979
FXUS66 KEKA 132227
AFDEKA
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE EUREKA CA
227 PM PST SAT DEC 13 2025
SYNOPSIS
HIGH PRESSURE DOMINATES THE WEEKEND WITH UNSEASONABLY WARM
AFTERNOONS AND COASTAL FOG. A PATTERN SHIFT BEGINS MONDAY AS A
PACIFIC FRONTAL SYSTEM INTRODUCES A WET AND BREEZY PERIOD THROUGH
MID-WEEK.
DISCUSSION
KEY MESSAGES:
* WARM AND CALM WEEKEND: UNSEASONABLY WARM TEMPERATURES AND DRY
CONDITIONS WILL PERSIST THROUGH SUNDAY, THOUGH COASTAL AREAS AND
VALLEYS WILL SEE NIGHT AND MORNING DENSE FOG.
* PATTERN CHANGE MONDAY: A WEAK-TO-MODERATE ATMOSPHERIC RIVER WILL
IMPACT THE NORTH COAST MONDAY THROUGH WEDNESDAY, BRINGING
BENEFICIAL RAIN AND BREEZY SOUTHERLY WINDS; SNOW LEVELS WILL
REMAIN VERY HIGH (ABOVE 6,000 FEET).
* STRONGER STORM POTENTIAL LATE WEEK: LONG-RANGE ENSEMBLES SIGNAL
THE POTENTIAL FOR A MORE SIGNIFICANT STORM SYSTEM LATE NEXT WEEK
(DEC 19-21), WITH FORECAST CONFIDENCE REGARDING IMPACTS
INCREASING, THOUGH SOME MODEL ENSEMBLE MEMBER SPREAD STILL EXISTS.
SCIENTIFIC RATIONALE & SYNOPTIC SETUP:
BROAD UPPER-LEVEL RIDGING CURRENTLY ANCHORED OVER THE GREAT BASIN IS
PROMOTING GENTLE OFFSHORE FLOW AND SUBSIDENCE, SUPPRESSING THE
MARINE LAYER TO A VERY SHALLOW DEPTH. THIS SETUP IS DRIVING 500MB
HEIGHTS WELL ABOVE CLIMATOLOGICAL MEANS, RESULTING IN THE
UNSEASONABLY WARM TEMPERATURES OBSERVED ACROSS THE INTERIOR.
HOWEVER, GLOBAL MODELS ARE IN EXCELLENT AGREEMENT REGARDING THE
BREAKDOWN OF THIS RIDGE BY LATE SUNDAY AS A LONGWAVE TROUGH OVER THE
NORTHEAST PACIFIC BEGINS TO APPROACH THE COAST. THIS TRANSITION
MARKS THE ONSET OF A PROGRESSIVE, WET PATTERN.
PROBABILISTICALLY-INFORMED DETAILS:
FOG/CLOUDS:
VISIBLE SATELLITE IMAGERY SHOWS A CLASSIC "SOUTHERLY SURGE" OF
STRATUS HUGGING THE IMMEDIATE COAST. RAPID ENSEMBLE FORECAST SYSTEM
(REFS) SUGGESTS A 40-70% PROBABILITY OF VISIBILITIES DROPPING BELOW
1/2 MILE ALONG THE REDWOOD COAST AND IN THE EEL RIVER VALLEY
OVERNIGHT INTO SUNDAY MORNING.
RAINFALL (MONDAY - WEDNESDAY):
CONFIDENCE IS HIGH (80%) THAT THE RIDGE WILL BE DISPLACED BY A
MOISTURE PLUME CHARACTERIZED BY INTEGRATED VAPOR TRANSPORT (IVT)
VALUES CONSISTENT WITH A WEAK-TO-MODERATE ATMOSPHERIC RIVER. KEPT
LOW CHANCE OF THUNDER FOR PARTS OF THE AREA MONDAY AS THE NBM V4.0
AND TO SOME EXTENT, THE NBM V5.0, INDICATE THIS, AND PERHAPS INTO
TUESDAY NIGHT AS WELL, ALTHOUGH THIS WAS HELD BACK FROM THE CURRENT
DETERMINISTIC FORECAST.
* TIMING: PRECIPITATION ONSET IS EXPECTED LATE MONDAY AFTERNOON FOR
DEL NORTE COUNTY, SPREADING SOUTH TO CAPE MENDOCINO BY MONDAY
EVENING.
* AMOUNTS: THE NATIONAL BLEND OF MODELS (NBM) SHOWS A TIGHT
CLUSTERING OF SOLUTIONS FOR THE NORTH COAST, FORECASTING 1.50 TO
3 INCHES OF RAIN FOR THE SMITH AND KLAMATH BASINS THROUGH
WEDNESDAY. FOR THE IMMEDIATE EUREKA/ARCATA AREA, THE PROBABILITY
OF EXCEEDING 1.00 INCH OF RAIN IN 48 HOURS IS APPROXIMATELY 75%.
* SOUTHERN EXTENT: THERE IS A SHARP GRADIENT SOUTH OF CAPE
MENDOCINO. LAKE AND SOUTHERN MENDOCINO COUNTIES HAVE ONLY A 50-70%
CHANCE OF EXCEEDING 0.50 INCHES THROUGH WEDNESDAY, AS THE BEST
DYNAMICS LIFT NORTH.
* IMPACTS: GIVEN THE ANTECEDENT DRY CONDITIONS, THE WPC EXCESSIVE
RAINFALL OUTLOOK (ERO) REMAINS LOW, WITH 12Z TUESDAY THROUGH 12Z
WEDNESDAY EXCESSIVE RAINFALL OUTLOOK IN THE MARGINAL CATEGORY (AT
LEAST 5%)AND LESS FOR WEDNESDAY. THE FLASH FLOOD THRESHOLD
PROBABILITIES ARE CURRENTLY LOW TO NEAR ZERO.
WIND:
AS THE COLD FRONT APPROACHES, THE PRESSURE GRADIENT WILL TIGHTEN.
NBM PROBABILISTIC GUIDANCE HIGHLIGHTS A 40-70% CHANCE OF SOUTH WIND
GUSTS (USING 24 HR WIND GUST PROBABILITIES) EXCEEDING 35 MPH ALONG
THE EXPOSED COASTAL HEADLANDS AND RIDGES OF DEL NORTE AND HUMBOLDT
COUNTIES MONDAY NIGHT AND MORE SO ON TUESDAY NIGHT. THURSDAY EVENING
WILL PROBABLY BE THE WINDIEST WITH A 70-90% CHANCE OF WIND GUSTS
GREATER THAN 35 MPH. HOWEVER, THE PROBABILITY OF DAMAGING WIND GUSTS
(>58 MPH) REMAINS LOW, GENERALLY UNDER 15%.
SNOW LEVELS:
THIS IS A WARM SYSTEM. DETERMINATION OF THERMAL PROFILES INDICATES
500MB HEIGHTS AND 850MB TEMPS THAT WILL KEEP SNOW LEVELS GENERALLY
WELL ABOVE PASS LEVELS, LIKELY HOVERING BETWEEN 6,000 AND 7,000
FEET, DIPPING TO 4,500 TO 6,000 FEET WEDNESDAY NIGHT. COLDER AIR
MOVES IN THURSDAY NIGHT INTO FRIDAY NIGHT AND SATURDAY NEXT WEEK
WITH FORECAST SNOW LEVELS OF 3,500 TO 4,500 FEET. SNOW ACCUMULATION
CHANCES ARE LOW (20%) TO NEAR ZERO WEDNESDAY NIGHT AT HIGHWAY PASS
LEVELS (BERRY SUMMIT, COLLIER TUNNEL, RIDGEWOOD SUMMIT), BUT BY
THURSDAY NIGHT, A NEAR 40% CHANCE OF AN INCH OF SNOWFALL AT SCOTT’S
MOUNTAIN PASS ON HIGHWAY 3.
LONG TERM (THURSDAY - WEEKEND):
LOOKING TOWARD THE END OF NEXT WEEK, THE 500MB CLUSTER ANALYSIS
STILL REVEALS SOME DIVERGENCE OF SOLUTIONS, BUT OVERALL, CONFIDENCE
IS INCREASING FOR NEXT WEEKEND TO BE RATHER WET AND WINDY.
* APPROX. 80% OF ENSEMBLE MEMBERS: SHOWS A DEEP TROUGH DIGGING
OFFSHORE WITH SOMEWHAT VARYING DEGREES OF EASTWARD PROGRESSION,
WHICH WOULD DIRECT A STRONG AR WITH HEAVY RAIN (2-6 INCHES) AND
POTENTIAL HIGH WINDS INTO NW CALIFORNIA.
* APPROX. 20% OF ENSEMBLE MEMBERS (MAINLY FROM THE ECMWF MEMBERS):
MAINTAINS A FLATTER FLOW OR BROADLY RIDGING, KEEPING THE HEAVIEST
PRECIP INTO OREGON/WASHINGTON. /HEINLEIN
AVIATION...(18Z TAFS)
AFTER THE PATCHY YET DENSE GROUND FOG
DISSIPATED THIS MORNING, MOSTLY VFR CONDITIONS BY 17Z AT KACV BUT
THIS COULD CHANGE THIS AFTERNOON WITH SOUTHERLY WINDS PUSHING CLOUDS
INTO THE TERMINALS. BROKEN TO OVERCASTS CLOUD GROUPS AND CEILINGS
BELOW 500FT AS OF 2130Z FOR BOTH KCEC AND KACV RESPECTIVELY,
POSSIBLY SIGNALING THE CONDITIONS TO COME. MODEL GUIDANCE SUGGEST
CEILINGS BELOW 500FT UNTIL SUNDAY AT 15Z-16Z FOR KCEC, WITH A LESS
PESSIMISTIC OUTLOOK FOR KACV UNTIL LATER TONIGHT AT 06-07Z WITH LIFR
LOW CEILINGS SETTING IN. THIS COULD MEAN THAT KACV WILL BE IN AND
OUT OF LIFR/IFR WITH OBSERVATIONS ALREADY IN LIFR AND SATELLITES
SHOWING GAPS IN OPTICAL DEPTH WITH SWATHS OF LESS DENSE STRATUS
BREAKING UP THE OVC SKIES. OTHERWISE KUKI IS LIKELY TO HAVE
PREVAILING VFR CONDITIONS BUT WITH THE SOUTHERLY PUSH, THERE COULD
BE ADVECTED STRATUS LATE TONIGHT DESPITE NO DEVELOPMENT PRESENTLY AS
OF 22Z. /EYS
MARINE
SOUTHERLY WINDS HAVE TAKEN OVER THE PREVAILING CARDINAL
DIRECTION TODAY, WITH THE NORTHERN WATERS A FEW KNOTS HIGHER AT 10-
12KTS AND ARE EXPECTED TO BUILD BY SUNDAY LATE AFTERNOON. CONDITIONS
WILL BE JUST UNDER SC.Y, SMALL CRAFT ADVISORY, THROUGH THE REST OF
THE DAY AND BECOME A BIT MORE HAZARDOUS SUNDAY EVENING AS
SOUTHERLIES STRENGTHEN JUST UNDER GALE FORCE CONDITIONS. MODERATE TO
STRONG SOUTHERLY WINDS ARE LIKELY NORTH OF CAPE MENDOCINO, WITH THE
STRONGEST OVER THE NORTHERN OUTER WATERS. LIGHTER WINDS ARE EXPECTED
SOUTH OF CAPE MENDOCINO. WINDS WILL GRADUALLY DIMINISH IN THE WAKE
OF THE FRONT MONDAY EVENING. A MID-PERIOD WESTERLY SWELL AROUND 8 TO
10 FEET IS EXPECTED TO BUILD IN MONDAY NIGHT INTO EARLY TUESDAY
MORNING. /ZVS/EYS
EKA WATCHES/WARNINGS/ADVISORIES
CA...
NONE.
NORTHWEST CALIFORNIA COASTAL WATERS...
NONE.
NOTE: THE FULL FORECAST DISCUSSION IS NOW REGULARLY ISSUED BETWEEN
10 AM/PM AND 1 PM/AM. ADDITIONAL UPDATES WILL BE ISSUED AS NEEDED.
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