811
NGUS86 KRSA 211614
HCMRSA
HYDROMETEOROLOGICAL COORDINATION MESSAGE
NATIONAL WEATHER SERVICE / CALIFORNIA NEVADA RFC / SACRAMENTO CA
815 AM PST SUN DEC 21 2025
...ATMOSPHERIC RIVER TO BRING WIDESPREAD MODERATE TO LOCALLY HEAVY
PRECIP THROUGH WEDS...
...A LARGE LOW WILL THEN APPROACH INTO THURS BRINGING ADDITIONAL
PRECIP TO THE REGION...
METEOROLOGICAL CONDITIONS (SUN AM - SAT AM)
AN ATMOSPHERIC RIVER CONTINUES TO IMPACT CA AS TPW IMAGERY SHOWS A
WIDE AREA OF 1-1.50" PW OF MOISTURE FROM THE CA/OR BORDER DOWN TO
POINT CONCEPTION AND INLAND UP TO SHASTA AND THE FOOTHILLS. IVT
VALUES AT THE 09Z CW3E SOUNDING LAUNCHED AT MARYSVILLE SHOWED 731
KG/MS AS KOAK 12Z SOUNDING REPORTED 1.32" PW. A DEEP UPPER TROUGH IS
MAKING ITS WAY ACROSS THE GULF OF ALASKA WHILE THE PERSISTENT RIDGE
WAS PUSHED TO THE SOUTH OVER BAJA AND MX. THESE TWO SYSTEMS WILL
CONTINUE TO ALLOW THE TROPICAL MOISTURE TO FUNNEL INTO THE AREA FROM
NEAR THE HAWAIIAN ISLANDS. RADAR IMAGERY SHOWS WIDESPREAD PRECIP
NORTH OF I-80 AND DOWN INTO THE CENTRAL SIERRA. OBSERVATIONS OVER
THE PAST 24 HOURS REPORTING TOTALS OF 1.5-5.50" ACROSS THE NRN
SIERRA, 2-4.50" OVER SHASTA, AND 1.5-3.50" FROM SONOMA COUNTY
NORTHWARD ALONG THE COAST. EXPECT THE MOISTURE PLUME TO CONTINUE TO
FOCUS AROUND THE SAME AREAS THROUGHOUT THE REST OF THE DAY AS IT
NARROWS BEFORE MOVING A BIT SOUTH OVERNIGHT. HREF EXCEEDANCE
PROBABILITIES SHOW STRONG CHANCES (30-90%) OF FAIRLY PERSISTENT
0.50+"/HR RAIN RATES FROM NOW THROUGH THE EARLY EVENING NORTH OF I-
80 AND PARTICULARLY OVER THE HIGHER TERRAIN AREAS. PARTS OF THE
NORTHERN SIERRA/SHASTA SHOW UP TO 10% CHANCES OF 1+"/HR RATES.
CHANCES OF THUNDERSTORMS ALSO IN THE FORECAST FOR THE SRN SAC/NRN SJ
VALLEY INTO THE SIERRA FOR TODAY ACCORDING TO THE SPC DAY 1 OUTLOOK.
TONIGHT/TOMORROW THE MOISTURE PLUME WILL START OUT FOCUSING PRECIP
AROUND THE SF BAY AREA AND OVER THE NORTHERN/CENTRAL SIERRA. OVERALL
EXPECT RAIN RATES TO DIMINISH, ESPECIALLY FOR THE LOWER ELEVATION
AREAS. BY LATE TOMORROW AFTERNOON, THE MOISTURE WILL CEASE ITS
SOUTHERLY MOVEMENT AND LIFT BACK NORTHWARD AS THE PLUME'S AXIS TILTS
INTO A STEEPER SLOPE. THIS WILL PULL THE FOCUS OF PRECIP BACK INTO
THE RUSSIAN AND NORTHERN SIERRA OVERNIGHT MOVING FURTHER NORTHWARD
INTO THE EEL/SHASTA EARLY TUESDAY. BY MID MORNING, EXPECTING THE
EASTERN EDGE OF THE MOISTURE'S CORE TO RETREAT OFFSHORE BEFORE BEING
CARRIED BACK IN BY A DEVELOPING OFFSHORE SURFACE LOW TO THE
SOUTHWEST. EXPECT CHANCES OF THUNDERSTORMS ALONG THE NRN/CNTRL
COASTS DUE TO THAT LOW AS WELL. THE TROUGH IN THE GULF ALSO WILL
CONTINUE TO STRENGTHEN AND PUSH TROUGHING OFFSHORE OF THE WEST
COAST.
THERE IS STILL SOME UNCERTAINTY FOR TUESDAY/WEDNESDAY AS MODELS
DISAGREE ON THE STRUCTURE OF THAT SURFACE LOW AND WHERE TO TAKE IT.
IT IS EXPECTED TO MOVE THROUGH NRN CA TO THE NORTHEAST SOMETIME LATE
TUESDAY/EARLY WEDNESDAY AS A COLD FRONT SENDS MODERATE TO LOCALLY
HEAVY PRECIP INTO CENTRAL/SOUTHERN CA JOINING FORCES WITH THE
TROPICAL MOISTURE PLUME. MODELS HAVE BEEN INCONSISTENT RUN TO RUN AS
TO HOW CLOSE TO BRING THAT LOW TO THE COAST ON TUESDAY. ANY SHIFT
WEST OR EAST WOULD EITHER PULL THE HEAVIEST PRECIP OFFSHORE OR SEND
IT FURTHER INLAND ACROSS NRN CA. THE QPF CLUSTERS ENDING 00Z WEDS
CAPTURE PART OF THIS. THEY ARE SPLIT INTO 4 CLUSTERS EITHER SHOWING
HEAVY PRECIP RIGHT ALONG THE COAST FROM CAPE MENDOCINO TO MARIN AND
OVER SHASTA/NRN SIERRA (CLUSTER 2: 25% OVERALL), LIGHTER AMOUNTS
OVER THOSE SAME AREAS (CLUSTER 1: 26% OVERALL), BROADER AND HIGHER
TOTALS PUSHING FURTHER INLAND (CLUSTER 3: 25% OVERALL), OR FOCUSING
HEAVIER PRECIP FROM SONOMA TO SANTA CRUZ AND INTO THE NRN SIERRA
(CLUSTER 4: 24% OVERALL). THERE IS A SPREAD AT ARCATA OF 0-2", 0-
2.50" AT KSFO, AND 0.50-2.50" AT THE SANTA ROSA AIRPORT IN SONOMA
COUNTY DURING THAT TIME.
AS THAT SYSTEM MOVES INLAND ON WEDNESDAY, A BROAD UPPER LOW AND
SURFACE LOW ARE EXPECTED JUST OFFSHORE. THE MOISTURE PLUME WILL
CONTINUE TO MOVE THROUGH SOUTHERN CA THE REST OF WEDNESDAY RESULTING
IN MODERATE TO LOCALLY HEAVY PRECIPITATION BEFORE EXITING TO THE
SOUTH OVERNIGHT INTO THURSDAY. MEANWHILE, THE LOW WILL ROTATE
OFFSHORE STRENGTHENING AND ROTATING ADDITIONAL PRECIP INTO NRN/CNTRL
CA THROUGH THURSDAY. BEYOND THAT THERE IS SOME DISAGREEMENT. THE
ECMWF STARTS TO MOVE THE SYSTEM TO THE SOUTH OFFSHORE LATE THURSDAY
BEFORE SPLITTING THE SYSTEM INTO TWO TROUGHS, ONE OVER THE PACNW AND
ONE MOVING THROUGH CENTRAL CA ON FRIDAY CLOSING OFF INTO A LOW OVER
SRN CA BY EARLY SATURDAY. THE GFS THROUGHOUT THIS TIME STILL HAS THE
UPPER LOW ROTATING OFFSHORE WEST OF NRN CA. THE RESULT EITHER WAY IS
CONTINUED WIDESPREAD PRECIPITATION TO VARYING EXTENTS. AMOUNTS ARE
STILL UNCERTAIN BUT EXPECT WET WEATHER TO PERSIST AFTER THE CURRENT
AR HEADS OUT THROUGH AT LEAST FRIDAY.
MODELS HAVE OVERALL SHIFTED NORTHWARD IN REGARDS TO THE
PRECIPITATION FROM THE CURRENT ATMOSPHERIC RIVER. THIS LEAD TO
PRETTY DECENT CHANGES IN THE QPF OVER THE FORECAST PERIOD. AMOUNTS
WENT DOWN BY 0.50-1" FOR A WIDE AREA FROM MONTEREY TO MARIN COUNTY
AND THROUGH TO THE FOOTHILLS WITH LOCALLY AMOUNTS DROPPING UP TO 2".
DECREASES GENERALLY ELSEWHERE SOUTH OF THAT AREA ON THE ORDER OF
0.10-0.50". TOTALS WENT UP BY 0.50-1.50" NORTH OF SONOMA COUNTY AND
WEST OF I-5 WITH SOME TERRAIN AREAS SEEING INCREASES UP TO 2-3"
COMPARED TO YESTERDAY THROUGH 12Z FRIDAY. HEAVIEST PRECIP IS STILL
EXPECTED ACROSS THE SIERRA, SHASTA, THE RUSSIAN/EEL BASINS, AND THE
COASTAL MOUNTAINS. QPF THROUGH 12Z SAT: 7-13" NORTH BAY, 8-21.50"
NORTHERN SIERRA, 7-16" CENTRAL SIERRA, 5-13" SOUTHERN SIERRA, 8.50-
16" SHASTA, 4-6.50" NORTH COAST (8-11.50" KINGS RANGE), 3-6.50"
CENTRAL COAST (UP TO 13" SANTA CRUZ MOUNTAINS), 4-9" SACRAMENTO
VALLEY INTO THE FOOTHILLS, 2-5" S TO N SJ VALLEY, 1.50-4.50" THE
REST OF NRN/NE CA, AND 2.50-6.50" FOR MOST OF COASTAL SRN CA (UP TO
11.50" TRANSVERSE RANGE).
FREEZING LEVELS 8-10.5 KFT NORTH OF I-80 AND 10-13.5 KFT FROM I-80
TO THE SOUTHERN SIERRA TODAY. LEVELS WILL RE-LOWER ON MONDAY DOWN TO
4.5-9 KFT NORTH OF I-80 AND 8.5-11.5 KFT TO THE SOUTHERN SIERRA.
MODERATE FREEZING LEVELS WILL THEN START TO SPREAD LATER TUESDAY
DOWN TO 8-10 KFT FOR MOST OF THE SIERRA BEFORE LOWERING ON WEDNESDAY
DOWN TO 6-9 KFT. THINGS WILL HOVER AROUND THERE UNTIL THURSDAY
MORNING BEFORE DROPPING DOWN TO 5.5-7.5 KFT AND LOWERING FURTHER ON
FRIDAY DOWN TO 5-7 KFT.
PLEASE NOTE: THIS PRODUCT MAY NOT BE ROUTINELY UPDATED.
PLEASE REFER TO THE FOLLOWING PRODUCT ISSUED BY THE CNRFC
WWW.CNRFC.NOAA.GOV/DAILY-BRIEFING FOR A GRAPHICAL SUMMARY OF
WEATHER AND HYDROLOGIC CONDITIONS.
QPF GRAPHICS ARE AVAILABLE AT WWW.CNRFC.NOAA.GOV/QPF.PHP
*********************** CONTACT INFORMATION ************************
CNRFC OFFICE HOURS PHONE: 1 (916) 979-3056 EXTENSION 338
AFTER-HOURS CALLING SERVICE: 1 (800) 218-0858
LEAVE NAME...OFFICE...AND PHONE NUMBER
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