586
FOUS30 KWBC 221550
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1050 AM EST MON DEC 22 2025
DAY 1
VALID 16Z MON DEC 22 2025 - 12Z TUE DEC 23 2025
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL IN EFFECT FOR
PORTIONS OF CENTRAL AND NORTHERN CALIFORNIA...
..16Z UPDATE
THE ATMOSPHERIC RIVER IMPACTING CALIFORNIA IS IN A BIT OF A LULL
TODAY. IVT VALUES, A MEASURE OF THE MOISTURE ADVECTION FROM OFF THE
PACIFIC, WILL BE DROPPING RAPIDLY THROUGH THE DAY. THIS MEANS THAT
THE WIND WILL BE LESS AND LESS COOPERATIVE WITH ADVECTING MOISTURE
INTO THE SIERRA NEVADA, RESULTING IN DIMINISHING UPSLOPE
ENHANCEMENT OF THE RAIN. THAT SAID, THE PLUME OF MOISTURE MOVING UP
I-80 FROM THE BAY AREA INTO THE CENTRAL SIERRA NEVADA RANGE WILL
TREND NORTH AS A DIGGING TROUGH IN THE UPPER LEVELS REALIGNS THE
FLOW TO MORE OUT OF THE SSW. THIS TOO WILL GRADUALLY DIMINISH THE
RAINFALL RATES IN THE HARD-HIT UPSLOPE REGIONS.
AS THE PLUME OF RAIN DRIFTS NORTH THIS AFTERNOON, EXPECT A RENEWED
ROUND OF RAIN INTO THE HARDEST HIT PORTIONS OF THE SIERRA NEVADA
FROM OROVILLE EAST. THIS IS LIKELY TO CAUSE ADDITIONAL OR RENEWED
FLOODING...HOWEVER THE DIMINISHING RAINFALL RATES SHOULD KEEP
IMPACTS BELOW MODERATE RISK CRITERIA. THE RAIN WILL CONTINUE NORTH
INTO THE NORTHERN SACRAMENTO VALLEY BY TONIGHT DUE TO THE
INCREASINGLY SOUTHERLY FLOW. WHILE SOILS ARE ALSO SATURATED FROM
RECENT RAINFALL IN THIS AREA, SINCE THE PREDOMINANT FLOW PRIOR TO
TODAY WAS WESTERLY, IT SHOULD BE DIFFERENT SLOPES OF THE NORTHERN
COAST RANGES AND THE KLAMATH MOUNTAINS THAT GET THE HEAVIEST RAIN
TODAY, AND EVEN THEN, DUE TO THE DIMINISHING PRESSURE GRADIENT,
SHOULD NOT BE ANYWHERE NEAR AS HEAVY AS PRIOR DAYS OVERNIGHT
TONIGHT.
GIVEN THE TRANSIENT NATURE OF THE RAIN PLUME TODAY, THE MODERATE
RISK HAS BEEN DOWNGRADED TO A SLIGHT, WITH NO CHANGES TO THAT
SLIGHT OR THE SURROUNDING MARGINAL. IMPACTS FROM THE RAIN THAT DOES
FALL TODAY COULD STILL INCLUDE LAND SLIDES, MUD SLIDES, AND FLASH
FLOODING IN SMALLER CREEKS THAT HAVE HAD SOME TIME TO DRAIN A BIT
THIS MORNING.
WEGMAN
DAY 2
VALID 12Z TUE DEC 23 2025 - 12Z WED DEC 24 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CALIFORNIA...
THE TRAJECTORY OF THE ATMOSPHERIC RIVER WILL BECOME MORE S/SW TO
N/NE AS IT SHIFTS SOUTHWARD DURING THIS PERIOD AS THE TROUGH
OFFSHORE IN THE PACIFIC ALSO DIGS TO THE SOUTH. 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. AS THE LOW DEEPENS THE ONSHORE FLOW WILL ALIGN PARALLEL TO
THE COASTLINE THUS SIGNIFICANTLY INCREASING THE AREAL EXTENT HEAVY
RAINFALL AND THE HIGHER TERRAIN SNOW ACCUMULATIONS. 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 LOS ANGELES BASIN. THE RENEWED ROUND OF HEAVY RAIN
WILL BE IN AREAS GETTING HARD HIT BY THE CURRENT ATMOSPHERIC RIVER
AND WILL CONTINUE WITH THE DEEPENING LOW OFFSHORE INTO THE
NORTHERN COASTAL RANGES. A SLIGHT RISK WAS MAINTAINED FOR THE
OREGON BORDER DOWN THROUGH LOS ANGELES.
CAMPBELL/WEGMAN
DAY 3
VALID 12Z WED DEC 24 2025 - 12Z THU DEC 25 2025
...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS SOUTHERN
CALIFORNIA...
WIDESPREAD HEAVY RAINFALL WILL RAMP UP EVEN FURTHER ACROSS THE
TRANSVERSE RANGE AND SURROUNDING LOCATIONS OF SOUTHERN CALIFORNIA.
AREAL AVERAGE OF 3 TO 5 INCHES ARE EXPECTED BUT ISOLATED LOCAL
MAXIMUMS MAY REACH AS MUCH AS 9 INCHES. A MODERATE RISK IS IN
EFFECT FOR A MAJORITY OF SANTA BARBARA, VENTURA, LOS ANGELES,
RANGE, SAN BERNARDINO, RIVERSIDE AND SAN DIEGO COUNTIES. ALTHOUGH
SNOW LEVELS WILL BE LOWERING ACROSS THE SIERRA NEVADA AND ADJACENT
LOCATIONS AND MUCH OF THE HIGHEST ELEVATIONS WILL HAVE SIGNIFICANT
SNOW, SOME OF THE LOWER ELEVATIONS COULD RECEIVE ADDITIONAL
RAINFALL OF 1 TO 4 INCHES, WHICH IS COVERED BY A SPRAWLING SLIGHT
RISK AREA. DEEP-LAYERED MOISTURE WILL ALSO TRACK EASTWARD ACROSS
EASTERN CALIFORNIA, SOUTHERN NEVADA AND INTO SOUTHERN UTAH AND
WESTERN ARIZONA WHERE AREAL AVERAGES UP TO 1 TO 2+ ARE FORECAST. A
NEW SLIGHT RISK AREA WAS RAISED FOR MUCH OF SOUTHERN NEVADA, ALONG
THE CALIFORNIA/NEVADA BORDER AND FOR WESTERN ARIZONA. RAINFALL
RATES WILL EASILY SURPASS 0.5-1.0 INCH/HR RATES, PARTICULARLY FOR
CALIFORNIA.
CAMPBELL
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
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