890
FXUS66 KMTR 211751
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
1051 AM PDT WED OCT 21 2026
..NEW AVIATION
KEY MESSAGES
UPDATED AT 801 PM PDT TUE OCT 20 2026
- COOLING TREND CONTINUES THROUGH THE END OF THE WEEK
- BENEFICIAL LIGHT RAIN SHOWERS EXPECTED WEDNESDAY THROUGH THURSDAY
- THUNDERSTORMS POSSIBLE WEDNESDAY AFTERNOON INTO THURSDAY MORNING
ACROSS CENTRAL COAST
- WARM AND DRY NEXT WEEKEND
UPDATE
ISSUED AT 939 AM PDT WED OCT 21 2026
WE ARE STARTING TO SEE THE SOME OF THE FIRST RAIN BANDS MOVING INTO
THE NORTH CENTRAL COAST THIS MORNING AS THE CORE OF THE CUT-OFF
UPPER LEVEL LOW APPROACHES THE REGION. THERE AREN'T CURRENTLY ANY
LIGHTING STRIKES AS OF YET ACROSS THE REGION THIS MORNING. HOWEVER,
WE ARE EXPECTING THAT TO CHANGE AS WE HEAD INTO LATE MORNING AND
MORESO THIS AFTERNOON AND EVENING. THIS IS AS INSTABILITY INCREASES
WITH UP TO 400-800 J/KG OF MUCAPE (MOST UNSTABLE CONVECTIVE
AVAILABLE POTENTIAL ENERGY). THE GREATEST INSTABILITY IS FORECAST TO
REMAIN OVER THE NORTH CENTRAL COAST AND POINTS SOUTHWARD. ALL EYES
REMAIN ON THE BURN SCARS OF THE TIMBER AND PLASKETT FIRES GIVEN THE
THREAT FOR DEBRIS FLOWS. RAIN RATES ACROSS THESE REGIONS MATTER AND
THE SCATTERED NATURE OF THESE SHOWERS AND/OR THUNDERSTORMS GIVES US
LESS CONFIDENCE OF EXACTLY HOW MUCH RAINFALL WILL OCCUR OVER ANY
GIVEN ONE AREA. PLEASE SEE THE AFD FOR THE SHORT TERM FOR ADDITIONAL
DETAILS.
RGASS
SHORT TERM
ISSUED AT 1230 AM PDT WED OCT 21 2026
(TODAY AND TONIGHT)
THE CENTER OF THE 500 MB CUT-LOW IS INCHING CLOSER TO THE COAST AS
THE SUB-TROPICAL JET STREAM SUPPORTS THE LEFT PORTION OF THE SYSTEM,
BRINGING ENHANCED MOMENTUM AND MOISTURE FLUX. THIS IS WHERE THE MOST
INSTABILITY AND LIFT CAN BE FOUND. AS OF 11:30 PM PDT, KMUX RADAR IS
PICKING UP ON A FEW CELLS OFFSHORE EXHIBITING A ~10-25% CHANCE OF
LIGHTNING WITH A FEW LIGHTNING STRIKES OBSERVED ALONG THE OCCLUDED
FRONT (~250-300 MILES SW FROM MONTEREY). THE CURRENT OFFSHORE
ACTIVITY IS FORESHADOWING WHAT IS TO COME EARLY WEDNESDAY MORNING.
THE FIRST FEW RAIN BANDS ARE EXPECTED TO MAKE THEIR WAY OVER THE
CENTRAL COAST JUST BEFORE SUNRISE. GOING INTO THE AFTERNOON,
PRECIPITATION GETS WRAPPED AROUND THE SYSTEM'S CENTER AND EVENTUALLY
SPREADS THROUGH THE REST OF THE BAY AREA. PRECIPITABLE WATER IS
EXPECTED TO RISE ABOVE THE 90TH PERCENTILE FOR THIS TIME OF YEAR,
WHICH PROMPTED WPC TO INCLUDE THE SOUTHERN CENTRAL COAST IN A
MARGINAL RISK OF EXCESSIVE RAINFALL FOR WEDNESDAY.
TO FURTHER ASSESS THE POTENTIAL FOR SEVERE THUNDERSTORMS, THREE HIGH-
RESOLUTION MODEL SOUNDINGS WERE ANALYZED: THE 3KM NAM, 10KM RDPS,
AND THE RRFS. KEEP IN MIND THAT THE NAM TENDS TO BE THE MOST
PESSIMISTIC MODEL OUT OF THE THREE IN TERMS OF MOISTURE. UPON
EVALUATION OF THE NEAR-STORM ENVIRONMENT FOR EACH, THERE ARE A FEW
INCONSISTENCIES ACROSS THE MODELS IN TERMS OF THE SEVERE
THUNDERSTORM THREAT AND WHETHER THE ENVIRONMENT IS CONDUCIVE TO
TORNADO SPIN-UPS.
BEFORE GETTING INTO WHAT *IS* CONSISTENT ACROSS THE MODELS, LET'S
RUN THROUGH A QUICK OVERVIEW OF A COUPLE NOTABLE STABILITY INDICES:
THE TOTAL-TOTALS AND K INDEX. THE TOTAL-TOTALS INDEX MEASURES
ATMOSPHERIC INSTABILITY BY TAKING THE THERMAL DIFFERENCE BETWEEN 850
TO 500 MB TO IDENTIFY THE POTENTIAL FOR SEVERE THUNDERSTORMS OR
TORNADOES. THE K INDEX IS USED TO ASSESS ATMOSPHERIC BUOYANCY BY
MEASURING THE LOW TO MID-LEVEL MOISTURE AS WELL AS THE 850-500 MB
THERMAL DIFFERENCE. THE GREATER THE K INDEX VALUE, THE GREATER THE
POTENTIAL IS FOR WIDESPREAD THUNDERSTORMS (K > 20 = SCATTERED
THUNDERSTORMS, K > 35 = NUMEROUS THUNDERSTORMS). SIMILARLY, HIGHER
TOTAL-TOTALS (TT) INDEX VALUES INDICATE AN ENHANCED POTENTIAL FOR
SEVERE THUNDERSTORMS AND/OR TORNADOES (TT > 48 = SEVERE
THUNDERSTORMS, TT > 50 = TORNADOES).
WHY AM I TALKING ABOUT STABILITY INDICES? WELL, ACCORDING TO THESE
INDICES AND OTHER ENVIRONMENTAL PARAMETERS, THE THREE HIGH-RES MODEL
SOUNDINGS THAT WERE ANALYZED ALL INDICATE A HEIGHTENED RISK FOR AT
LEAST SCATTERED, SEVERE THUNDERSTORMS ALONG THE BIG SUR COASTLINE.
THE NAM ESTIMATES TT ~ 56 AND A K INDEX ~ 36, RDPS ESTIMATES TT ~ 54
AND A K INDEX ~ 30, AND RRFS ESTIMATES TT ~ 50 AND K INDEX ~ 31.
ANOTHER CONSISTENCY FOUND ACROSS THE MODEL SOUNDINGS IS THE AMOUNT
OF LOW-LEVEL WIND SHEAR. FOR OUR AREA, A SMALL AMOUNT OF SHEAR GOES
A LONG WAY AND CAN SUPPORT LOW-TOPPED SUPERCELLS AND/OR BRIEF SPIN-
UPS OF WATERSPOUTS/TORNADOES. FROM WEDNESDAY AFTERNOON THROUGH EARLY
THURSDAY MORNING, THE MODELS AGREE ON LOW-LEVEL 0-1 KM SHEAR IN
EXCESS OF 10-15 KT (12-17 MPH). THIS TIME FRAME COINCIDES WITH WHEN
THE GREATEST SEVERE WEATHER THREAT MAY OCCUR. THE NAM WANTS TO FULLY
LOCK IN THE TORNADO THREAT, THOUGH THE RDPS AND RRFS ARE SPLIT
50/50. THE SPC ISSUED A 2-4% TORNADO PROBABILITY FOR THE BIG SUR
COASTLINE, THEREBY INDICATING INCREASED POTENTIAL FOR SOME LOW-
TOPPED SUPERCELLS AND NEAR-COASTAL, POTENTIALLY LANDFALLING,
WATERSPOUTS.
IF SEVERE WEATHER DOES DEVELOP, HERE ARE SOME SAFETY REMINDERS:
* HAVE MULTIPLE WAYS TO RECEIVE EMERGENCY ALERTS (I.E. NOAA WEATHER
RADIO, FOLLOW NWS BAY AREA ON SOCIAL MEDIA, AND/OR ENSURING YOUR
PHONE'S EMERGENCY ALERT SYSTEM IS ON)
* MOVE TO AN INTERIOR ROOM AWAY FROM WINDOWS, IF A WATERSPOUT MAKES
LANDFALL
* HAVE AN EMERGENCY ACTION PLAN READY IF YOU LIVE DOWNSLOPE OF
WILDFIRE BURN SCARS
* AVOID FLOODED ROADS. TURN AROUND, DON'T DROWN!
BY LATE THURSDAY MORNING, SOME LINGERING SHOWERS PERSIST THROUGH THE
REGION WITH THE POTENTIAL FOR GENERAL THUNDERSTORMS AND NO
WATERSPOUT/TORNADO THREAT ACROSS THE CENTRAL COAST. MAX TEMPERATURES
REMAIN IN THE MID 50S TO LOW 70S AS ONSHORE WINDS GRADUALLY INCREASE
THROUGH THE EVENING.
LONG TERM
ISSUED AT 1230 AM PDT WED OCT 21 2026
(THURSDAY THROUGH TUESDAY)
THE RAIN AND THUNDERSTORM THREAT ARE GONE BY FRIDAY. WITH THE TROUGH
TO OUR EAST, BUILDING HIGH PRESSURE BRINGS WARM AND DRY CONDITIONS
INTO EARLY NEXT WEEK. EURO AND GFS ENSEMBLE MEMBERS SUGGEST WARMING
TO NEAR SEASONAL AVERAGES MEANWHILE PRECIPITABLE WATER VALUES ARE
FORECAST TO DIP BELOW THE 25TH PERCENTILE. MONDAY ONWARD, THERE IS A
GREAT DEAL OF UNCERTAINTY ON WHEN THE WEAK RIDGE BREAKS DOWN. AN
ENERGETIC CENTRAL PACIFIC PLUS THE MOST RECENT 6-10 DAY CLIMATE
PREDICTION CENTER PRECIPITATION OUTLOOK SUGGESTS ANOTHER TROUGH
COMING OUR WAY WITH THE POTENTIAL FOR MORE RAIN.
AVIATION
(18Z TAFS)
ISSUED AT 1049 AM PDT WED OCT 21 2026
CURRENTLY VFR WITH CLOUDS AND RAIN SHOWERS PUSHING IN FROM THE
SOUTH. THE BEST CHANCES FOR THUNDERSTORMS ARE FOR THE MONTEREY BAY
TERMINALS THIS AFTERNOON AND THIS EVENING, 22Z TO 02Z. ELSEWHERE,
RAIN SHOWERS ARE EXPECTED THROUGH THE TAF PERIOD, SPREADING FROM
SOUTH TO NORTH. THE BEST CHANCES FOR RAIN SHOWERS TO OCCUR AT A
TERMINAL WILL BE THIS AFTERNOON AND THIS EVENING AS COVERAGE AND
INTENSITY INCREASES; HOWEVER, CHANCES LINGER THROUGH THE TAF PERIOD
WITH WRAP AROUND RAIN SHOWERS EXPECTED OVERNIGHT INTO TOMORROW
MORNING. WINDS WILL BE LIGHT AND DIURNAL EXCEPT WHEN RAIN
SHOWER/THUNDERSTORM DRIVEN. LOW CONFIDENCE ON SUB-VFR CEILINGS
DEVELOPING OVERNIGHT; HOWEVER, SUB-VFR CEILINGS AND VISIBILITIES CAN
BE EXPECTED BRIEFLY IF A RAIN SHOWER PASSES DIRECTLY OVER A
TERMINAL.
VICINITY OF SFO...CURRENTLY VFR WITH NORTHWESTERLY FLOW. CLOUDS WILL
CONTINUE TO LOWER THROUGH THE MORNING WITH RAIN SHOWERS EXPECTED BY
THIS AFTERNOON. THE BEST CHANCES FOR RAIN SHOWERS DIRECTLY AT THE
TERMINAL TODAY ARE BETWEEN 22Z AND 04Z WITH WRAP AROUND RAIN SHOWERS
CONTINUING THROUGH THE END OF THE TAF PERIOD. CONDITIONS MAY IMPROVE
TOWARDS THE TAIL END OF THE TAF PERIOD AT OR AFTER 20Z; HOWEVER,
ANOTHER CHANCE OF RAIN SHOWERS IS EXPECTED DURING THIS TIME AS
THE UPPER-LEVEL LOW PULLS EAST.
SFO BRIDGE APPROACH...RELATIVELY HIGHER CHANCE FOR THUNDERSTORMS;
HOWEVER, THE BEST CHANCES REMAIN SOUTH OF SJC. CLOUDS WILL LOWER AND
RAIN SHOWERS WILL OCCUR SOONER THAN SFO. CLOUDS AT OR BELOW FL070
ARE EXPECTED THROUGH THE TAF PERIOD; HOWEVER, CONDITIONS MAY BRIEFLY
IMPROVE BETWEEN RAIN SHOWERS.
MONTEREY BAY TERMINALS...CURRENTLY VFR WITH SOUTHEASTERLY FLOW AT
BOTH TERMINALS AND RAIN BEGINNING TO BE OBSERVED AT MRY. THERE IS A
CHANCE FOR THUNDERSTORMS TODAY WITH THE BEST CHANCES BEING BETWEEN
22Z AND 02Z. RAIN SHOWERS WILL INCREASE IN COVERAGE AND INTENSITY
THIS EVENING. WRAP AROUND RAIN SHOWERS ARE EXPECTED OVERNIGHT AND
INTO TOMORROW MORNING, LIKELY LESS IN COVERAGE AND INTENSITY. WINDS
WILL BE LIGHT AND DIURNAL EXCEPT WHEN RAIN SHOWER/THUNDERSTORM
DRIVEN.
MARINE
(TODAY THROUGH MONDAY)
ISSUED AT 939 AM PDT WED OCT 21 2026
THERE IS A CHANCE FOR THUNDERSTORMS TODAY, ESPECIALLY SOUTH OF
PIGEON POINT. RAIN SHOWERS AND THUNDERSTORMS WILL BE CAPABLE OF
PRODUCING LOCAL GALE FORCE WIND GUSTS, ROUGH SEAS, HEAVY RAINFALL
WITH REDUCED VISIBILITY, AND WATERSPOUTS. GENTLE VARIABLE BREEZES
WILL PREVAIL TODAY. MODERATE TO FRESH NORTHERLY BREEZES WILL
PREVAIL THURSDAY THROUGH SUNDAY WITH LOCALLY STRONG TO NEAR GALE
BREEZES ACROSS THE OUTER WATERS THURSDAY THROUGH SATURDAY.
MODERATE SEAS BUILD TO BECOME ROUGH ACROSS THE OUTER WATERS
FRIDAY.
MTR WATCHES/WARNINGS/ADVISORIES
CA...NONE.
PZ...SMALL CRAFT ADVISORY FROM 9 PM THURSDAY TO 9 AM PDT FRIDAY FOR
PIGEON PT TO PT PINOS 10-60 NM-PT ARENA TO PT REYES 0-10 NM.
SMALL CRAFT ADVISORY FROM 3 PM THURSDAY TO 9 AM PDT FRIDAY FOR
PT ARENA TO PT REYES 10-60 NM.
SHORT TERM...NAVARRETE
LONG TERM....NAVARRETE
AVIATION...SARMENT
MARINE...SARMENT
VISIT US AT WWW.WEATHER.GOV/SANFRANCISCO
FOLLOW US ON FACEBOOK, X, AND YOUTUBE AT:
WWW.FACEBOOK.COM/NWSBAYAREA
X.COM/NWSBAYAREA
WWW.YOUTUBE.COM/NWSBAYAREA
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page