGE AND UP TO 0.25-0.5 INCH AMOUNTS ACROSS THE HIGHER TERRAIN FROM THE SHASTA/TRINITY MTNS TO THE NRN SIERRA NEVADA. .METEOROLOGICAL CONDITIONS... IR IMAGERY SHOWS A COMPLEX OF UPPER LOWS OVER THE NE PACIFIC NORTH OF 40N BETWEEN 135W AND 150W. TO THE SOUTH OF THESE FEATURES...AN UPPER JET OF 110+ KTS IS AIMED INTO WRN WASHINGTON AND A MOISTURE PLUME EXTENDS ROUGHLY ALONG THE SAME LINE WITH PEAK VALUES AROUND 0.75". THE UPPER TROUGH THAT MOVED ACROSS THE REGION ON WED HAS NOW PROGRESSED TO THE FOUR-CORNERS REGION...LEAVING WEAK RIDGING AND DRY CONDITIONS IN PLACE ACROSS THE FORECAST AREA FOR TODAY AND TONIGHT. MODELS DIG A TROUGH OFF THE WEST COAST BY FRI AND BY SAT THE PATTERN BECOMES HIGHLY AMPLIFIED WITH A TROUGH NEAR 130-140W AND A RIDGE AXIS NEAR THE ROCKIES. UPPER-LEVEL FLOW WILL REMAIN NEARLY PARALLEL TO THE MOISTURE PLUME...AND THE MOISTURE PLUME IS EXPECTED TO MAKE VERY SLOW PROGRESS TOWARD THE COAST THROUGH FRI NIGHT. THE ECMWF AND NAM ARE SLOWER THAN THE GFS BRINGING THE TROUGH THE REGION... AND THE SLOWER SOLN SEEMS REASONABLE GIVEN THE AMPLIFIED NATURE OF THE PATTERN. BY SAT...THE TROUGH TAKES ON MORE OF A NEGATIVE TILT...AND MODELS FINALLY SPREAD PRECIP INLAND. THE NORTH COAST SHOULD SEE A SHOT OF AT LEAST MODERATE PRECIP BY LATE MORNING OR EARLY AFTERNOON SAT AS A PW PLUME 0.75-1" REACHES THE COAST ALONG WITH STRONG WINDS 50-60 KTS AT H85. DUE TO THE STRONG SOUTH WINDS AND THE LOCATION OF THE MAXIMUM MOISTURE WITHIN THE PLUME...SHADOWING IS LIKELY NORTH OF CAPE MENDOCINO WITH THE HEAVIEST PRECIPITATION EXPECTED TO THE SOUTH. AS THE TROUGH PROGRESSES ACROSS THE REGION...THE PRECIP WILL SPREAD SOUTH AND EAST AND AFFECT THE SIERRAS AND SOCAL BY SAT EVENING. WITH SLY GOOD MOISTURE FLUX INTO THE HIGHER TERRAIN OF SRN CA...A PERIOD OF HEAVY RAIN IS POSSIBLE THERE SAT EVENING AND SAT NIGHT. BY SUN...THE TROUGH WILL MOVE EAST OF THE REGION WITH DRYING EXPECTED OVER MOST OF THE FORECAST AREA. DAY 1 QPF (THU 23/12Z TO FRI 24/12Z): VERY LIGHT AMOUNTS SW CA HIGHER TERRAIN AND SE NV...OTHERWISE DRY. DAY 2 QPF (FRI 24/12Z TO SAT 25/12Z): LIGHT AMOUNTS ALONG THE NORTH COAST FROM THE EEL BASIN NWD WITH UP TO 0.15" SMITH BASIN... OTHERWISE DRY. DAY 3 QPF (SAT 25/12Z TO SUN 26/12Z): 1-1.8" SMITH AND EEL BASINS AND 0.75-1.5" RUSSIAN/NAPA BASINS AS WELL AS THE SANTA CRUZ MTNS. GENERALLY 0.5-0.9" THROUGH THE SANTA LUCIA MTNS EXCEPT UP TO 1.75" FAR NRN PORTION. 1-1.5" ACROSS THE TRANSVERSE RANGES OF SRN CA WITH 0.75-1.25" SAN GABRIEL MTNS AND AROUND 0.5-0.65" SAN BERNADINO MTNS. GENERALLY 0.25-0.5" REMAINDER OF SRN CA COAST. 0.15-0.5" SAN JOAQUIN VALLEY AND 0.4-1" SACRAMENTO VALLEY. 1-1.4" NEAR LAKE SHASTA AND UP TO AROUND 1.25" ABOVE THE LAKE. 0.1-0.4" FAR NRN CA AND SRN OREGON. 0.75-1.5" NRN AND CNTRL SIERRAS AND TRAILING OFF TO AROUND 0.5" FAR SRN SIERRAS. UP TO 0.5" ON THE EAST SIDE OF THE SIERRAS AND 0.25" NEAR THE NV BORDER. 0.1-0.25" NE CA AND LIGHT AMOUNTS LESS THAN 0.1" REMAINDER OF WRN NV AS WELL AS SE CA DESERT AREAS. .HYDROLOGICAL CONDITIONS... MANY SOUTHERN CALIFORNIA RIVERS AND STREAMS ARE STILL FLOWING HIGH...BUT THE MOJAVE RIVER IS THE ONLY ONE ABOVE FLOOD STAGE. THE SAN DIEGO RIVER IS CURRENTLY ABOVE MONITOR STAGE...BUT BELOW FLOOD STAGE. RIVERS IN SOUTHERN CALIFORNIA WILL CONTINUE RECEDING WITH THE MOJAVE RIVER EXPECTED TO DROP BELOW FLOOD STAGE BY THIS EVENING WHILE THE SAN DIEGO RIVER SHOULD DROP BELOW MONITOR STAGE BY THIS EVENING AS WELL. IN NORTHERN AND CENTRAL CALIFORNIA MOST RIVERS AND STREAMS REMAIN WELL ABOVE SEASONAL NORMALS BUT AT MANAGEABLE FLOW VOLUMES. OVERFLOW CONTINUES INTO THE SUTTER BYPASS AT TISDALE WEIR. BOTH TISDALE AND COLUSA WEIRS ARE EXPECTED TO HAVE PERIODIC OVERFLOW DURING THE NEXT FEW DAYS. OVERFLOW INTO THE YOLO BYPASS AT FREMONT WEIR IS EXPECTED TO END THIS EVENING. IN THE DELTA...THE STAGE AT RIO VISTA IS EXPECTED TO TOP MONINTOR STAGE BRIEFLY LATE THIS AFTERNOON. OTHERWISE...RIVERS IN NORTHERN AND CENTRAL CALIFORNIA WILL EXPERIENCE MODERATE RISES IN RESPONSE TO PRECIPITATION THIS WEEKEND. MORE INFORMATION ON THE CNRFC WEBSITE AT HTTP://WWW.CNRFC.NOAA.GOV ME/KL/ASH