380
FOUS30 KWBC 010029
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
729 PM EST WED DEC 31 2025
DAY 1
VALID 01Z THU JAN 01 2026 - 12Z THU JAN 01 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN CALIFORNIA...
GENERALLY LIGHT RAIN HAS FALLEN THIS AFTERNOON ACROSS PORTIONS OF SOUTHERN
CALIFORNIA...AND THAT IS EXPECTED TO PERSIST A FEW MORE HOURS
BEFORE THERE IS AN INCREASE IN RAINFALL RATES WITH A CORRESPONDING
INCREASE IN THE RISK OF EXCESSIVE RAINFALL. THE 18Z SUITE OF
GLOBAL MODELS AND ENSEMBLE RUNS HAS REMAINED CONSISTENT WITH THE
BROAD IDEA ALTHOUGH THERE HAS CONTINUED TO BE MINOR SHIFTS IN THE
TIMING OF THE ARRIVAL OF INSTABILITY AND WHEN THE HIGHER RATES
ARRIVE. RATES IN THE 18Z HREF NEVER HAVE 1 HOUR RAINFALL EXCEEDING
FLASH FLOOD GUIDANCE PROBABILITIES BUT DO GENERATE SOME 3-HOUR QPF
EXCEEDANCE OF 3-HOUR FLASH FLOOD GUIDANCE IN THE 06Z TO 09Z TIME
FRAME WHICH CORRESPONDS WITH AN UPTICK IN INSTABILITY AND THE
ARRIVAL OF AN IVT PLUME. WITH THE QPF OVERLAPPING SOME OF THE AREAS
WHICH ARE STILL HYDROLOGICALLY SENSITIVE DUE TO THE MOST RECENT
EXCESSIVE RAINFALL EVENT...WILL MAINTAIN WHAT IS EFFECTIVELY A
HIGH-END SLIGHT RISK AREA. THE CHANGES MADE IN THE EARLIER DAY 1
UPDATE STILL LOOK GOOD AND NO NEW CHANGES WERE NEEDED. THE RISK OF
EXCESSIVE RAINFALL THEN CONTINUES BEYOND THE END OF THE DAY 1
PERIOD AT 01/12Z.
BANN
..16Z OUTLOOK UPDATE
THE ONGOING FORECAST IS LARGELY ON TRACK, WITH AREAS OF MODERATE TO
HEAVY RAINFALL EXPECTED ACROSS THE TRANSVERSE RANGES ESPECIALLY
AFTER 06Z TONIGHT.
THE ONLY MINOR CHANGE TO THE ONGOING FORECAST FOR THIS UPDATE WAS
TO BROADEN MARGINAL ACROSS PORTIONS OF SAN LUIS OBISPO AND SANTA
BARBARA COUNTIES, WHERE GREATER RAINFALL TOTALS (AROUND/OVER 1
INCH) COULD OCCUR ATOP SENSITIVE AREAS/BURN SCARS DURING THE
FORECAST PERIOD). ISOLATED AREAS OF EXCESSIVE RUNOFF CANNOT BE
RULED OUT IN THESE AREAS TODAY AND TONIGHT.
SEE THE PREVIOUS DISCUSSION BELOW FOR MORE FORECAST INFORMATION.
COOK
..PREVIOUS DISCUSSION
A SLOW-MOVING UPPER LEVEL LOW CENTERED WELL WEST OF THE CALIFORNIA
COAST WILL MOVE TOWARDS CALIFORNIA THROUGH THE DAY 1/NEW YEAR'S
PERIOD. THE UPPER LOW WILL REMAIN NEGATIVELY TILTED AS IT
APPROACHES THE COAST. THIS WILL ALLOW A SURFACE LOW, ALREADY FORMED
AT THE START OF THE PERIOD, TO MOVE NORTH ALONG THE COAST.
SOUTHERLY WINDS AHEAD OF THE LOW WILL DRIVE PLENTIFUL PACIFIC
MOISTURE INTO SOUTHERN CALIFORNIA. UPSLOPE INTO THE TRANSVERSE
RANGES WILL WRING OUT MUCH OF THAT MOISTURE, RESULTING IN MULTIPLE
INCHES OF RAINFALL, ESPECIALLY NEW YEAR'S NIGHT, INTO THE L.A.
BASIN AND ADJACENT TRANSVERSE RANGES. ONCE THE CENTER OF THE
SURFACE LOW MOVES NORTH OF THE BAY AREA, ITS TRAILING COLD FRONT
WILL MOVE INTO SOUTHERN CALIFORNIA. THIS WILL TURN THE FLOW TO OUT
OF THE SOUTHWEST, WHICH WILL FURTHER ENHANCE THE UPSLOPE FLOW INTO
THE TRANSVERSE RANGES, CAUSING HEAVIER RAINFALL WEDNESDAY NIGHT.
THE INHERITED SLIGHT RISK FOR MUCH OF THE L.A. BASIN AND HIGHER-END
SLIGHT FOR THE TRANSVERSE RANGES THEMSELVES WAS LEFT LARGELY
UNCHANGED WITH THIS UPDATE. THE FORECAST WEATHER PATTERN REMAINS
LARGELY UNCHANGED, WITH MORE PARALLEL SOUTHEASTERLY FLOW INTO THE
RANGES SUPPORTING MOSTLY LIGHT RAIN THROUGH MUCH OF NEW YEAR'S
EVE AND INTO THE EVENING, THEN THE RAIN TURNING HEAVIER AROUND THE
STROKE OF MIDNIGHT FOR THE L.A. BASIN. WHILE SOILS ACROSS SOUTHERN
CALIFORNIA HAVE DRIED OUT A FAIR BIT SINCE THE CHRISTMAS RAINFALL
EVENT, BOTH SOILS AND RIVERS ARE STILL RUNNING ABOVE NORMAL EVEN
SEVERAL DAYS LATER. THUS, THE RENEWAL OF HEAVY RAINFALL WEDNESDAY
NIGHT SHOULD QUICKLY REFILL ANY RIVERS, WITH MUCH OF THE LIGHTER
RAIN DURING THE DAY WEDNESDAY SATURATING THE SOILS IN TIME FOR THE
HEAVIER RAINS WEDNESDAY NIGHT TO LARGELY CONVERT TO RUNOFF. THUS,
THE SLIGHT RISK LOOKS TO BE IN GOOD SHAPE, WITH NO MAJOR CHANGES TO
RAINFALL AMOUNTS EXPECTED REQUIRING ANY FURTHER MAJOR CHANGES TO
THE EXCESSIVE RAINFALL FORECAST.
WEGMAN
DAY 2
VALID 12Z THU JAN 01 2026 - 12Z FRI JAN 02 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN CALIFORNIA...
..21Z OUTLOOK UPDATE
THE ONGOING FORECAST PHILOSOPHY IS ON TRACK AND NO CHANGES ARE
NEEDED. WHILE QPFS HAVE DECREASED SLIGHTLY ACROSS THE EASTERN
TRANSVERSE RANGES AND VICINITY, HEAVY RAINFALL REMAINS LIKELY TO
CAUSE ISSUES WITH EXCESSIVE RUNOFF, ROCK/MUDSLIDES, AND FLASH
FLOODING IN SEVERAL AREAS ESPECIALLY BETWEEN 12-18Z THURSDAY. SEE
THE PREVIOUS DISCUSSION BELOW FOR MORE INFORMATION.
COOK
..PREVIOUS DISCUSSION
HEAVY RAIN WILL BE ONGOING AT THE START OF THE PERIOD THURSDAY
MORNING ACROSS MUCH OF SOUTHERN CALIFORNIA. AS THE UPPER LEVEL
SHORTWAVE MOVES INTO THE COAST, THE WINDS WILL TURN OUT OF THE
WEST-SOUTHWEST, OR EXACTLY ORTHOGONAL TO THE TRANSVERSE RANGES.
THIS WILL MAXIMIZE THE UPSLOPE ENHANCEMENT OF THE RAINFALL JUST AS
THE MAXIMUM FORCING IS MOVING ASHORE. THROUGH 18Z, THIS PERIOD NEW
YEAR'S MORNING WILL BE WHEN THE HEAVIEST RAIN OF THE EVENT IS
EXPECTED INTO SOUTHERN CALIFORNIA. PWATS BETWEEN 1 AND 1.25 INCHES
WILL BE BETWEEN 3 AND 4 SIGMA ABOVE NORMAL FOR THIS TIME OF YEAR, A
SIGN OF HOW MUCH MOISTURE WILL BE AVAILABLE TO CONVERT INTO
RAINFALL AND RUNOFF FROM THE TRANSVERSE AND TO A LESSER EXTENT,
PENINSULAR RANGES. ONCE THE SHORTWAVE MOVES ASHORE AND INLAND BY
THURSDAY EVENING, THE COMBINATION OF WEAKENING OF BOTH UPPER LEVEL
AND SURFACE LOWS, AS WELL AS RAPID WEAKENING OF THE ONSHORE WEST-
SOUTHWESTERLY FLOW BEHIND THE LOW, WILL LEAD TO RAPID DIMINISHING
OF THE RAINFALL ACROSS SOUTHERN CALIFORNIA, SUCH THAT BY THURSDAY
EVENING, LITTLE IF ANY RAINFALL WILL BE LEFT.
THE SLIGHT RISK INHERITED IS LARGELY UNCHANGED BOTH FROM THE
PREVIOUS FORECAST AND FROM THE DAY 1/WEDNESDAY SLIGHT RISK AREA.
BOTH DAYS' SLIGHT RISKS WILL BE SPLITTING THE TWO HALVES OF THE
SAME EVENT, SO LITTLE SHOULD BE DIFFERENT. THERE IS A HIGHER-END
SLIGHT IN EFFECT FOR MOST OF THE TRANSVERSE RANGES FROM ABOUT
MALIBU EAST THROUGH SAN BERNARDINO. THIS WILL LARGELY BE FOR THE
MORNING HOURS, AS ONCE THE RAIN WEAKENS, THE FLOODING THREAT
ASSOCIATED WITH ADDITIONAL RAINFALL SHOULD ALSO RAPIDLY DIMINISH,
LIKELY RESULTING IN AN EARLY DOWNGRADE OF THE SLIGHT RISK THURSDAY
AFTERNOON.
FURTHER NORTH, ALONG THE TRACK OF THE AFOREMENTIONED UPPER LEVEL
SHORTWAVE, RAINFALL WILL PERSIST FOR MUCH LONGER INTO THE NORTHERN
COASTAL RANGES AND MUCH OF THE SIERRA NEVADA FOOTHILLS, ESPECIALLY
THOSE ALONG THE SACRAMENTO VALLEY. HOWEVER, THIS AREA HAS MORE
THOROUGHLY RECOVERED FROM THE CHRISTMAS RAINFALL EVENT AND ALSO IS
BETTER EQUIPPED NATURALLY TO HANDLE LONG DURATION RAINFALL. AS
SUCH, ONLY AN ISOLATED FLASH FLOODING THREAT, CONSISTENT WITH THE
UNCHANGED MARGINAL, IS EXPECTED.
WEGMAN
DAY 3
VALID 12Z FRI JAN 02 2026 - 12Z SAT JAN 03 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CALIFORNIA...
..21Z OUTLOOK UPDATE
THE ONGOING FORECAST PHILOSOPHY REMAINS ON TRACK AND NO CHANGES ARE
NEEDED TO THE OUTLOOK AT THIS TIME. MODELS REMAIN CONSISTENT IN
DEPICTING AREAS OF 1-3 INCHES OF RAINFALL ACROSS NORTHERN
CALIFORNIA AS ANOTHER LARGE MID-LEVEL WAVE APPROACHES THE REGION
LATE IN THE FORECAST PERIOD. A FEW INSTANCES OF FLOODING/FLASH
FLOODING AND LANDSLIDES ARE POSSIBLE.
SOME CONSIDERATION WAS GIVEN TO ADDING A MARGINAL RISK ACROSS
PORTIONS OF ALABAMA AND NORTHWESTERN GEORGIA AS SCATTERED
CONVECTION SHOULD TRAVERSE THOSE AREAS BETWEEN 00Z-12Z SATURDAY.
MODELS SEEM TO HINT AT POSSIBLE TRAINING OF CELLS DURING THAT
TIMEFRAME. CONCERNS ABOUT STORM SPEEDS, MARGINAL
INSTABILITY/MOISTURE, MESOSCALE UNCERTAINTY, AND ANTECEDENT DRYNESS
PRECLUDES ANY RISK AREAS/PROBABILITIES AT THIS TIME.
SEE THE PREVIOUS DISCUSSION BELOW FOR MORE INFORMATION.
COOK
..PREVIOUS DISCUSSION
LIGHT RAIN OVER MOST OF NORTHERN CALIFORNIA FROM ABOUT 06Z FRIDAY
THROUGH 18Z FRIDAY WILL END AS ANOTHER LARGE UPPER LEVEL LOW BEGINS
TO MAKE ITS WAY TOWARDS NORTHERN CALIFORNIA AND OREGON. THIS UPPER
LOW, WHILE BIGGER, WILL BE LESS AMPLIFIED/ELONGATED AS COMPARED
WITH THE LAST UPPER LOW. THAT SHOULD FOCUS MOST OF THE RAINFALL
INTO NORTHERN CALIFORNIA. FURTHER, THE SURFACE LOW WILL BE A
VERTICALLY STACKED ONE, WHOSE OCCLUDED FRONT HAS WRAPPED CLEAR
AROUND THE LOW CENTER. IT IS THIS OCCLUDED FRONT MAKING ITS WAY
ASHORE THAT WILL RESTART ANY HEAVIER RAIN INTO THE SOUTHWARD FACING
SLOPES OF THE COASTAL RANGES, KLAMATH, AND THE NORTHERN SIERRA
NEVADA FOOTHILLS. RAINFALL TOTALS WILL GENERALLY FALL IN THE 2-3
INCH RANGE IN MOST OF THE MOUNTAINS, WITH AROUND AN INCH INTO THE
SACRAMENTO VALLEY. THIS AMOUNT OF RAIN SHOULD BE ENOUGH TO RAISE
RIVER LEVELS WITH PERHAPS AN ISOLATED INSTANCE OR TWO OF FLASH
FLOODING IN SOME OF THE FLASHIER CREEKS AND STREAMS, BUT OVERALL
IMPACTS SHOULD STILL BE MINIMAL, DESPITE SOME PRECIPITATION
OCCURRING PRIOR TO THIS ON DAY 2. MUCH OF NORTHERN CALIFORNIA IS
WELL-SUITED TO HANDLE RAINFALL AMOUNTS SUCH AS THESE, SO WIDESPREAD
IMPACTS ARE NOT EXPECTED. THE AREA'S FORECAST WILL BE MONITORED
FOR ANY POTENTIAL SLIGHT RISK UPGRADES NEEDED.
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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