974
FOUS30 KWBC 240027
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
727 PM EST TUE DEC 23 2025
DAY 1
VALID 01Z WED DEC 24 2025 - 12Z WED DEC 24 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CALIFORNIA...
..01Z UPDATE
ONCE AGAIN...FEW TWEAKS NEEDED TO THE PREVIOUSLY-ISSUED EXCESSIVE
RAINFALL OUTLOOK VALID INTO THE EARLY-MORNING HOURS OF
WEDNESDAY.LATEST RUNS OF THE CAM GUIDANCE HAVE REMAINED VERY
CONSISTENT THAT A SURGE OF IVT WILL ARRIVE LATE TONIGHT. THERE
HAVE BEEN AREAS OF RAINFALL ACROSS SOUTHERN CALIFORNIA THROUGHOUT
MUCH OF THE DAY...BUT AN INCREASE IN AREAL COVERAGE AND RAINFALL
INTENSITY IS EXPECTED OVERNIGHT ONCE THE IVT PLUME ARRIVES AND
CONTINUE TO BUILD BEYOND THE END OF THE DAY 1 PERIOD. THIS WILL
LEAD TO AN INCREASING THREAT FOR FLASH FLOODING...WITH GREATEST
CONCERN IN REGIONS OF COMPLEX TERRAIN OR HIGHLY-URBANIZED AREAS.
BANN
..16Z UPDATE
ONLY MINOR TWEAKS WERE NEEDED FOR THE SLIGHT AND MARGINAL RISK
AREAS ACROSS MUCH OF CALIFORNIA TODAY. THE ATMOSPHERIC RIVER (A.R.)
IS MAKING ITS LONG EXPECTED TURN FROM WESTERLY TO SOUTH-
SOUTHWESTERLY. FOR NOW, THAT HAS RESULTED IN THE STEADIEST RAIN TO
SHIFT NORTH TO OVER FAR NORTHWESTERN CALIFORNIA AND SOUTHWESTERN
OREGON. HOWEVER, EVEN HERE RATES ARE GENERALLY BELOW A HALF INCH
PER HOUR, SO FLOODING CONCERNS LOOK TO BE MINOR FOR MUCH OF THE
REST OF THE DAY.
A RAPIDLY DEEPENING LOW AND ITS ATTENDANT COLD FRONT WILL APPROACH
THE CALIFORNIA COAST TONIGHT. THE LOW AND ITS COLD FRONT WILL
GREATLY INCREASE RAINFALL RATES OVER MUCH OF CALIFORNIA. TO THE
NORTH RAIN WILL BE ASSOCIATED WITH THE LOW ITSELF, AS IT TRACKS NNE
ALONG THE COAST. TO THE SOUTH, THE RAIN WILL BE ASSOCIATED WITH THE
TRAILING COLD FRONT. WITH THE TRANSVERSE RANGES ALIGNED GENERALLY
WEST-EAST, AND THE FLOW SSW TO NNE, THERE WILL BE A SIGNIFICANT
ORTHOGONAL COMPONENT TO THE MEAN FLOW WHICH WILL FURTHER ENHANCE
RAINFALL RATES ON THE SOUTH FACING SLOPES OF THE WESTERN TRANSVERSE
RANGES TONIGHT, INCLUDING THE SANTA YNEZ AND SAN RAFAEL RANGES NEAR
SANTA BARBARA. LATE TONIGHT, RAIN RATES COULD APPROACH AN INCH PER
HOUR, WHICH WHEN COMBINED WITH THE NUMEROUS BURN SCARS IN THE AREA,
WILL INCREASE THE FLOODING THREAT SIGNIFICANTLY. THUS, HAVE OPTED
TO HIGHLIGHT THOSE RANGES IN A HIGHER-END SLIGHT RISK FOR THIS
UPDATE.
WEGMAN
..PREVIOUS DISCUSSION
THE S/SW TO N/NE ORIENTATED ATMOSPHERIC RIVER IS EXPECTED TO
MAINTAIN SCATTERED TO WIDESPREAD RAINFALL ALONG THE COASTLINE AND
INLAND TO THE SIERRA NEVADA RANGE WITH RAIN FOR THE LOWER
ELEVATIONS AND SNOW FOR THE HIGHER ELEVATIONS. CYCLOGENESIS WILL
OCCUR 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 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 ALONG
WITH A MARGINAL RISK ENCOMPASSING MUCH OF THE SAME AREA.
CAMPBELL/WEGMAN
DAY 2
VALID 12Z WED DEC 24 2025 - 12Z THU DEC 25 2025
...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL ACROSS SOUTHERN
CALIFORNIA...
..2030Z UPDATE
RELATIVELY FEW CHANGES WERE NEEDED FOR THE PEAK OF THE ATMOSPHERIC
RIVER (A.R.) EVENT ACROSS SOUTHERN CALIFORNIA. IN COORDINATION WITH
LOX/OXNARD, CA FORECAST OFFICE, THE MODERATE RISK WAS EXPANDED
NORTH UP THE COAST TO SAN LUIS OBISPO COUNTY. EXPECT THE HEAVY RAIN
ON TOP OF THE NUMEROUS BURN SCARS IN THE AREA TO RESULT IN NUMEROUS
INSTANCES OF FLASH FLOODING. ELSEWHERE, THE FORECAST IS LITTLE
CHANGED SO THE CHANGES TO THE ERO RISK AREAS WERE ALSO MINIMAL.
WEGMAN
..PREVIOUS DISCUSSION
HIGHLY ANOMALOUS 850-700 MB MOISTURE FLUX ON THE ORDER OF 5+
STANDARD DEVIATIONS ABOVE THE MEAN WILL YIELD EXTREMELY HEAVY
RAINFALL AHEAD OF THE AMPLIFYING MID/UPPER TROUGH LOCATED OFFSHORE
OF THE CALIFORNIA COAST. THE HEAVY RAINFALL TOTALS FOR SOUTHERN CA
WILL LIKELY OCCUR OVER AN 18 HOUR PERIOD, WITH WIDESPREAD 4-7 INCH
TOTALS, WITH ISOLATED MAX AMOUNTS OF 9"+. EXPECT A WELL DEFINED
SOUTH TO NORTH PRECIP BAND IN THIS ANOMALOUS MOISTURE FLUX AXIS
WITH HOURLY RAINFALL RATES OF 0.50-1.5"+ LIKELY. THE POTENTIAL FOR
HIGH HOURLY RAINFALL TOTALS WILL ACCENTUATE RUNOFF OVER SOIL THAT
IS RELATIVELY DRY AND OR OVER RECENT BURN SCAR AREAS. THIS WILL
POSE A SIGNIFICANT THREAT OF FLASH FLOODING, LANDSLIDES, ROCK FALLS
AND MUD SLIDES. THE GREATER THAN NORMAL TRAVEL DURING THE
CHRISTMAS HOLIDAY WILL LIKELY EXPOSE A POTENTIALLY LARGER NUMBER OF
PEOPLE TO THESE LIFT THREATENING HAZARDS AND WAS TAKEN INTO
CONSIDERATION WITH THE UPGRADE TO HIGH RISK.
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/ORAVEC
DAY 3
VALID 12Z THU DEC 25 2025 - 12Z FRI DEC 26 2025
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN CALIFORNIA...
..2030Z UPDATE
IN COORDINATION WITH LOX/OXNARD, CA FORECAST OFFICE, A MODERATE
RISK UPGRADE WAS INTRODUCED WITH THIS UPDATE FOR MUCH OF THE COAST
FROM SAN LUIS OBISPO THROUGH MUCH OF THE TRANSVERSE RANGES NORTH OF
L.A. EXPECT MUCH OF THE HEAVIEST RAIN TO OCCUR ON D2/WEDNESDAY, BUT
BY CHRISTMAS DAY, THERE WILL STILL BE ONE MORE FRONT THAT WILL
CROSS THE AREA FROM WEST TO EAST, WHICH WILL RESULT IN ANOTHER
ROUND OF HEAVY RAINFALL IN SOUTHERN CALIFORNIA. SINCE THIS NEXT
FRONT WILL ARRIVE SOON AFTER THE HEAVIEST RAINS FROM WEDNESDAY,
EXPECT THERE WILL BE OUTSIZED IMPACTS FROM THE HEAVY RAINFALL
ASSOCIATED THEREWITH. SHOWERS AND MAYBE EVEN A FEW THUNDERSTORMS
WILL IMPACT MUCH OF COASTAL SOUTHERN CALIFORNIA AND THE ADJACENT
MOUNTAINS. WHILE EACH ONE WILL BE FAST MOVING, THE RIVERS AND
STREAMS ALL ACROSS SOUTHERN CALIFORNIA WILL LIKELY ALREADY BE
FLOODING, SO ANY ADDITIONAL RAINFALL, ESPECIALLY WHERE HEAVY, WILL
MAKE ONGOING FLOODING WORSE.
ELSEWHERE, FEW CHANGES WERE NEEDED. ONSHORE FLOW AND PERIODS OF
HEAVY RAIN WILL IMPACTS MUCH OF CENTRAL AND COASTAL NORTHERN
CALIFORNIA ON CHRISTMAS DAY. SNOW LEVELS WILL COME DOWN SUCH THAT
MUCH OF THE SIERRA NEVADA WILL SEE FEET OF NEW SNOW. THIS SHOULD
KEEP DOWNSTREAM LOWER-ELEVATION FLOODING A BIT BETTER SINCE THIS
SNOW WILL NOT CONTRIBUTE TO THOSE HIGHER RIVER LEVELS.
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
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