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FXUS66 KMTR [Monterey CA] 201132 AFDMTR 

FXUS66 KMTR 201132
AFDMTR [HEAD]

National Weather Service San Francisco Bay Area 4:32am PDT Sun Sep 20 2020 

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
432 AM PDT SUN SEP 20 2020 [HEAD]

Synopsis. Warm temperatures will continue through today across the region. A mild cool down is forecast to follow during the first half the work week with seasonal conditions expected late in the week. Dry conditions will persist at least through part of next week. 

.SYNOPSIS...WARM TEMPERATURES WILL CONTINUE THROUGH TODAY ACROSS
THE REGION. A MILD COOL DOWN IS FORECAST TO FOLLOW DURING THE
FIRST HALF THE WORK WEEK WITH SEASONAL CONDITIONS EXPECTED LATE
IN THE WEEK. DRY CONDITIONS WILL PERSIST AT LEAST THROUGH PART OF
NEXT WEEK. [DISC]

Discussion. As of 03:08am PDT Sunday… both the Fort Ord and Bodega Bay profilers reporting shallow depths this morning as the marine layer becomes compressed under the weak upper-level West Coast ridge. GOES [Geostationary Operational Environmental Satellite]-17 is already capturing the development of more low stratus and fog over the North Bay coast, while fog and low stratus have gotten more apparent from the San Mateo County coast down to Big Sur. Visibilities have also started falling along the Central Coast and the Salinas Valley, with Salinas reporting visibilities at 3/4 mile. As such, expecting for areas of patchy and dense fog to develop along the coast due to the shallow nature of this marine layer which should mix out by mid-morning. The interior will get a head start with diurnal heating but as soon as the coastal fog burns off temps will also steadily climb there. Expecting temps to be 5–8°F warmer than yesterday in several interior locations, including San Jose and Livermore while coastal locations will run 3–5°F warmer. As for air quality concerns, low to moderate levels of near-surface smoke associated with the August complex have settled over parts of Napa County this morning. HRRR-smoke runs this morning picked up the northerly flow and continue to bring smoke down to both Napa and Sonoma Counties throughout the morning, with elevated smoke potentially making it down to parts of Marin County and the Bay. Thankfully, winds will become lighter through the afternoon due to the upper-level ridging, which should diminish the possibility of smoke making it too far south. 

.DISCUSSION...AS OF 03:08 AM PDT SUNDAY...BOTH THE FORT ORD AND
BODEGA BAY PROFILERS REPORTING SHALLOW DEPTHS THIS MORNING AS THE
MARINE LAYER BECOMES COMPRESSED UNDER THE WEAK UPPER-LEVEL WEST
COAST RIDGE. GOES-17 IS ALREADY CAPTURING THE DEVELOPMENT OF MORE
LOW STRATUS AND FOG OVER THE NORTH BAY COAST, WHILE FOG AND LOW
STRATUS HAVE GOTTEN MORE APPARENT FROM THE SAN MATEO COUNTY COAST
DOWN TO BIG SUR. VISIBILITIES HAVE ALSO STARTED FALLING ALONG THE
CENTRAL COAST AND THE SALINAS VALLEY, WITH SALINAS REPORTING
VISIBILITIES AT 3/4 MILE. AS SUCH, EXPECTING FOR AREAS OF PATCHY
AND DENSE FOG TO DEVELOP ALONG THE COAST DUE TO THE SHALLOW NATURE
OF THIS MARINE LAYER WHICH SHOULD MIX OUT BY MID-MORNING. THE
INTERIOR WILL GET A HEAD START WITH DIURNAL HEATING BUT AS SOON AS
THE COASTAL FOG BURNS OFF TEMPS WILL ALSO STEADILY CLIMB THERE.
EXPECTING TEMPS TO BE 5-8 DEGREES F WARMER THAN YESTERDAY IN
SEVERAL INTERIOR LOCATIONS, INCLUDING SAN JOSE AND LIVERMORE WHILE
COASTAL LOCATIONS WILL RUN 3 TO 5 DEGREES F WARMER. AS FOR AIR
QUALITY CONCERNS, LOW TO MODERATE LEVELS OF NEAR-SURFACE SMOKE
ASSOCIATED WITH THE AUGUST COMPLEX HAVE SETTLED OVER PARTS OF NAPA
COUNTY THIS MORNING. HRRR-SMOKE RUNS THIS MORNING PICKED UP THE
NORTHERLY FLOW AND CONTINUE TO BRING SMOKE DOWN TO BOTH NAPA AND
SONOMA COUNTIES THROUGHOUT THE MORNING, WITH ELEVATED SMOKE
POTENTIALLY MAKING IT DOWN TO PARTS OF MARIN COUNTY AND THE BAY.
THANKFULLY, WINDS WILL BECOME LIGHTER THROUGH THE AFTERNOON DUE TO
THE UPPER-LEVEL RIDGING, WHICH SHOULD DIMINISH THE POSSIBILITY OF
SMOKE MAKING IT TOO FAR SOUTH. [DISC]

For more details on air quality concentrations in your area, please visit airnow and/or the Bay Area air quality district. 

FOR MORE DETAILS ON AIR QUALITY CONCENTRATIONS IN YOUR AREA,
PLEASE VISIT AIRNOW AND/OR THE BAY AREA AIR QUALITY DISTRICT. [DISC]

Water vapor imagery is picking up on the moisture associated with the subtle shortwave trough that will be making its way into the Pacific Northwest during the first part of the upcoming workweek. As its axis moves over the CWA [County Warning Area; see the LWP home page (link at bottom of page) for a CWA map] Monday and Tuesday, expect a cooling trend, especially across the interior, that will bring temps back to seasonal averages. These seasonal conditions look forecast to continue through the second-half of the work week as a more pronounced upper-level trough moves into the Pacific Northwest during that time. GFS [Global Forecast System: global spectral model used primarily for aviation weather forecasts; provides guidance out to 384 hours at 00, 06, 12 and 18UTC.] run brings POPs [Probability of Precipitation] as far south as parts of Northern California, but not looking like precipitation will make it as far south as our CWA [County Warning Area; see the LWP home page (link at bottom of page) for a CWA map]. Nevertheless, the passage of both the shortwave and the upper-level trough will greatly help increase marine layer depths. These depths along with cooler temps will help bring better humidity recoveries to our region, which is good news for burn areas. 

WATER VAPOR IMAGERY IS PICKING UP ON THE MOISTURE ASSOCIATED WITH
THE SUBTLE SHORTWAVE TROUGH THAT WILL BE MAKING ITS WAY INTO THE
PAC NW DURING THE FIRST PART OF THE UPCOMING WORKWEEK. AS ITS AXIS
MOVES OVER THE CWA MONDAY AND TUESDAY, EXPECT A COOLING TREND,
ESPECIALLY ACROSS THE INTERIOR, THAT WILL BRING TEMPS BACK TO
SEASONAL AVERAGES. THESE SEASONAL CONDITIONS LOOK PROGGED TO
CONTINUE THROUGH THE SECOND-HALF OF THE WORK WEEK AS A MORE
PRONOUNCED UPPER-LEVEL TROUGH MOVES INTO THE PAC NW DURING THAT
TIME. GFS RUN BRINGS POPS AS FAR SOUTH AS PARTS OF NORTHERN
CALIFORNIA, BUT NOT LOOKING LIKE PRECIP WILL MAKE IT AS FAR SOUTH
AS OUR CWA. NEVERTHELESS, THE PASSAGE OF BOTH THE SHORTWAVE AND
THE UPPER-LEVEL TROUGH WILL GREATLY HELP INCREASE MARINE LAYER
DEPTHS. THESE DEPTHS ALONG WITH COOLER TEMPS WILL HELP BRING
BETTER HUMIDITY RECOVERIES TO OUR REGION, WHICH IS GOOD NEWS FOR
BURN AREAS. [DISC]

ECMWF [European Center for Medium-Range Weather Forecasts model] ens, GEFS [NCEP Global Ensemble Forecast System], and CMC [Canadian Meteorological Centre] ensemble members continue to suggest the possibility for an offshore wind event next weekend. Still a lot of uncertainty in the latest GFS [Global Forecast System: global spectral model used primarily for aviation weather forecasts; provides guidance out to 384 hours at 00, 06, 12 and 18UTC.] run regarding the fate of the upper-level trough during the second half of the workweek; the run deepens it into a cut-off low over the Great Basin and ejects it into the high Northern Plains sometime after Sunday, while the ECMWF [European Center for Medium-Range Weather Forecasts model] deepens the trough. As such, variation in the strength of the upper-level ridge that looks forecast to build off the coast. Any subtle variations in this set up could substantially increase the likelihood of the offshore wind event verifying. 925 hPa [hectopascal (1 hPa = 1 mb)] winds on the GFS [Global Forecast System: global spectral model used primarily for aviation weather forecasts; provides guidance out to 384 hours at 00, 06, 12 and 18UTC.] pick up on the wind shift across the North and East Bay hills Friday through Sunday night, while the ECMWF [European Center for Medium-Range Weather Forecasts model] suggests a weaker offshore signature. For now, will have to wait until there is greater confidence in the medium-range models. Stay tuned. 

ECMWF ENS, GEFS, AND CMC ENSEMBLE MEMBERS CONTINUE TO SUGGEST THE
POSSIBILITY FOR AN OFFSHORE WIND EVENT NEXT WEEKEND. STILL A LOT
OF UNCERTAINTY IN THE LATEST GFS RUN REGARDING THE FATE OF THE
UPPER-LEVEL TROUGH DURING THE SECOND HALF OF THE WORKWEEK; THE RUN
DEEPENS IT INTO A CUT-OFF LOW OVER THE GREAT BASIN AND EJECTS IT
INTO THE HIGH NORTHERN PLAINS SOMETIME AFTER SUNDAY, WHILE THE
ECMWF DEEPENS THE TROUGH. AS SUCH, VARIATION IN THE STRENGTH OF
THE UPPER-LEVEL RIDGE THAT LOOKS PROGGED TO BUILD OFF THE COAST.
ANY SUBTLE VARIATIONS IN THIS SET UP COULD SUBSTANTIALLY INCREASE
THE LIKELIHOOD OF THE OFFSHORE WIND EVENT VERIFYING. 925 HPA WINDS
ON THE GFS PICK UP ON THE WIND SHIFT ACROSS THE NORTH AND EAST
BAY HILLS FRIDAY THROUGH SUNDAY NIGHT, WHILE THE ECMWF SUGGESTS A
WEAKER OFFSHORE SIGNATURE. FOR NOW, WILL HAVE TO WAIT UNTIL THERE
IS GREATER CONFIDENCE IN THE MEDIUM- RANGE MODELS. STAY TUNED. [DISC]

Aviation. As of 04:32am PDT Sunday… for 12Z [5am PDT] TAFs [Terminal Aerodrome Forecasts]. A shallow marine layer has formed along the coast at about 800 ft AGL [Above Ground Level] per the Fort Ord profiler [instrument designed to measure horizontal winds directly above its location, and thus measure the vertical wind profile. Profilers operate on the same principles as Doppler radar.]. Generally VFR [Visual Flight Rules] conditions early this morning aside from patchy fog around the Monterey Bay with a small amount of stratus offshore and up the San Mateo coast. LIFR [Low Instrument Flight Rules] conditions around the Monterey terminals this morning due to low clouds and patchy fog. Patches of stratus/fog may develop elsewhere early this morning. VFR [Visual Flight Rules] conditions this afternoon and evening. There is a chance of some low cloud re-development overnight though widespread stratus is not anticipated. The latest HRRR smoke model still shows northerly flow drifting smoke into the North Bay this morning and into the east/South Bay this afternoon. Slant range visibility may be impacted at some terminals as well as haze at the surface, but no major visibility issues expected. Light winds this morning increasing and turning onshore this afternoon around 10–15 kt [12–17 mph] with locally higher winds possible at KSFO [San Francisco CA]

.AVIATION...AS OF 04:32 AM PDT SUNDAY...FOR 12Z TAFS. A SHALLOW
MARINE LAYER HAS FORMED ALONG THE COAST AT ABOUT 800 FT AGL PER
THE FORT ORD PROFILER. GENERALLY VFR CONDITIONS EARLY THIS MORNING
ASIDE FROM PATCHY FOG AROUND THE MONTEREY BAY WITH A SMALL AMOUNT
OF STRATUS OFFSHORE AND UP THE SAN MATEO COAST. LIFR CONDITIONS
AROUND THE MONTEREY TERMINALS THIS MORNING DUE TO LOW CLOUDS AND
PATCHY FOG. PATCHES OF STRATUS/FOG MAY DEVELOP ELSEWHERE EARLY
THIS MORNING. VFR CONDITIONS THIS AFTERNOON AND EVENING. THERE IS
A CHANCE OF SOME LOW CLOUD RE-DEVELOPMENT OVERNIGHT THOUGH
WIDESPREAD STRATUS IS NOT ANTICIPATED. THE LATEST HRRR SMOKE MODEL
STILL SHOWS NORTHERLY FLOW DRIFTING SMOKE INTO THE NORTH BAY THIS
MORNING AND INTO THE EAST/SOUTH BAY THIS AFTERNOON. SLANT RANGE
VIS MAY BE IMPACTED AT SOME TERMINALS AS WELL AS HAZE AT THE
SURFACE, BUT NO MAJOR VIS ISSUES EXPECTED. LIGHT WINDS THIS
MORNING INCREASING AND TURNING ONSHORE THIS AFTERNOON AROUND 10-15
KT WITH LOCALLY HIGHER WINDS POSSIBLE AT KSFO. [AVIA]

Vicinity of KSFO [San Francisco CA]VFR [Visual Flight Rules] prevailing, though some low clouds may develop early this morning. Increasing W/NW winds this afternoon around 15 kt [17 mph]

VICINITY OF KSFO...VFR PREVAILING, THOUGH SOME LOW CLOUDS MAY
DEVELOP EARLY THIS MORNING. INCREASING W/NW WINDS THIS AFTERNOON
AROUND 15 KT. [AVI2]

SFO [San Francisco CA] bridge approach… similar to SFO [San Francisco CA]

SFO BRIDGE APPROACH...SIMILAR TO SFO. [AVI2]

Monterey Bay terminals… intermittent VFR [Visual Flight Rules]/LIFR [Low Instrument Flight Rules] conditions around the Bay due to low clouds and fog. Current observations show fog has cleared out for the moment, but fog may re-form before mid morning. VFR [Visual Flight Rules] conditions this afternoon and evening before some low clouds and fog will be possible again late tonight. Light winds turning W/NW this afternoon. 

MONTEREY BAY TERMINALS...INTERMITTENT VFR/LIFR CONDITIONS AROUND
THE BAY DUE TO LOW CLOUDS AND FOG. CURRENT OBS SHOW FOG HAS
CLEARED OUT FOR THE MOMENT, BUT FOG MAY RE-FORM BEFORE MID
MORNING. VFR CONDITIONS THIS AFTERNOON AND EVENING BEFORE SOME LOW
CLOUDS AND FOG WILL BE POSSIBLE AGAIN LATE TONIGHT. LIGHT WINDS
TURNING W/NW THIS AFTERNOON. [AVI2]

Marine. As of 02:01am PDT Sunday… a broad area of surface high pressure over the eastern Pacific will keep breezy north to northwest winds over the northern waters through Monday evening. Generally light to locally moderate winds expected elsewhere. Mixed seas will persist with a shorter period northwest swell and a longer period southwest swell arriving today. Expect northwest swell to build late this week. 

.MARINE...AS OF 02:01 AM PDT SUNDAY...A BROAD AREA OF SURFACE
HIGH PRESSURE OVER THE EASTERN PACIFIC WILL KEEP BREEZY NORTH TO
NORTHWEST WINDS OVER THE NORTHERN WATERS THROUGH MONDAY EVENING.
GENERALLY LIGHT TO LOCALLY MODERATE WINDS EXPECTED ELSEWHERE.
MIXED SEAS WILL PERSIST WITH A SHORTER PERIOD NORTHWEST SWELL AND
A LONGER PERIOD SOUTHWEST SWELL ARRIVING TODAY. EXPECT NORTHWEST
SWELL TO BUILD LATE THIS WEEK. [AVIA]

MTR Watches/Warnings/Advisories

  • Today: Small Craft AdvisoryPt Arena [Mendocino CA] to Pigeon Pt [Pescadero CA] 10–60 nm [69 miles] Small Craft AdvisoryPt Arena [Mendocino CA] to Pt Reyes [San Francisco CA] 0–10 nm [11 miles] from 3pm 
.MTR WATCHES/WARNINGS/ADVISORIES...
.TDAY...SCA...PT ARENA TO PIGEON PT 10-60 NM
SCA...PT ARENA TO PT REYES 0-10 NM FROM 3 PM [WARN]

Public Forecast: Diaz/Dykema; Aviation: AS; Marine: AS 

PUBLIC FORECAST: DIAZ/DYKEMA
AVIATION: AS
MARINE: AS [CRED]
VISIT US AT WWW.WEATHER.GOV/SANFRANCISCO [LINK]

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WWW.YOUTUBE.COM/NWSBAYAREA [DISC]

Point Forecast

Half Moon Bay Airport

20 Sep 09:15am PDT

Today Patchy dense fog before 10am. Otherwise, mostly sunny, with a high near 70°. NW wind 6 to 11 mph.
Tonight Partly cloudy, with a low around 57°. WNW wind 5 to 10 mph.
Monday Mostly sunny, with a high near 66°. West wind 5 to 9 mph.
Monday Night Increasing clouds, with a low around 57°. WNW wind 7 to 10 mph.
Tuesday Partly sunny, with a high near 66°. West wind 6 to 11 mph.
Tuesday Night Partly cloudy, with a low around 56°.
Wednesday Mostly sunny, with a high near 67°.
Wednesday Night Partly cloudy, with a low around 58°.
Thursday Mostly sunny, with a high near 68°.
Thursday Night Partly cloudy, with a low around 57°.
Friday Partly sunny, with a high near 67°.
Friday Night Mostly cloudy, with a low around 56°.
Saturday Mostly sunny, with a high near 69°.
Today
Patchy dense fog before 10am. Otherwise, mostly sunny, with a high near 70. Northwest wind 6 to 11 mph.
Tonight
Partly cloudy, with a low around 57. West northwest wind 5 to 10 mph.
Monday
Mostly sunny, with a high near 66. West wind 5 to 9 mph.
Monday Night
Increasing clouds, with a low around 57. West northwest wind 7 to 10 mph.
Tuesday
Partly sunny, with a high near 66. West wind 6 to 11 mph.
Tuesday Night
Partly cloudy, with a low around 56.
Wednesday
Mostly sunny, with a high near 67.
Wednesday Night
Partly cloudy, with a low around 58.
Thursday
Mostly sunny, with a high near 68.
Thursday Night
Partly cloudy, with a low around 57.
Friday
Partly sunny, with a high near 67.
Friday Night
Mostly cloudy, with a low around 56.
Saturday
Mostly sunny, with a high near 69.

This page is generated dynamically from the National Weather Service's NWS point forecast and Area Forecast Discussion, reformatted for readability. NWS glossary; about point forecasts. For more information, visit the Lobitos Weather Project home page. Comments and corrections are welcome: