253
FXUS66 KMTR 211142
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
442 AM PDT THU MAY 21 2026
..NEW AVIATION, MARINE
KEY MESSAGES
UPDATED AT 1201 AM PDT THU MAY 21 2026
- DIURNALLY DRIVEN WINDS, WITH ONSHORE WINDS DURING THE AFTERNOON
AND EARLY EVENING
- WARM AND DRY WEATHER TODAY LEADS TO ELEVATED FIRE WEATHER
CONCERNS FOR THE INTERIOR BAY AREA AND CENTRAL COAST
- GRADUAL COOLING TREND BEGINS FRIDAY WITH SEASONAL TEMPERATURES
FOR THE UPCOMING WEEKEND
SHORT TERM
ISSUED AT 1201 AM PDT THU MAY 21 2026
(TODAY AND TONIGHT)
COASTAL STRATUS LINGERS ACROSS THE CENTRAL COAST AND HAS FILLED IN
ON THE WESTERN COAST OF SAN MATEO COUNTY. A DECAYING OTTER EDDY
WITHIN MONTEREY BAY COMBINED WITH A DRY SLOT PARALLELING THE BIG SUR
COAST IS MAKING THE STRATUS FORECAST SOMEWHAT UNCERTAIN WITH A
CHANCE THAT THE IMMEDIATE VICINITY OF MONTEREY REMAINS CLEAR
OVERNIGHT. CONFIDENCE IN INLAND STRATUS DEVELOPMENT, PARTICULARLY
AWAY FROM THE IMMEDIATE BAYSHORE OR SOUTH OF SALINAS, IS LOW.
STRATUS SHOULD GENERALLY CLEAR BACK TO THE IMMEDIATE COAST IN THE
HOURS AFTER SUNRISE.
THE LARGE SCALE PATTERN OVER CENTRAL CALIFORNIA IS A BALANCE BETWEEN
A RIDGE OVER THE NORTHEAST PACIFIC AND EDGING INTO THE PACIFIC
NORTHWEST AND A TROUGH LINGERING OVER THE ROCKY MOUNTAINS WITH THE
AXIS EXTENDING INTO SOUTHERN CALIFORNIA AND BAJA CALIFORNIA. THIS
PATTERN KEEPS THE UPPER LEVEL WINDS OFFSHORE, IF LIGHT, BUT AT THE
SURFACE, A THERMAL TROUGH OVER THE CENTRAL VALLEY KEEPS THE WINDS
GENERALLY ONSHORE WITH COASTAL BREEZES DEVELOPING ACROSS THE REGION
DURING THE AFTERNOON AND EVENING.
LOW TEMPERATURE THIS MORNING RANGE FROM THE UPPER 40S TO LOW 50S
NEAR THE COAST, LOWER TO MIDDLE 50S IN THE INLAND VALLEYS, AND THE
MIDDLE 50S TO LOWER 60S IN THE HIGHER ELEVATIONS. THE NBM MODEL
OUTPUT IS SLIGHTLY OVERSHOOTING THE HIGH TEMPERATURES, ESPECIALLY IN
THE IMMEDIATE COASTAL AREAS UNDER THE MARINE LAYER INFLUENCE, SO I
HAVE TAMPED DOWN TODAY'S COASTAL HIGHS TO THE UPPER 50S TO THE
MIDDLE 60S. THESE HIGH TEMPERATURES ARE LIABLE TO BE AN
UNDERESTIMATE OF AS MUCH AS 5 DEGREES OR SO IF STRATUS CLEARS OUT
EARLIER THAN THE CURRENT FORECAST, OR INDEED NEVER DEVELOPS AT ALL.
AWAY FROM THE IMMEDIATE COAST, TEMPERATURES REMAIN NEAR TO WELL
ABOVE THE SEASONAL AVERAGE, WITH HIGHS IN THE 70S TO THE LOWER 80S
NEAR THE BAYS, WHILE ACROSS THE INLAND VALLEYS, HIGH TEMPERATURES
RANGE FROM THE MIDDLE 80S TO THE MIDDLE 90S. THE CONTINUING HOT AND
DRY CONDITIONS WILL CONTRIBUTE TO ANOTHER DAY OF ELEVATED FIRE
WEATHER CONCERNS IN THE INTERIOR REGIONS, AND INDEED A COUPLE OF
GRASS FIRES WERE REPORTED ACROSS THE REGION, ONE IN CONTRA COSTA
COUNTY AND ONE IN SAN BENITO COUNTY. FOR THOSE LIVING OR VISITING
THOSE HOT AND DRY REGIONS, KEEP THE FOLLOWING IN MIND:
* PROPERLY DISPOSE OF CIGARETTES AND MATCHES
* DO NOT USE A LAWN MOWER ON WEEDS OR DRY GRASS
* AVOID USING EQUIPMENT THAT CREATES SPARKS
* MAKE SURE CAMPFIRES ARE COMPLETELY PUT OUT
* MAKE SURE NO VEHICLE PARTS DRAG ON THE GROUND, INCLUDING TOWING
CHAINS
* OBEY BURN BANS & ONLY BURN WITH A PERMIT
LONG TERM
ISSUED AT 1201 AM PDT THU MAY 21 2026
(FRIDAY THROUGH WEDNESDAY)
A GRADUAL COOLING TREND BEGINS ON FRIDAY WITH THE UPPER LEVEL RIDGE
RETREATING WESTWARD, ALLOWING FOR MORE ZONAL UPPER LEVEL FLOW ACROSS
THE WEST COAST AND A GRADUALLY THICKENING MARINE LAYER. BY SATURDAY,
TEMPERATURES SHOULD BE NEAR THE SEASONAL AVERAGE WITH HIGHS IN THE
MIDDLE 70S TO THE MIDDLE 80S INLAND. BY THE EARLY PART OF NEXT WEEK,
A COLD FRONT ASSOCIATED WITH A DEEP UPPER LEVEL LOW IS LIKELY TO
SWEEP THROUGH THE STATE, WHICH WOULD CAUSE HIGHS TO DROP INTO THE
70S ACROSS THE INLAND VALLEYS. CLOSE ANALYSIS OF THE ENSEMBLE MODEL
CLUSTERS REVEALS A SMALL, BUT SIGNIFICANT FRACTION OF THE ENSEMBLE
MEMBERS (AROUND 10-20%) KEEPING SOME LEVEL OF RIDGING OVER
CALIFORNIA. THERE IS EVEN A SLIGHT CHANCE OF LIGHT RAIN OR DRIZZLE
ASSOCIATED WITH AN INCOMING FRONT, BUT IT'S A LOW CONFIDENCE
FORECAST AT THIS POINT WITH AROUND A 20-30% PROBABILITY OF SEEING
ANY WATER FALLING OUT OF THE SKY, AND THAT IS TILTED TOWARDS THE
COASTAL REGIONS WHERE TOPOGRAPHY WOULD TEND TO ENHANCE THE RAINFALL
AMOUNTS. CPC EXTENDED GUIDANCE DOES SHOW A SLIGHT LEAN TOWARDS
PRECIPITATION TOTALS ABOVE SEASONAL AVERAGES INTO THE BEGINNING OF
JUNE, WITH THE CAVEAT THAT FOR THE LAST WEEKS OF MAY, JUST A
COUPLE HUNDREDTHS OF AN INCH WILL TIE OR EXCEED THE DAILY AVERAGE
TOTALS FOR DOWNTOWN SAN FRANCISCO.
AVIATION
(12Z TAFS)
ISSUED AT 436 AM PDT THU MAY 21 2026
SHALLOW MARINE LAYER DEVELOPED INITIALLY IN MONTEREY BAY BEFORE
SPREAD UP THE SALINAS VALLEY AND THEN NORTHWARD ALONG THE
COASTLINE. GIVEN THE SHALLOW NATURE DENSE FOG/MIST HAS BEEN
REPORTED. AT THIS TIME IMPACTS LOOK TO BE CONFINED TO SNS, HAF,
AND AT TIMES MRY. CLOUDS WILL HUG THIS COAST TODAY, BUT DO EXPECT
VFR CONDITIONS AT ALL TERMINALS AT SOME POINT THIS AFTERNOON.
MARINE LAYER INCREASE SLIGHTLY IN COVERAGE TONIGHT, BUT STILL NOT
A BIG INLAND INTRUSION. OVERALL CONF IS MODERATE.
VICINITY OF SFO...VFR. ONSHORE FLOW THIS AFTERNOON. TONIGHT CIGS
MOVE THROUGH GG AND HEAD TOWARD KOAK, BUT GUIDANCE SUGGEST CIGS DO
REMAIN NORTH OF TERMINAL. THEREFORE, VFR TONIGHT AS WELL.
SFO BRIDGE APPROACH...SIMILAR TO SFO.
MONTEREY BAY TERMINALS... LOW CIGS AND FOG AT SNS, BUT LOCAL WIND
FLOW AT MRY HAS LED TO IN-OUT STRATUS PUSHES. VFR BY THIS
AFTERNOON, BUT EARLY RETURN THIS EVENING.
MARINE
(TODAY THROUGH TUESDAY)
ISSUED AT 436 AM PDT THU MAY 21 2026
PERSISTENT HIGH PRESSURE OFF THE WEST COAST WILL MAINTAIN FRESH TO
STRONG NORTHERLY BREEZES OVER THE OUTER WATERS NORTH OF PIGEON
POINT. THE STRONGER WINDS WILL RESULT IN HAZARDOUS CONDITIONS AND
ROUGH SEAS THROUGH THURSDAY. MODERATE NORTHWESTERLY BREEZES AND
MODERATE SEAS WILL PREVAIL FOR THE REST OF THE WATERS. CONDITIONS
IMPROVE INTO THE WEEKEND AS SEAS AND NORTHWESTERLY BREEZES BECOME
MODERATE.
MTR WATCHES/WARNINGS/ADVISORIES
CA...NONE.
PZ...SMALL CRAFT ADVISORY UNTIL 3 AM PDT FRIDAY FOR PT ARENA TO PT
REYES 10-60 NM.
SHORT TERM...DIALH
LONG TERM....DIALH
AVIATION...MM
MARINE...MM
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