877
FXUS66 KMTR 131743
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
1043 AM PDT WED AUG 13 2025
..NEW AVIATION
KEY MESSAGES
UPDATED AT 1200 AM PDT WED AUG 13 2025
- A PATTERN CHANGE WILL BRING A MORE ROBUST MARINE LAYER, COOLER
TEMPERATURES, AND SOME MORNING DRIZZLE TODAY.
- STRONG NW WINDS WILL ARRIVE TO THE COAST THURSDAY AND SPREAD
INLAND FRIDAY.
- TEMPERATURES WILL CLIMB SLIGHTLY THROUGH FRIDAY BEFORE COOLING
AGAIN THIS WEEKEND.
UPDATE 2
ISSUED AT 858 AM PDT WED AUG 13 2025
A VERY MOIST MARINE LAYER HAS LED TO ACCUMULATING DRIZZLE THIS
AM. A QUICK SAMPLING OF BUCKET TIPS OVER THE LAST SIX HOURS SHOWS
A FEW HUNDREDTHS OF PRECIP BEING RECORDED ALONG THE COAST. MORNING
VIS SATELLITE ALSO SHOWS THE SOLID MARINE LAYER (AS NOTED BELOW IN
UPDATE 1) EXTENDING WELL INLAND. SPEAKING OF SATELLITE IMAGERY,
CURIOUS WEATHER WATCHERS MAY ALSO NOTICE SOME MID-LEVEL CU PUSHING
THROUGH THE BAY AREA THIS MORNING. IF YOU READ THE AFD FROM
YESTERDAY AFTERNOON IT'S THE LEFTOVER MOISTURE FROM IVO STILL
TRACKING NORTHWARD. FARTHER UPSTREAM THERE ARE MORE DEFINED CU
(DUE WEST OF SAN NICOLAS ISLAND) ENOUGH LIFT/MOISTURE WAS PRESENT
IN THIS REGION TO SPARK A FEW LIGHTNING STRIKES OVER THE LAST 3
HRS. WE'LL STILL BE KEEPING AN EYE ON IT THROUGH OUT THE DAY, BUT
INDICATIONS STILL SHOW THIS ACTIVITY STAYING SOUTH OF THE
FORECAST AREA.
NO UPDATE AT THIS TIME.
MM
UPDATE 1
ISSUED AT 454 AM PDT WED AUG 13 2025
AS THE WEATHER BALLOON RISES THIS MORNING THE MARINE LAYER WAS
REVEALED TO BE AROUND 2,100 FEET. WHILE SOME EXPANSION WAS
EXPECTED, THIS IS ADMITTEDLY DEEPER THAN ANTICIPATED. AS A
RESULT, THE STRATUS EXPANSION IS MUCH FARTHER INLAND THAN
YESTERDAY AND AND DRIZZLE IS NOW BEING REPORTED ACROSS SAN
FRANCISCO AND ALONG THE PENINSULA COAST. THE SOUTHERN MONTEREY BAY
HAS ALSO RECORDED SOME MEASURABLE DRIZZLE EARLY THIS MORNING.
FLYNN
SHORT TERM
ISSUED AT 1200 AM PDT WED AUG 13 2025
(TODAY AND TONIGHT)
LET'S TALK ABOUT MOISTURE FIRST. TROPICAL STORM IVO DISSIPATED
EARLY MONDAY IN THE TROPICAL EASTERN PACIFIC AND MOST OF THE
MOISTURE IS BEING REABSORBED BY THE MONSOON TROUGH. DESPITE THIS,
A LONG LOOP OF WATER VAPOR IMAGERY SHOWS THAT A SMALL BAND OF THIS
MOISTURE SPLIT OFF AND PUSHED NORTH TOWARDS THE BAY AREA. THIS
MOISTURE IS NOW MANIFESTING AS UPPER LEVEL CLOUDS CURRENTLY
STREAMING ACROSS THE BAY AREA (WELL ABOVE THE MARINE LAYER STRATUS).
THESE CLOUDS ARE SOMEWHERE BETWEEN 20,000 AND 25,000 FEET BASED
ON A COMPARISON BETWEEN IR IMAGERY CLOUD TOP TEMPERATURES (-11C)
AND THE LATEST VERTICAL SOUNDING (TEMPERATURE VS. HEIGHT). OUTSIDE
OF THESE HIGH CLOUDS, THIS MOISTURE IS TOO HIGH AND THIN TO HAVE
ANY OTHER EFFECT ON THE WEATHER AT THE GROUND. THE VERTICALLY
INTEGRATED PRECIPITABLE WATER IS ONLY 0.61". THAT PUTS US IN THE
25TH PERCENTILE FOR THIS TIME OF YEAR. SO THE OVERALL MOISTURE IS
STILL WELL BELOW NORMAL. THE REASON FOR THIS IS VERY DRY AIR
BETWEEN THE MARINE LAYER AND THESE HIGH CLOUDS. THE PGE 2KM WRF
CROSS SECTION ACROSS THE BAY AREA IS HIGHLIGHTING RH AROUND 10%
ABOVE 3,000 FEET. THIS DRY AIR IS BEING DRAGGED TO LOWER
ELEVATIONS EAST OF THE DIABLO RANGE DUE TO MOUNTAIN WAVE
ACTIVITY. AS A RESULT, A FEW INLAND STATIONS ARE REPORTING RED
FLAG CONDITIONS WITH RH STILL IN THE TEENS AT MIDNIGHT AND
MODERATE WESTERLY WINDS AROUND 15 MPH.
OTHERWISE THE SHORT TERM FORECAST WILL BRING A PATTERN CHANGE. A
500 MB TROUGH IS SLOWLY APPROACHING FROM THE WEST. THIS WILL BRING
A DEEPER MARINE LAYER, HIGHER HUMIDITY, AND LOWER TEMPERATURES.
BOTTOM LINE: TEMPERATURES WILL BE ABOUT 5 DEGREES COOLER THAN
YESTERDAY WITH SIMILAR MODERATE ONSHORE WINDS.
LONG TERM
ISSUED AT 1200 AM PDT WED AUG 13 2025
(THURSDAY THROUGH TUESDAY)
BY THURSDAY AFTERNOON THE 500 MB TROUGH AXIS SHOULD BE OVER THE
BAY AREA AND WILL STALL. THIS WILL GENERALLY BRING COOLER
TEMPERATURES, A DEEPER MARINE LAYER, AND HIGHER HUMIDITY. AT THE
SURFACE, HOWEVER, A NARROW RIDGE OF HIGH PRESSURE WILL MOVE OVER
FAR NORTHERN CALIFORNIA/SOUTHERN OREGON THU-FRI. THE EUREKA-SFO
PRESSURE GRADIENT WILL INCREASE FROM 0.5 MB TONIGHT TO 4-6 MB BY
EARLY FRIDAY MORNING. THIS WILL INCREASE NORTHERLY WINDS, CAUSING
A BRIEF PERIOD OF OFFSHORE WINDS IN THE NORTH BAY. THESE WINDS
WILL BRING SLIGHTLY HIGHER TEMPS FRIDAY, WHEN THE NORTH BAY WILL
BE ABOUT 5 DEGREES WARMER THAN NORMAL. OUTSIDE OF THE NORTH BAY,
WIND DIRECTION SHOULD REMAIN ONSHORE, AND THIS GRADIENT WILL BE
FELT MORE IN THE WIND STRENGTH. THE MTR WRF IS SHOWING LARGE AREAS
OF 20+ MPH GUSTS ACROSS THE REGION FRIDAY AFTERNOON, WITH STRONGER
WINDS EXPECTED ALONG THE COAST AND IN HIGHER ELEVATIONS. AT THIS
TIME WE DO NOT EXPECT TO REACH WIND ADVISORY CRITERIA, BUT THIS
BREEZE WILL BE NOTICEABLE. AS THE N-S GRADIENT RELAXES OVER THE
WEEKEND, THE WIND WILL DECREASE AND TEMPERATURES FALL BACK BELOW
NORMAL ACROSS THE CWA. THE FORECAST THEN LOOKS FAIRLY STABLE AS
THE 500 MB TROUGH PATTERN REMAINS IN PLACE. LOOKING AHEAD, 500 MB
ENSEMBLE CLUSTER ANALYSIS REVEALS A SWITCH BACK TO RIDGING IS
LIKELY BY THE 21ST, THOUGH THERE IS SIGNIFICANT UNCERTAINTY
REGARDING THE INTENSITY AT THE MOMENT.
AVIATION
(18Z TAFS)
ISSUED AT 1040 AM PDT WED AUG 13 2025
A MIX BAG OF IFR/MVFR CONDITIONS ACROSS THE REGION AS THE MARINE
LAYER HAS DEEPENED TO AROUND 2000 FEET. EXPECTING INLAND TERMINALS
TO RETURN TO VFR WITHIN THE NEXT FEW HOURS, HOWEVER THERE IS
MODERATE CONFIDENCE FOR MVFR CEILINGS TO PERSIST THROUGHOUT MUCH
OF THE DAY NEAR THE COAST. ONSHORE WINDS INCREASE ONCE AGAIN THIS
AFTERNOON, SLIGHTLY STRONGER THAN YESTERDAY. HOWEVER, WINDS ARE
FORECAST TO EASE ACROSS THE MONTEREY BAY TERMINALS AFTER SUNSET
AND THE BAY AREA TERMINALS EARLY THURSDAY MORNING. THERE IS HIGH
CONFIDENCE FOR IFR/MVFR CEILINGS TO MAKE AN EARLY RETURN THIS
EVENING AND LOWER INTO THURSDAY MORNING AROUND THE MONTEREY BAY
TERMINALS.
VICINITY OF SFO...MVFR. LOW CLOUDS ARE FORECAST TO SCATTER OUT
AND/OR CLEAR BY 19Z-21Z AND RETURN TO VFR. ONSHORE FLOW WILL BE
SLIGHTLY STRONGER TODAY COMPARED TO YESTERDAY AS A TROUGH OF LOW
PRESSURE IMPACTS THE REGION. AS SUCH, EXPECTING A SIMILAR RETURN
OF MVFR CEILINGS THIS EVENING THAT WILL LOWER TO IFR THROUGH MID-
MORNING ON THURSDAY. ONCE LOW CLOUDS CLEAR, ONSHORE WINDS ARE
FORECAST TO INCREASE ONCE AGAIN THURSDAY AFTERNOON.
SFO BRIDGE APPROACH...SIMILAR TO SFO.
MONTEREY BAY TERMINALS...IFR/MVFR. LOW CLOUDS ARE FORECAST TO
SCATTER OUT AT SNS WITHIN THE NEXT HOUR. MEANWHILE, IFR CONDITIONS
PERSIST AT MRY AND SHOULD SCATTER OUT BY AROUND 21Z. HOWEVER,
HIGH CONFIDENCE FOR AN EARLY RETURN OF IFR CEILINGS THIS EVENING
BEFORE LOWERING TO LIFR EARLY THURSDAY MORNING.
MARINE
(TODAY THROUGH MONDAY)
ISSUED AT 858 AM PDT WED AUG 13 2025
MODERATE NORTHWESTERLY BREEZES TODAY WILL INCREASE TO BECOME FRESH
TO STRONG ON THURSDAY. WIDESPREAD HAZARDOUS CONDITIONS WILL BE
PRESENT THURSDAY WITH GALE FORCE GUSTS ACROSS THE OUTER WATERS
AND ALONG THE COASTAL JET REGION OF POINT REYES AND NEAR-GALE
FORCE GUSTS ACROSS THE REST OF THE INNER WATERS. MODERATE SEAS
WILL INCREASE TO BECOME ROUGH FOR THE INNER WATERS AND OUTER
WATERS THROUGH FRIDAY.
MTR WATCHES/WARNINGS/ADVISORIES
CA...NONE.
PZ...SMALL CRAFT ADVISORY UNTIL 3 AM PDT THURSDAY FOR SF BAY N OF BAY
BRIDGE.
SMALL CRAFT ADVISORY FROM 9 AM TO 3 PM PDT THURSDAY FOR PT ARENA
TO PT REYES 0-10 NM.
GALE WARNING FROM 3 PM THURSDAY TO 3 AM PDT FRIDAY FOR PT ARENA
TO PT REYES 0-10 NM-PT ARENA TO PT REYES 10-60 NM.
SMALL CRAFT ADVISORY FROM 3 PM THURSDAY TO 3 AM PDT FRIDAY FOR
PT REYES TO PIGEON PT 0-10 NM.
SMALL CRAFT ADVISORY FROM 3 PM TO 9 PM PDT THURSDAY FOR PIGEON
PT TO PT PINOS 0-10 NM.
SMALL CRAFT ADVISORY UNTIL 3 AM PDT FRIDAY FOR PT PINOS TO PT
PIEDRAS BLANCAS 0-10 NM.
SMALL CRAFT ADVISORY FROM 9 PM THIS EVENING TO 3 PM PDT THURSDAY
FOR PT ARENA TO PT REYES 10-60 NM.
SMALL CRAFT ADVISORY FROM 9 AM TO 9 PM PDT THURSDAY FOR PIGEON
PT TO PT PINOS 10-60 NM.
GALE WARNING FROM 9 PM THURSDAY TO 3 AM PDT FRIDAY FOR PIGEON PT
TO PT PINOS 10-60 NM.
SHORT TERM...FLYNN
LONG TERM....FLYNN
AVIATION...RGASS
MARINE...RGASS
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