658
AGUS76 KRSA 241612
HMDRSA
HYDROMETEOROLOGICAL DISCUSSION
NATIONAL WEATHER SERVICE / CALIFORNIA NEVADA RFC / SACRAMENTO CA
830 AM PST WED DEC 24 2025
...ONGOING ATMOSPHERIC RIVER IS SPREADING HEAVY PRECIPITATION
ACROSS MUCH OF THE FORECAST AREA...
...ADDITIONAL ACTIVITY TO IMPACT THE REGION TOMORROW BEFORE RIDGING
STARTS TO BUILD ON SUNDAY...
METEOROLOGICAL CONDITIONS (SHORT TERM: WED AM - SAT AM)
THE CURRENT ATMOSPHERIC RIVER IS WELL UNDERWAY WITH RADAR
REFLECTIVITY SHOWING HEAVY PRECIPITATION SPREADING ACROSS MUCH OF
THE FORECAST AREA. A NARROW COLD FRONTAL RAINBAND WITH REFLECTIVITY
VALUES ABOVE 45 DBZ, ORIENTED SOUTHWEST TO NORTHEAST, IS CURRENTLY
MOVING EASTWARD INTO THE SIERRA FOOTHILLS AROUND THE I-80 CORRIDOR
AND OVER THE NORTHERN SAN JOAQUIN VALLEY. THIS CONVECTIVE FEATURE
PRODUCED HEAVIER PRECIPITATION ACROSS THE CENTRAL VALLEY THAN
GUIDANCE INITIALLY PREDICTED RESULTING IN AN INCREASE IN QPF IN THE
FIRST 6-HOUR PERIOD ACROSS THE CENTRAL COAST FROM MONTEREY TO POINT
CONCEPTION AND UP THE CENTRAL VALLEY TO YUBA COUNTY.
STRONG SOUTHERLY ATMOSPHERIC RIVER CONDITIONS (IVT >750 UNITS) HAVE
BEGUN SPREADING ACROSS THE SOUTHERN CALIFORNIA BIGHT PRODUCING HEAVY
PRECIPITATION ACROSS THE SOUTH FACING SLOPES OF THE WESTERN
TRANSVERSE RANGE. SOME LOCATIONS ARE CURRENTLY EXPERIENCING RAIN
RATES EXCEEDING 1-1.5 IN./HR, RESULTING IN WIDE SPREAD FLASH
FLOODING IMPACTS FROM SANTA BARBARA THROUGH VENTURA AND INTO LOS
ANGELES COUNTY. MODERATE TO HEAVY PRECIPITATION IS FORECAST TO
CONTINUE ACROSS SOUTHERN CALIFORNIA OVER THE NEXT SEVERAL HOURS WITH
THE POTENTIAL FOR ONGOING HYDROLOGIC IMPACTS. 6 HOUR PRECIPITATION
TOTALS ENDING 18Z TODAY OF >4 INCHES ARE FORECAST FOR THE HIGHER
ELEVATIONS OF THE TRANSVERSE MOUNTAINS ABOVE VENTURA AND LA COUNTIES
WITH POCKETS OF HEAVIER ACCUMULATIONS LIKELY WHERE CONVECTIVE
ACTIVITY INITIATES AND TRACKS. THE SOUTHERLY AR CONDITIONS WILL FEED
INTO THE SOUTHERN SIERRA THROUGHOUT THE DAY TODAY, PRODUCING 24-HOUR
TOTALS UPWARDS OF 5 INCHES AT SOME HIGHER ELEVATIONS. 24-HOUR TOTALS
ACROSS SOUTHERN CALIFORNIA RANGE FROM 2 INCHES AT LOWER ELEVATIONS
TO 10+ INCHES OVER SOUTH FACING MOUNTAIN SLOPES ACROSS THE
TRANSVERSE. THE SOUTHERLY FLOW UP THE SACRAMENTO VALLEY ALSO FAVORS
STRONG UPSLOPE MOISTURE FLUX OVER THE SHASTA DRAINAGE, WHICH IS
CURRENTLY FORECAST TO SEE 3-4 INCHES OF PRECIPITATION.
AS THE CURRENT ATMOSPHERIC RIVER SPREADS INLAND TO THE FOUR CORNERS
REGION AND INTERMOUNTAIN WEST OVERNIGHT TONIGHT AND INTO TOMORROW
MORNING, ANOTHER LOW PRESSURE SYSTEM SPINS UP OVER THE EASTERN
PACIFIC, SPREADING ANOTHER ROUND OF SOUTHERLY AR CONDITIONS ACROSS
CALIFORNIA. AS THIS LOW PINWHEELS OFF THE NORTHERN CALIFORNIA COAST,
THE SOUTHERLY AR CONDITIONS WILL FAVOR HEAVIER PRECIPITATION ACROSS
MUCH OF THE SAME REGIONS AS THE CURRENT AR WITH 3+ INCHES FORECAST
OVER THE SHASTA DRAINAGE, DOWN THE CREST OF THE SIERRA AND ACROSS
THE WESTERN TRANSVERSE. ACCUMULATIONS MAY REACH AS HIGH AS 5+ INCHES
OVER THE NORTHERN SIERRA WITH STRONG MOISTURE FLUX EXTENDING INLAND
THROUGHT THE SAN FRANSCISCO BAY GAP. THE NORTH COAST COULD SEE AN
ADDITIONAL 1 TO 2 INCHES WHILE THE BIG SUR COAST IS CURRENTLY
FORECAST TO RECEIVE ANOTHER 2-3 INCHES. GUIDANCE CURRENTLY SHOWS
HIGHER UNCERTAINTY WITH THIS SECOND SYSTEM AS ANY SHIFT IN WHERE THE
LOW DECIDES TO PINWHEEL OVER NORTHERN CALIFORNIA COAST WILL PLAY A
ROLE ON TIMING AND INTENSITY OF THE MOISTURE PLUME MOVING ONSHORE.
CONDITIONS BEGIN TO LIGHTEN UP LATE FRIDAY INTO SATURDAY MORNING AS
THE LOW BEGINS TO WEAKEN AND THE AR STARTS DISSIPATING. ADDITIONAL
SHOWER ACTIVITY IS FORECAST TO BRING AN ADDITIONAL 1 TO 1.5 INCHES
ACROSS THE HIGHER ELEVATIONS OF THE NORTH COAST, SHASTA DRAINAGE,
SIERRA AND EASTERN TRANSVERSE RANGE (ENDING 12Z SATURDAY). SIMILIAR
TO THE PREVIOUS DAY, UNCERTAINTY IS RELATIVELY HIGHER ON THE
SPECIFICS AND GUIDANCE DISAGREES IN EXACT LOCATION AND DISSIPATION
SPEED OF THE LOW AND MOISTURE PLUME.
FREEZING LEVELS ARE FORECAST TO LOWER ACROSS THE SIERRA OVER THE
NEXT FEW DAYS FROM ~8K FEET TO ~5K FEET BY SATURDAY MORNING
SWITCHING PRECIPITATION OVER THE SIERRA FROM MOSTLY LIQUID TO A
LARGER PROPORTION OF SNOW. FOR SOUTHERN CALIFORNIA, FREEZING LEVELS
ARE FORECAST TO START THE PERIOD ABOVE 9K FEET BEFORE LOWERING TO
~6K FEET ON SATURDAY.
METEOROLOGICAL CONDITIONS (LONG TERM: SAT AM - TUE AM)
THE UPPER LOW THAT ARRIVED IN THE SHORT TERM TIME FRAME IS PROGGED
TO OPEN INTO A TROUGH EARLY SATURDAY CENTERED SOMEWHERE OVER CENTRAL
CA. THE TROUGH WILL SWING INLAND TO THE SOUTH EAST ACROSS SOUTHERN
CA THE REST OF THE DAY JOINING WITH A LARGER SYSTEM CENTERED OVER
CANADA BY SUNDAY MORNING AS ANOTHER LOW DEVELOPS OFFSHORE SOUTHWEST
OF CA. AS THE TROUGH MOVES ACROSS CA, EXPECT SOME LIGHT SCATTERED
SHOWERS, MAINLY ALONG THE COAST AND ACROSS THE SIERRA FOR SATURDAY.
AMOUNTS LOOK MINIMAL AT GENERALLY 0.10" OR LESS. HIGH PRESSURE WILL
THEN BUILD IN NORTH OF THE NEWLY FORMED UPPER LOW OVER THE EASTERN
PACIFIC AND INTO THE PACNW RETURNING DRY CONDITIONS TO THE REGION
THE REST OF THE FORECAST PERIOD.
TAKING A PEAK INTO THE FAR BEYOND, CPC OUTLOOKS HAVE CHANCES AGAIN
OF ABOVE NORMAL PRECIPITATION FOR MUCH OF THE REGION IN THE 6-10 TO
8-14 DAY OUTLOOKS. IVT PROBABILITIES DO SHOW INCREASED CHANCES OF
ACTIVITY OVER SRN CA AROUND NEW YEARS AND OVER NRN CA THE FIRST WEEK
OF JANUARY. STAY TUNED.
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.
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
CH/AS
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