908
FXUS66 KSGX 201738
AFDSGX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN DIEGO CA
938 AM PST SAT DEC 20 2025
SYNOPSIS
WARM AND DRY WEATHER THROUGH MONDAY WITH HIGH TEMPERATURES
REMAINING 10-15 DEGREES ABOVE NORMAL INLAND. PATCHY FOG WILL
CONTINUE NEAR THE COAST AND WESTERN VALLEYS INTO EARLY NEXT WEEK.
A LOW PRESSURE SYSTEM AND ASSOCIATED WEAK TO MODERATE ATMOSPHERIC
RIVER WILL BRING WIDESPREAD HEAVY RAIN AND HIGH ELEVATION MOUNTAIN
SNOW LATE TUESDAY INTO CHRISTMAS DAY. SHOWER CHANCES LOWER TO
AROUND 50-60% ON FRIDAY AND 20-30% ON SATURDAY.
DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
KEY POINTS:
* HIGH TEMPERATURES AS MUCH AS 10-15 DEGREES ABOVE NORMAL INLAND
THROUGH MONDAY.
* REDUCED VISIBILITY IN FOG FOR THE COASTAL AREAS AND WESTERN
VALLEYS EACH NIGHT AND MORNING THROUGH TUESDAY COULD RESULT IN
LOCALLY HAZARDOUS TRAVEL CONDITIONS.
* WIDESPREAD MODERATE TO HEAVY RAINFALL AND GUSTY SOUTHERLY WINDS
CHRISTMAS EVE AND CHRISTMAS DAY WITH THE POTENTIAL FOR ROADWAY
FLOODING, ROCK AND MUDSLIDES ALONG MOUNTAIN HIGHWAYS, AND DEBRIS
FLOWS IN AND BELOW RECENT BURN SCARS. RAPID RISES IN SMALL
STREAMS AND INCREASED FLOW IN MAIN STEM RIVERS.
* ADDITIONAL CHANCES OF SHOWERS FRIDAY AND SATURDAY WITH LOWER
SNOW LEVELS, CREATING SLICK CONDITIONS ON MOUNTAIN HIGHWAYS.
MARINE LAYER LOW CLOUDS HAVE SPREAD INTO THE WESTERN VALLEYS THIS
MORNING WITH ONLY ISOLATED DENSE FOG BEING REPORTED IN THE
VALLEYS. OTHERWISE A FEW HIGH CLOUDS ARE MOVING OVER THE REGION.
LOW CLOUDS AND FOG WILL CLEAR INLAND BY LATE MORNING, BUT COULD
LINGER ALONG THE COAST THROUGH THE AFTERNOON.
NOT MUCH CHANGE IN THE WEATHER OVER THE NEXT THREE DAYS AS WE
REMAIN UNDER MOSTLY ZONAL FLOW. AN UPPER LEVEL HIGH TO OUR SOUTH
SLOWLY SHIFTS EAST THROUGH MONDAY, BUT THE END RESULT IS LITTLE
CHANGE IN HEIGHTS AND FLOW ALOFT BECOMING SLIGHTLY MORE
SOUTHWESTERLY. A PERSISTENT MARINE LAYER WITH NIGHT AND MORNING
LOW CLOUDS AND FOG FOR THE COASTAL AREAS AND WESTERN VALLEYS WILL
HELP MODERATE HIGH TEMPERATURES IN THOSE AREAS, WHILE INLAND
AREAS REMAIN WARM. HIGHS ALONG THE COAST WILL BE NEAR NORMAL,
AROUND 5-10 DEGREES ABOVE NORMAL FOR THE VALLEYS AND LOW DESERTS,
AND AS MUCH AS 15 DEGREES ABOVE NORMAL FOR THE LOWER ELEVATION
MOUNTAINS AND HIGH DESERTS. OTHERWISE OCCASIONAL HIGH CLOUDS WILL
PASS OVERHEAD.
A BIG SHIFT IN THE WEATHER PATTERN WILL OCCUR BY MIDWEEK. AN
UPPER LEVEL TROUGH AND ASSOCIATED ATMOSPHERIC RIVER WILL AMPLIFY
OFF THE WEST COAST. A SHORT WAVE EMBEDDED IN THE TROUGH LIFTS INTO
SO CAL TUESDAY NIGHT, 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. THERE ARE STILL DIFFERENCES IN IVT FROM THE
GLOBAL ENSEMBLES, WITH THE EPS HAVING A 60% CHANCE OF A MODERATE
AR AND A 10% CHANCE OF A STRONG AR IN OUR FORECAST AREA, WHILE THE
GFS PROBABILITIES ARE A BIT LOWER - AROUND 30% AND 0%
RESPECTIVELY. EVEN WITHIN EACH ENSEMBLE SYSTEM, THE SPREAD IN
RAINFALL AMOUNTS REMAINS LARGE - ON THE ORDER OF INCHES EVEN IN
THE COASTAL AREAS, THOUGH THE VAST MAJORITY OF MEMBERS ARE ON THE
WETTER END. THIS AMPLE MOISTURE COMBINED WITH STRONG SOUTHERLY
MID-LEVEL FLOW OF 30-40 KT WILL LEAD TO STRONG OROGRAPHIC LIFT AND
CONSIDERABLY HIGHER RAINFALL AMOUNTS ALONG THE SOUTH SLOPES OF THE
SAN BERNARDINO MOUNTAINS ON CHRISTMAS EVE. SEE THE HYDROLOGY
SECTION FOR POTENTIAL RAINFALL AMOUNTS. GUSTY SOUTH WINDS ARE
ALSO EXPECTED ACROSS THE COASTAL AREAS, VALLEYS, AND THE SAN
BERNARDINO MOUNTAINS INTO THE ADJACENT DESERT FOOTHILLS. THE SNOW
LEVEL WILL BE ABOVE 8000 FT THROUGH CHRISTMAS MORNING.
FOR CHRISTMAS DAY, ENSEMBLES BEGIN TO DIVERGE ON THE EVOLUTION OF
THE TROUGH AND THE TRAJECTORY OF THE NEXT SHORT WAVE. AROUND 30%
OF THE GLOBAL ENSEMBLE MEMBERS LIFT THE SHORT WAVE FURTHER NORTH
INTO CENTRAL CA, RESULTING IN LOWER PRECIPITATION AMOUNTS HERE,
41% DIG THE LOW FURTHER SOUTH WITH THE NEXT SURGE OF MOISTURE
AIMED AT SO CAL FOR HIGHER RAINFALL AMOUNTS, AND THE REMAINING 26%
SOMEWHERE IN BETWEEN. THEN AS WE HEAD INTO FRIDAY AND SATURDAY,
AROUND 57% OF THE MEMBERS HAVE THE TROUGH PROGRESSING EASTWARD
WITH PRECIPITATION COMING TO AN END SOONER, WHILE THE REMAINING
43% HAVE ANOTHER SHORT WAVE DIVING DOWN THE BACK SIDE OF THE
TROUGH, CAUSING IT TO AMPLIFY FURTHER AND SLOWING THE EASTWARD
PROGRESSION FOR MORE PROLONGED CHANCES OF SHOWERS. 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 COULD BECOME HAZARDOUS TO SMALL CRAFT TUESDAY
NIGHT THROUGH WEDNESDAY.
SGX WATCHES/WARNINGS/ADVISORIES
CA...NONE.
PZ...NONE.
PUBLIC...CO
AVIATION/MARINE...MUNYAN
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