407
FXUS66 KMTR 131121
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
421 AM PDT WED AUG 13 2025
..NEW AVIATION, MARINE
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.
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
(12Z TAFS)
ISSUED AT 419 AM PDT WED AUG 13 2025
CURRENTLY A MIXED BAG OF LIFR-MVFR AT THE TERMINALS WITH A MARINE
LAYER OF 1,800 FEET. WITH THE EXCEPTION OF HAF, HIGH CONFIDENCE IN
AT LEAST BRIEF VFR BEING ACHIEVED BY THIS AFTERNOON. IT IS LIKELY
(60% CHANCE) THAT SUB-VFR CEILINGS RETURN TO ALL TERMINALS TONIGHT
WITH THE PATTERN CHANGING VERY LITTLE THROUGH THE TAF PERIOD.
VICINITY OF SFO...CURRENTLY MVFR WITH WESTERLY FLOW. HIGH (80%)
CONFIDENCE IN VFR BY THE AFTERNOON WITH HIGH (70%) CONFIDENCE ON A
SUB-VFR CEILING LIKELY ON THE CUSP OF MVFR/IFR RETURNING TO THE
TERMINAL TONIGHT. WESTERLY WINDS WILL PREVAIL THROUGH THE TAF PERIOD.
SFO BRIDGE APPROACH...10% PROBABILITY THAT OAK DOES NOT ACHIEVE VFR
AND THAT LOW STRATUS CLOUDS LINGER THROUGH THE TAF PERIOD.
MONTEREY BAY TERMINALS...CURRENTLY LIFR WITH WESTERLY FLOW AT MRY
AND LIFR WITH NORTHWESTERLY FLOW AT SNS. BRIEF VFR WILL BE ACHIEVED
THIS AFTERNOON WITH LOW STRATUS CLOUDS STICKING CLOSE TO THE COAST.
CEILINGS ON THE CUSP OF LIFR/IFR WILL RETURN TO BOTH TERMINALS
TONIGHT.
MARINE
(TODAY THROUGH MONDAY)
ISSUED AT 419 AM PDT WED AUG 13 2025
MODERATE NORTHWESTERLY BREEZES TODAY WILL INCREASE TO BECOME FRESH
TO STRONG TOMORROW. WIDESPREAD HAZARDOUS CONDITIONS WILL BE
PRESENT TOMORROW 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 FROM 1 PM THIS AFTERNOON TO 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...SARMENT
MARINE...SARMENT
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