San Bruno, California Weather Forecast Discussion

723
FXUS66 KMTR 171146
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
446 AM PDT Wed Sep 17 2025
...New AVIATION...
.KEY MESSAGES...
Updated at 406 AM PDT Wed Sep 17 2025
- Dense Fog Advisory in effect until 10AM along the Pacific Coast
   and the waters of Monterey Bay
- Above normal temperatures with widespread Moderate HeatRisk,
   the last of our three day heat wave
- Showers with isolated to scattered thunderstorms as early as
   late tonight through Friday
&&
.SHORT TERM...
Issued at 406 AM PDT Wed Sep 17 2025
(Today and tonight)
Areas of dense fog have developed across the Pacific Coast and the
waters of Monterey Bay with a Dense Fog Advisory in effect until
10AM once again this morning.
Thermal belts remain active with a majority of sites above 1500ft at
70 degrees or higher and RH values mostly between 25%-40%, with Oak
Ridge the lowest at 16% as of 3AM. As we warm to above normal
temperatures for the last day of this relatively short heat wave,
we`ll also be keeping an eye on the tropical plume of moisture
working its way north from SOCAL. Vandenberg radar is picking up
an area of elevated showers over the open water moving due north
past the Channel Islands and Point Conception at this hour, which
may be a little ahead of schedule with a number of individual
HiRes CAMS seeming to pick up on the general activity so far. The
bulk of the first wave of shower activity and isolated
thunderstorms is expected to remain offshore as it works north
toward our latitude later today and tonight. By late tonight into
the early hours of Thursday morning we see increasing chances of
showers and isolated thunderstorms moving into southern Monterey
County and San Benito County. There is still a slight chance of
dry thunderstorm activity and lightning initially as the activity
moves into our area late tonight, but we quickly shift toward more
wetting rainfall with ensemble mean PWATs indicating around
1.40", associated with the leading edge of the remnants of Mario
over our southern zones by sunrise Thursday morning. That mean
value is 200% of normal, giving high confidence that we`ll see
near record PWATs with this event for this time of the year.
&&
.LONG TERM...
Issued at 406 AM PDT Wed Sep 17 2025
(Thursday through Tuesday)
As mentioned above, the leading edge of the remnants of Mario move
into our area by early Thursday morning. Any thunderstorm activity
early Thursday morning is expected to remain mostly offshore, but
things get more interesting by late morning into Thursday afternoon
as the environment becomes more favorable for scattered
thunderstorms to develop across the Central Coast into the South
Bay. Given the near record PWATs, MUCAPE values between 200-400
J/kg, moderate PVA, and bulk shear up to 30kts by early Thursday
afternoon, a few strong thunderstorms can`t be ruled out
especially along the Santa Lucia and the Big Sur Coastline where
terrain could also enhance lift at times near the surface. The
primary concern would be strong wind gusts up to 50 MPH, cloud-to-
ground lightning and brief downpours given the high PWATs through
early Thursday evening. Thunderstorm activity is expected to
gradually decline after sunset with scattered shower activity
persisting through the overnight into Friday as the center of the
remnants of Mario shifts into NORCAL. Weak ridging builds north
out of the desert southwest for the weekend, as another weak
disturbance approaches the west coast for the beginning of next
week. Rain chances increase once again Monday afternoon and
persist into Tuesday.
&&
.AVIATION...
(12Z TAFS)
Issued at 444 AM PDT Wed Sep 17 2025
Currently VFR at all terminals with the exception of HAF, MRY, and
SNS which are experiencing dense fog. Widespread VFR is expected by
this afternoon with clouds lowering through the TAF period as
remnant moisture from Post-Tropical Cyclone Mario spreads north. The
aforementioned feature will bring chances for scattered
thunderstorms at least over the next two days. Thunderstorms are
underway off Point Conception and are moving northward; however, no
thunderstorms are in the TAFs as of now as chances today are best
over the coastal waters and Interior Central Coast. Rain showers
will approach the Monterey Bay terminals towards the end of their
24 hour TAF period and OAK and SFO towards the end of their 30
hour TAF period.
Vicinity of SFO...Currently VFR with northwesterly flow. High
confidence in VFR and westerly flow prevailing through the TAF
period. Rain showers will approach the terminal towards the end of
the 30 hour TAF period.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Currently LIFR and calm at MRY and LIFR
with drainage flow at SNS. Dense fog will mix out by late-morning
with low stratus sticking close to the coast. Moderate confidence on
LIFR ceilings returning to the terminals tonight. Rain showers will
approach the terminals towards the end of the 24 hour TAF period.
&&
.MARINE...
(Today through Monday)
Issued at 406 AM PDT Wed Sep 17 2025
Hazardous conditions will exist over the outer waters through
Friday due to strong northerly breezes and rough seas that are
developing today. Conditions will be locally worse in the northern
outer waters where gale force gusts and very rough seas are
likely. Scattered thunderstorms are possible today and tomorrow,
posing the risks of lightning, locally heavy rainfall, and erratic
and gusty winds. Isolated rain showers will linger through
Friday.
&&
.FIRE WEATHER...
Issued at 225 PM PDT Tue Sep 16 2025
- Near critical fire weather conditions through Wednesday
- Threat for dry lightning has diminished, but didn`t completely
   go away.
Near critical fire weather conditions continue into Wednesday due
to hot, dry, and afternoon breezy onshore winds. Tuesday afternoon
featured interior temps of a hot 101 and a crispy 11% at Arroyo
Seco. Expect another repeat away from the coast on Wednesday.
Don`t expect much relief tonight either as thermal belts will be
very pronounced with mild and dry conditions over the higher
elevations. Overnight humidity recovery will be moderate to poor
over the highest peaks. Still have concerns regarding low
confidence high impact scenario as tropical moisture brings a
chance for elevated/high- based thunderstorms. After assessing the
latest guidance and coordinating with other offices and fire
partners decided to NOT issue any Fire Weather watch at this time.
The initial threat for dry lightning is decreasing with focus
over the ocean on Wednesday and Wednesday night. If the threat
shifts east we`ll have to re-evaluate. Once convection chances
increase over land Thursday enough moisture should be present
that storms will be wetter vs drier.
MM
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Dense Fog Advisory until 10 AM PDT this morning for CAZ006-505-
     509-529-530.
PZ...Dense Fog Advisory until 10 AM PDT this morning for Mry Bay.
Small Craft Advisory from 9 AM this morning to 3 PM PDT this
     afternoon for Pt Arena to Pt Reyes 10-60 NM.
Gale Warning from 3 PM this afternoon to 9 AM PDT Thursday for
     Pt Arena to Pt Reyes 10-60 NM.
Small Craft Advisory from 3 PM this afternoon to 9 AM PDT
     Thursday for Pigeon Pt to Pt Pinos 10-60 NM.
&&
$$
SHORT TERM...JM
LONG TERM....JM
AVIATION...Sarment
MARINE...Sarment
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NWS MTR Office Area Forecast Discussion