376
FOUS30 KWBC 220059
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
759 PM EST SUN DEC 21 2025
DAY 1
VALID 01Z MON DEC 22 2025 - 12Z MON DEC 22 2025
...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL IN EFFECT FOR
PORTIONS OF THE SIERRA NEVADA IN NORTHERN CALIFORNIA...
..16Z UPDATE
MODERATE TO HEAVY RAIN WAS ONGOING ACROSS NORTHERN CALIFORNIA THIS
MORNING WITH THE ATMOSPHERIC RIVER ORIENTED SOUTHWEST TO NORTHEAST
INTO AND ACROSS THE COASTLINE. THROUGH 15Z, 24 HOUR LOCALIZED
TOTALS OF 3 TO 5 INCHES HAVE BEEN REPORTED ACROSS THE KING RANGE OF
SOUTHERN HUMBOLDT COUNTY AND 3 TO 6+ INCHES ACROSS THE NORTHERN
SIERRA NEVADA. IVT VALUES PEAKING NEAR 800 KG/M/S WERE ESTIMATED
THIS MORNING AT 15Z IN/AROUND MENDOCINO COUNTY WITH SLOW BUT STEADY
SOUTHWARD TRANSLATION AND WEAKENING FORECAST OVER THE NEXT 12-24
HOURS. ADDITIONAL 24 HOUR TOTALS OF 4 TO 8 INCHES (PERHAPS LOCAL
MAXIMA NEAR 10 INCHES) ARE ANTICIPATED FOR THE NORTHERN SIERRA
NEVADA WITH 2 TO 4 INCHES (LOCAL 5+) FOR THE COASTAL RANGES FROM
CAPE MENDOCINO TO MONTEREY BAY. VALLEY LOCATIONS WILL RECEIVE MUCH
LESS, BUT HIGHER THAN THE 0 TO 0.1 INCH FORECAST VALUES DEPICTED BY
THE HIRES GUIDANCE.
A FEW LANDSLIDES/DEBRIS FLOWS HAVE ALREADY BEEN REPORTED AND
ADDITIONAL FLOOD RELATED IMPACTS WILL BE LIKELY WITHIN REGIONS
THAT RECEIVE ADDITIONAL HIGH RAINFALL TOTALS WITH PEAK HOURLY RATES
OVER 0.5 INCHES AT TIMES, LOCALLY UP TO 1 INCH IN AN HOUR.
OTTO
..PREVIOUS DISCUSSION FOLLOWS
A SIGNIFICANT SURGE IN PACIFIC MOISTURE WILL OVERSPREAD MUCH OF
NORTHERN CALIFORNIA DURING THIS PERIOD WITHIN THE REINFORCED
ATMOSPHERIC RIVER AND WILL LEAD TO WIDESPREAD HEAVY RAINFALL.
WITHIN THE MODERATE RISK AREA OVER THE PORTIONS OF THE SIERRA
NEVADA RANGE THERE ARE SEVERAL HOURS OF >50% HREF PROBABILITIES OF
AT LEAST 0.50"/HR RATES BETWEEN 12Z SUN AND 03Z MON, WHILE HREF
PROBABILITIES OF >8" WITHIN THE 24 HOUR DAY 2 PERIOD ARE >60% FOR
MUCH OF THE AREA. THERE WILL LIKELY BE MANY AREAS WHERE FLASH
FLOODING MAY ARISE, ALONG WITH SLOUGHING AND DEBRIS FLOWS. FOR
ADDITIONAL DETAILS PLEASE REFER TO WPC MPD #1267.
CAMPBELL/HURLEY
DAY 2
VALID 12Z MON DEC 22 2025 - 12Z TUE DEC 23 2025
...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL IN EFFECT FOR
PORTIONS OF THE SIERRA NEVADA IN NORTHERN CALIFORNIA...
..20Z UPDATE
12Z GUIDANCE TRENDS ARE NOT AS FAR SOUTH WITH THE PEAK RAINFALL
ACROSS THE NORTHERN SIERRA FOOTHILLS; IN FACT, THERE IS QUITE A BIT
OF OVERLAP WITH THE DAY 2 MODERATE RISK AREA WITH TODAY'S D1
MODERATE. GIVEN THIS, WE HAVE INCLUDED AN 'ENHANCED' OR 'HIGHER-
END' MODERATE AREA ACROSS PORTIONS OF THE NORTHERN SIERRA
FOOTHILLS, INCLUDING THE COMMUNITIES OF COLFAX, GRASS VALLEY, AND
BLUE CANYON, WHERE THE NEIGHBORHOOD PROBABILITIES OF MORE RAPID
INUNDATION (PARTICULARLY OVER BURN SCARS AND/OR MORE FLOOD PRONE
AREAS) CLIMBS TO ABOVE 50%.
THE ATMOSPHERIC RIVER WILL MOVE SOUTHWARD DURING THIS PERIOD AS
WILL THE HEAVY RAINFALL FOOTPRINT SPREAD FURTHER DOWN THE SIERRA
NEVADA RANGE AND ALONG THE COASTLINE. MUCH OF THE SAME AREA OF THE
SIERRA NEVADA IS PROJECTED TO RECEIVE ADDITIONAL 2-3 INCHES OVER
THE WATER LOGGED AREA FROM DAY 1. A MODERATE RISK WAS MAINTAINED
ALONG WITH EXPANDING A TOUCH TO THE SOUTH. MULTI-DAY ACCUMULATIONS
WILL LIKELY BE WELL INTO THE DOUBLE DIGITS BY THIS TIME AND LEADING
TO SEVERAL INSTANCES OF RAPID RUNOFF, SLOUGHING AND DEBRIS FLOWS
ALONG THE FOOTHILLS/ADJACENT LOCATIONS.
HURLEY/CAMPBELL
DAY 3
VALID 12Z TUE DEC 23 2025 - 12Z WED DEC 24 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CALIFORNIA...
..20Z UPDATE
NOT TOO MANY CHANGES WERE NEEDED FOR THE DAY 3 PERIOD. THE
ATMOSPHERIC RIVER THAT HAD BEEN IMPACTING PORTIONS OF NORTHERN
CALIFORNIA WILL BEGIN TO SHIFT SOUTH AND REALIGN MORE SOUTH-
SOUTHWEST TO NORTH-NORTHEAST BY DAY 3/TUESDAY. THIS WILL OCCUR AS A
DEEP LONGWAVE TROUGH DIGS SOUTHWARD ACROSS THE PACIFIC OFF THE
COAST OF CALIFORNIA. IN CONTRAST TO THE AIR MASS CONTAINING THE
ATMOSPHERIC RIVER, WHICH WILL BE WARM AND MOISTURE-LADEN, THIS
CONTRAST WILL SUPPORT CYCLOGENESIS ALONG THE GREATEST
GRADIENT/BOUNDARY BETWEEN THE COLD AIR MASS WITHIN THE TROUGH AND
THE MUCH WARMER AIR MASS IN THE RIDGE/ATMOSPHERIC RIVER. THE LOW
THAT FORMS WILL DEEPEN TO AS LOW AS THE UPPER 970S MB. MEANWHILE,
THE DIGGING TROUGH WILL CAUSE THE A.R. TO TURN MORE
LONGITUDINAL/PARALLEL TO THE CALIFORNIA COAST. THIS MEANS MUCH OF
THE STATE WILL BE IMPACTED BY HEAVY PRECIPITATION, AND MORE
SNOW/FALLING SNOW LEVELS ARE EXPECTED INTO NORTHERN CALIFORNIA
WITHIN THE MUCH BROADER LONGWAVE TROUGH. THE ONSHORE FLOW
ASSOCIATED WITH THE LOW ITSELF WILL CONTINUE VERY HEAVY RAIN AND
MOUNTAIN SNOW ACROSS NORTHERN CALIFORNIA, WHILE THE TRAILING COLD
FRONT MOVES INTO SOUTHERN CALIFORNIA STARTING TUESDAY NIGHT.
THE AREAS OF GREATEST IMPACT WILL BE INTO THE NORTHERN COASTAL
RANGES AND THE TRANSVERSE RANGES. BOTH THESE RANGES ARE
SUFFICIENTLY ALIGNED PERPENDICULAR TO THE SOUTH-SOUTHWESTERLY FLOW
TO HAVE A SIGNIFICANT LOCAL UPSLOPE CONTRIBUTION TO RAINFALL RATES.
IN THE TRANSVERSE RANGES, LOCAL BURN SCARS WILL ALSO WORSEN IMPACTS
FROM RESULTANT FLOODING OVER AND DOWNSTREAM OF ANY BURN SCARS NORTH
AND WEST OF THE L.A. BASIN. MEANWHILE, UP NORTH THE RENEWED ROUND
OF HEAVY RAIN WILL BE IN AREAS GETTING HARD HIT BY THE A.R. NOW AND
WILL CONTINUE WITH THE DEEPENING LOW OFFSHORE INTO THE NORTHERN
COASTAL RANGES. THE SLIGHT RISK INHERITED WAS COMBINED TO INCLUDE
ALL OF THE CENTRAL CALIFORNIA COAST, AND WILL NOW EXTEND FROM CLOSE
TO THE OREGON BORDER DOWN THROUGH LOS ANGELES. MEANWHILE ALONG THE
SIERRAS, IN COORDINATION WITH HNX/HANFORD, CA FORECAST OFFICE, THE
SLIGHT RISK WAS EXTENDED SOUTH ALONG THE SOUTHERN SIERRA NEVADA
RANGE DUE TO EXPECTED IMPACTS FROM THE HEAVY RAIN THAT COULD BE
SUPPORTED BY INCREASING INSTABILITY AS WARMER AND MORE MOIST AIR IS
ADVECTED INTO THE REGION AS THE A.R. REALIGNS.
WEGMAN
DAY 1 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/94EPOINTS.TXT
DAY 2 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/98EPOINTS.TXT
DAY 3 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/99EPOINTS.TXT
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page