697
FXUS66 KMTR 130803
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
103 AM PDT WED AUG 13 2025
..NEW SHORT TERM, LONG TERM
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
(06Z TAFS)
ISSUED AT 942 PM PDT TUE AUG 12 2025
NORTH BAY TERMINALS ARE CURRENTLY VFR BUT SATELLITE TRENDS INDICATE
THAT LOW STRATUS WILL CONTINUE TO FILL IN THE VALLEYS NEAR KSTS AND
KAPC OVER THE NEXT COUPLE OF HOURS. BAY AREA TERMINALS ARE EXPECTED
TO HOVER BETWEEN MVFR/IFR THROUGH THE OVERNIGHT INTO LATE WEDNESDAY
MORNING. SIMILAR FORECAST FOR THE MONTEREY TERMINALS WHERE THE
WINDOW FOR VFR WEDNESDAY AFTERNOON IS EXPECTED TO BE SHORTER.
VICINITY OF SFO...BREEZY ONSHORE WINDS HAVE EASED LATE THIS EVENING
BUT ARE EXPECTED TO INCREASE AGAIN WEDNESDAY AFTERNOON THROUGH
SUNSET. MVFR/IFR CEILINGS ARE PROGGED TO PERSIST THROUGH THE
OVERNIGHT AND SCATTER OUT AT SIMILAR TIMES AS THE PREVIOUS FEW
DAYS, BY MID/LATE MORNING WEDNESDAY WITH SIMILAR RETURN TIMES IN
THE EVENING.
SFO BRIDGE APPROACH...SIMILAR TO SFO.
MONTEREY BAY TERMINALS...CURRENT IFR CONDITIONS ARE EXPECTED TO DROP
TO LIFR DURING THE OVERNIGHT HOURS, LIFTING BACK UP TO IFR AFTER
SUNRISE. SOME CLEARING IS EXPECTED BY LATE MORNING/EARLY AFTERNOON,
ESPECIALLY AT KSNS, BUT KMRY WILL SEE SOME LINGERING LOW STRATUS
NEAR THE COASTLINE MOST OF THE DAY EVEN WHEN VFR CONDITIONS RETURN
WEDNESDAY AFTERNOON. EXPECT SIMILAR RETURN TIMES FOR MVFR/IFR
CEILINGS LATE IN THE AFTERNOON/EARLY EVENING.
MARINE
(TONIGHT THROUGH NEXT MONDAY)
ISSUED AT 856 PM PDT TUE AUG 12 2025
MODERATE NORTHWESTERLY BREEZES WILL PERSIST TONIGHT, BECOMING
FRESH TO STRONG WEDNESDAY, WITH INCREASING STRONG GUSTS ON
THURSDAY. MODERATE SEAS WILL RESPOND TO THE STRONG GUSTS BECOMING
ROUGH ON THURSDAY. NORTHWESTERLY WINDS WILL INCREASE FURTHER ON
FRIDAY, APPROACHING GALE FORCE SPEEDS IN THE OUTER WATERS.
MTR WATCHES/WARNINGS/ADVISORIES
CA...NONE.
PZ...SMALL CRAFT ADVISORY FROM 1 PM THIS AFTERNOON TO 9 PM PDT THIS
EVENING FOR SF BAY N OF BAY BRIDGE.
SMALL CRAFT ADVISORY FROM 3 PM THURSDAY TO 3 AM PDT FRIDAY FOR
PT ARENA TO PT REYES 0-10 NM-PT PINOS TO PT PIEDRAS BLANCAS
0-10 NM.
SMALL CRAFT ADVISORY UNTIL 3 AM PDT THURSDAY FOR PT PINOS TO PT
PIEDRAS BLANCAS 0-10 NM.
SMALL CRAFT ADVISORY FROM 9 PM THIS EVENING TO 3 AM PDT FRIDAY
FOR PT ARENA TO PT REYES 10-60 NM.
SHORT TERM...FLYNN
LONG TERM....FLYNN
AVIATION...JM
MARINE...JM
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