468
FXUS66 KSGX 202025
AFDSGX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN DIEGO CA
1225 PM PST SAT DEC 20 2025
SYNOPSIS
DRY CONDITIONS WITH ABOVE AVERAGE TEMPERATURES INLAND WILL CONTINUE
INTO EARLY THIS WEEK. PERIODS OF PATCHY FOG EXPECTED NEAR THE COAST
AND VALLEYS THROUGH SUNDAY. AN ATMOSPHERIC RIVER WILL BRING
WIDESPREAD HEAVY RAIN AND MOUNTAIN SNOW ABOVE 8000 FT LATE TUESDAY
INTO THURSDAY (CHRISTMAS DAY). ADDITIONALLY GUSTY SOUTHERLY WINDS
CAN BE EXPECTED ACROSS MUCH OF THE AREA, INCLUDING THE COAST. THERE
ARE ADDITIONAL CHANCES FOR SHOWERS FRIDAY AND NEXT SATURDAY BUT
CHANCES ARE LOWER.
DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
KEY POINTS:
* WIDESPREAD MODERATE TO HEAVY RAINFALL CHRISTMAS EVE AND CHRISTMAS
DAY WITH THE POTENTIAL FOR ROADWAY FLOODING. RAPID RISES IN SMALL
STREAMS AND INCREASED FLOW IN MAIN STEM RIVERS EXPECTED.
* SNOW LEVELS ARE FORECAST TO REMAIN ABOVE 8000 FT THROUGH CHRISTMAS
DAY, INCREASING POTENTIAL FOR ROCK AND MUDSLIDES ALONG MOUNTAIN
HIGHWAYS, AND DEBRIS FLOWS IN AND BELOW BURN SCARS.
* GUSTY SOUTHERLY WINDS ARE EXPECTED ACROSS THE AREA WITH 80-90%
CHANCE OF WIND GUSTS EXCEEDING 35 MPH ALONG THE COAST TUESDAY
NIGHT INTO WEDNESDAY. MAKE SURE TO SECURE OUTDOOR FURNITURE. TREE
DAMAGE IS POSSIBLE.
* ADDITIONAL CHANCES OF SHOWERS FRIDAY AND SATURDAY WITH LOWER SNOW
LEVELS, CREATING SLICK CONDITIONS ON MOUNTAIN HIGHWAYS.
AT 12 PM A COASTAL EDDY WAS SPINNING OVER THE COASTAL WATERS, WHICH
HAS HELPED MAINTAIN FAIRLY WIDESPREAD LOW CLOUD COVERAGE AT THE
COAST AND IN PORTIONS OF THE WESTERN VALLEYS. WITH MINIMAL CHANGES
IN THE WEATHER PATTERN EXPECTED INTO SUNDAY THE MARINE LAYER SHOULD
REMAIN DEEP ENOUGH FOR AREAS OF DENSE FOG TO DEVELOP ON HIGHER
COASTAL TERRAIN TONIGHT INTO SUNDAY MORNING. HIGHS WILL REMAIN 5 TO
10 DEGREES ABOVE AVERAGE FOR INLAND LOCATIONS THROUGH MONDAY, WITH
HIGHS ALONG THE COAST NEAR AVERAGE.
A SIGNIFICANT PATTERN CHANGE IS EXPECTED BY THE MIDDLE OF THE WEEK.
AN UPPER LEVEL TROUGH AND ASSOCIATED ATMOSPHERIC RIVER WILL AMPLIFY
OFF THE WEST COAST. A SHORT WAVE EMBEDDED IN THE TROUGH LIFTS INTO
SOUTHERN CALIFORNIA TUESDAY NIGHT INTO WEDNESDAY, USHERING IN THE
FIRST SURGE OF MOISTURE AND WIDESPREAD MODERATE TO HEAVY
PRECIPITATION. CURRENT FORECASTS FROM CW3E WRF MODELS ARE SHOWING
IVT FORECASTS AS HIGH AS 600-700 KG/M/S AIMED AT SOUTHERN CALIFORNIA
TUESDAY NIGHT THROUGH MUCH OF THE DAY WEDNESDAY. EVEN WITH THE
DIFFERENCES BETWEEN GLOBAL MODELS AND UNCERTAINTY IN EXACT DETAILS,
THERE IS INCREASING CERTAINTY THAT PERIODS OF MODERATE TO LOCALLY
HEAVY RAIN WILL OCCUR BETWEEN THE OVERNIGHT HOURS OF DECEMBER 23
THROUGH DECEMBER 25. SNOW LEVELS ARE EXPECTED TO STAY ABOVE 7500 FT
THROUGH DECEMBER 25, WITH SNOW LEVELS ABOVE 8500 FT AT THE ONSET OF
THE PRECIPITATION. THIS INCREASES THE CHANCES FOR FLOODING, DEBRIS
FLOWS, AND MUD SLIDES IN THE MOUNTAINS, ESPECIALLY NEAR BURN SCARS.
ADDITIONALLY IMPACTS FROM RIVER FLOODING, ESPECIALLY NEAR THE SAN
DIEGO RIVER, ARE POSSIBLE. SEE THE HYDROLOGY SECTION BELOW FOR
ADDITIONAL INFORMATION. GUSTY SOUTH WINDS ARE ALSO EXPECTED ACROSS
THE COASTAL AREAS, VALLEYS, AND THE SAN BERNARDINO MOUNTAINS INTO
THE ADJACENT DESERT FOOTHILLS LIKELY STARTING LATE TUESDAY
NIGHT/EARLY WEDNESDAY CONTINUING THROUGH WEDNESDAY AFTERNOON.
FROM THURSDAY ONWARDS, THERE IS INCREASING UNCERTAINTY IN THE
EVOLUTION OF THE UPPER LEVEL PATTERN, IN BOTH THE FORWARD
PROGRESSION OF THE LOW AND HOW FAR SOUTH IT WILL DIG. ABOUT 45
PERCENT OF ENSEMBLE SOLUTIONS DIG THE TROUGH OF LOW PRESSURE SOUTH
WITH THE AXIS OFF TO OUR WEST WHICH WOULD CONTINUE TO PULL MOISTURE
INTO THE AREA AND CONTINUE THE WETTER PATTERN FOR SOUTHERN
CALIFORNIA FOR THURSDAY AND FRIDAY. BY SATURDAY, 60 PERCENT OF
SOLUTIONS STILL HAVE THE TROUGH IN POSITION TO CONTINUE BRINGING
PRECIPITATION TO SOUTHERN CALIFORNIA. CURRENT FORECAST MAINTAINS
CHANCES OF PRECIPITATION THROUGH THE END OF THE FORECAST PERIOD
(NEXT SATURDAY). THE CLOSED LOW THAT MAY BRING ADDITIONAL
PRECIPITATION COULD BE COLDER, WHICH WOULD INCREASE CHANCES OF SNOW
IN THE MOUNTAINS ALTHOUGH UNCERTAINTY IN WHAT THE SNOW LEVELS WOULD
BE REMAINS QUITE HIGH. ADDITIONALLY, WESTERLY WINDS COULD INCREASE
DEPENDING ON THE FINAL TRACK OF THE LOW. REGARDLESS, MULTIPLE DAYS
OF PRECIPITATION, WHICH WILL LIKELY BE HEAVY AT TIMES, IS BRINGING A
RISK OF FLOODING AND DIFFICULT TRAVEL CONDITIONS AROUND THE HOLIDAY.
AVIATION
201730Z...COAST/WESTERN VALLEYS...LOW CLOUDS AND FOG THAT SPREAD
INTO INLAND VALLEYS THIS MORNING HAVE LIFTED SOME AFTER SUNRISE,
THOUGH WIDESPREAD CIGS REMAIN. GRADUAL VIS IMPROVEMENTS SEEN THIS
MORNING, THOUGH LOCALIZED VIS BELOW 1SM REMAINS WITHIN INLAND
VALLEYS WHILE LOCATIONS NEAR SEA LEVEL HAVE SEEN VIS RISE TO
GENERALLY 6SM AND HIGHER. CLOUD BASES ARE AROUND 900-1100FT MSL AND
WILL CONTINUE TO RISE 100-200FT BEFORE SCATTERING OUT TO THE BEACHES
BY 19-20Z. LOW CLOUDS PUSH BACK ASHORE AFTER 02Z SUNDAY, EVENTUALLY
FILLING BACK IN TO 15-20 MILES INLAND, SIMILAR EXTENT TO THIS
MORNING. BASES INITIALLY NEAR 800-1000FT MSL MAY SETTLE CLOSER TO
500-700FT MSL OVERNIGHT WITH A WEAKER COASTAL EDDY, THOUGH
CONFIDENCE IN THESE LOWER CLOUD BASES IS LOW. IF BASES REMAIN NEAR
800-1000FT MSL, VIS RESTRICTIONS DOWN TO 0-3 SM FOR INLAND VALLEYS
AND HIGHER INLAND TERRAIN, WITH 4-6 SM ALONG THE COASTS AND COASTAL
MESAS. CLOUDS SCATTERING BACK TO THE COASTS 17-19Z SUNDAY MORNING.
OTHERWISE...FEW-SCT HIGH CLOUDS WITH UNRESTRICTED VIS THROUGH
SATURDAY EVENING. BKN-OVC HIGH CLOUDS ACROSS THE DESERTS.
MARINE
NO HAZARDOUS MARINE CONDITIONS ARE EXPECTED THROUGH TUESDAY MORNING.
STRENGTHENING SOUTH WINDS LATE TUESDAY IN ADVANCE OF A PACIFIC
STORM. WINDS AND SEAS ARE EXPECTED TO INCREASE AS A RESULT, WITH
SEAS NEAR 8-10 FT AND WIND GUSTS UPWARD OF 30-35 KTS.
HYDROLOGY
FOR LATE TUESDAY THROUGH FRIDAY MORNING, THE CHANCE FOR 5 INCHES OR
MORE OF RAINFALL FOR THE WESTERN PORTIONS OF THE TO SAN BERNARDINO
COUNTY MOUNTAINS(MT BALDY TO I-15) ARE 85 PERCENT WITH AN 75 PERCENT
CHANCE FOR 6 INCHES OR MORE, AND THE CHANCE FOR 8 INCHES OR MORE OF
RAINFALL HAS INCREASED TO 50 PERCENT. STRONG SOUTHERLY FLOW ALOFT
(30-50 KT) WILL ALLOW FOR OROGRAPHIC ENHANCEMENT OF RAINFALL ON THE
COASTAL SLOPES OF THE SAN BERNARDINO MOUNTAINS. IN THIS TYPE OF
WEATHER PATTERN, THIS AREA TENDS TO RECEIVE THE MOST RAINFALL. FOR
THE EASTERN PART OF THE SAN BERNARDINO COUNTY MOUNTAINS (AREAS EAST
OF THE CAJON PASS) CHANCES OF EXCEEDING 5 INCHES ARE 70 PERCENT,
EXCEEDING 6 INCHES ARE 60 PERCENT AND EXCEEDING 8 INCHES ARE 15
PERCENT.
PROBABILITIES FOR SEEING RAINFALL TOTALS OVER 2 INCHES TUESDAY NIGHT
THROUGH THURSDAY NIGHT ARE LISTED BELOW:
- OC/SANTA ANA MOUNTAINS: 90-95%
- INLAND EMPIRE: 60-90%, HIGHEST WEST
- RIVERSIDE/SD CO MTS: 60-80%
- SD COAST/VALLEYS: 60-80%, HIGHEST NORTHWEST SD CO
- DESERTS: 15-25% (LOWER DESERTS), 40-55% (HIGH DESERT)
PROBABILITIES FOR SEEING RAINFALL TOTALS OVER 3 INCHES TUESDAY
THROUGH THURSDAY NIGHT ARE LISTED BELOW:
- OC/SANTA ANA MOUNTAINS: 70-85%
- INLAND EMPIRE: 40-85%, HIGHEST WEST
- RIVERSIDE/SD CO MTS: 40-70%
- SD COAST/VALLEYS: 30-60%, LOCALLY HIGHER IN NORTHWEST SD CO
- DESERTS: 10-20% (LOWER DESERTS), 20-30% CHANCE (HIGH DESERT)
CURRENT DETERMINISTIC FORECAST FOR TUESDAY THROUGH FRIDAY MORNING:
- ORANGE COUNTY: 3.5 - 4.5", HIGHEST NORTH
- INLAND EMPIRE: 3-4", HIGHEST WEST
- SAN DIEGO COUNTY COASTS/VALLEYS: 2-3", HIGHEST NORTH
- SAN BERNARDINO COUNTY MOUNTAINS: 4-10", HIGHEST WEST
- SANTA ANA MOUNTAINS: 4-6"
- RIVERSIDE/SAN DIEGO COUNTY MOUNTAINS: 2-4.5"
- HIGH DESERT: 1.5-3", HIGHEST WEST
- LOW DESERTS: 0.9-1.5", LOCALLY 2.5" NEAR THE SAN GORGONIO PASS
MOST OF THE PRECIPITATION ACROSS SOUTHWESTERN CALIFORNIA THROUGH
CHRISTMAS MORNING IS EXPECTED TO FALL AS RAIN WITH SNOW LEVELS
REMAINING ABOVE 8000 FEET. BY LATE EVENING ON THE 25TH, SNOW LEVELS
MAY DROP TO 7500 FT, BUT PRECIPITATION IS EXPECTED TO BE LESS
WIDESPREAD. SNOW LEVELS COULD DROP EVEN LOWER FOR FRIDAY AND
SATURDAY, BUT EXACT LEVELS AND LOCATIONS REMAINS UNCERTAIN.
THE SAN DIEGO RIVER AT FASHION VALLEY IS CURRENTLY EXPECTED TO REACH
ACTION/MONITOR STAGE BY EARLY THURSDAY MORNING. BASED ON CURRENT
FORECAST, THE RIVER IS EXPECTED TO PEAK AT 7.8 FT.
SKYWARN
SKYWARN ACTIVATION IS NOT REQUESTED. HOWEVER WEATHER SPOTTERS ARE
ENCOURAGED TO REPORT SIGNIFICANT WEATHER CONDITIONS.
SGX WATCHES/WARNINGS/ADVISORIES
CA...NONE.
PZ...NONE.
PUBLIC...CO
AVIATION/MARINE...MUNYAN
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