San Bruno, California Weather Forecast Discussion

032
FXUS66 KMTR 220447
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
947 PM PDT Sun Sep 21 2025
...New AVIATION...
.KEY MESSAGES...
Updated at 136 PM PDT Sun Sep 21 2025
- Warming trend Monday and into Tuesday, with Moderate HeatRisk
   across the interior
- Weak offshore flow Monday through Tuesday in the higher terrain
- Rain showers and isolated thunderstorm chances return Tuesday and
   lingering through midweek
&&
.UPDATE...
Issued at 730 PM PDT Sun Sep 21 2025
The warming and drying trend will continue through Tuesday
afternoon. Light offshore flow will develop tonight, further
drying things out while effectively compressing the marine layer.
The initial start for rain and thunderstorms continues to be
pushed back to Tuesday night for the Central Coast (Wednesday
morning elsewhere) with activity continuing through Wednesday
night. Due to the proximity of the cutoff low, the Central Coast
is expected to have the highest rainfall totals and highest
probability for thunderstorms. Outside of thunderstorms, rainfall
is expected to be beneficial. Within thunderstorms, lightning,
locally heavy rainfall, and erratic/gusty winds can be expected.
Sarment
&&
.SHORT TERM...
Issued at 136 PM PDT Sun Sep 21 2025
(This evening through Monday)
Stratus is currently retreating to the coast this afternoon with
mostly sunny skies across inland areas. As such, temperatures will
warm into the mid 80s to lower 90s across the interior, upper 70s to
lower 80s around the San Fransico Bayshore and Santa Cruz region,
while mid 60s to lower 70s will prevail near the coast. Meanwhile,
cumulus clouds are developing this afternoon across the higher
elevations.
Tonight, expecting much less in the way of stratus as the airmass
aloft continues to warm and with light offshore flow in the higher
terrain across the Bay Area and Central Coast. This will lead to a
quicker warm up on Monday with temperatures reaching 5-10 degrees F
above seasonal averages (warmer than today) with warming 850 MB
temperatures. Offshore winds will continue into Tuesday afternoon
across the higher terrain, yet are not forecast to be all that
strong. This will result in elevated fire weather concerns as
minimum relative humidity values will be between 15-25% in these
higher elevations.
&&
.LONG TERM...
Issued at 136 PM PDT Sun Sep 21 2025
(Monday night through next Saturday)
Tuesday will likely be the warmest day of the week as mid/upper
level clouds forecast to approach the Central Coast have been
delayed by about 6-12 hours. Thus, afternoon temperatures are
forecast to reach the upper 80s to upper 90s across the interior,
upper 70s to lower 80s for San Fransico (away from the Pacific
Ocean) and around the Santa Cruz region, low 80s to low 90s around
the San Francisco Bayshore, and upper 60s to upper 70s near the
coast. This will result in Moderate HeatRisk across most inland
areas on Tuesday.
Sub-tropical and/or monsoonal moisture will be advected northward as
a cut-off mid/upper level low pressure nears the Central Coast from
the south, brining the potential for rain showers and isolated
thunderstorms to the Central Coast on Tuesday afternoon. With PWAT
values between 1.00"-1.40" and MUCAPE values between 100-300 J/kg
expected, the threat for dry lightning will be very limited. This is
also supported by the CAMs (Convection-Allowing Models). The
potential for rain showers and isolated thunderstorms will extend
northward into the North Bay by Wednesday morning and may linger
into Wednesday night before the upper level feature shifts inland.
Dry conditions are anticipated by Thursday afternoon in wake of the
exiting cut-off low. A gradual cooling trend, to near seasonal
averages, is expected late in the upcoming week and into the
following weekend as troughing lingers over the West Coast. Be sure
to keep up to date on the latest forecast information as we near
this upcoming pattern shift.
&&
.AVIATION...
(06Z TAFS)
Issued at 945 PM PDT Sun Sep 21 2025
VFR continues as of this writing with VFR probable to be the
dominant flight category at most terminals as overall wind
trajectories will tend to hinder stratus/mist/fog. The exception
will be KHAF and for 1-2 hours potentially KAPC and KOAK.
Confidence is highest that IFR to LIFR ceilings/visibility will
transpire at KHAF, but the timing is uncertain. If stratus does
invade KAPC and KOAK, the most likely timeframe would be just
prior to sunrise Monday. VFR should be the rule at all TAFs by
18Z Monday.
Vicinity of SFO...VFR is advertised, however, we`ll need to
monitor trends upstream across the San Bruno Gap. Current
indications are that stratus may approach the terminal, but the
latest model guidance indicates around a 15-20% chance for
MVFR/IFR cigs. Should stratus make it to the terminal, it`ll
likely be between the 13Z-19Z time frame. For now, I`ll advertise
a scattered deck of clouds around FL015.
SFO Bridge Approach...VAPS may be supported through the entire
TAF cycle and scattered clouds are more probable at the terminal.
Monterey Bay Terminals...A eddy/circulation has formed across
Monterey Bay which has pulled in a little dry air to result in
VFR at KMRY and KSNS. This is anticipated to be relatively short-
lived as satellite trends indicate re-development of IFR to near
LIFR ceilings across northern portions of the bay. Cigs are
forecast to fill back in after 07Z, though confidence in the
exact timing is low. LIFR cigs to intermittent LIFR visibility are
still advertised at KMRY, with IFR cigs at KSNS. VFR is probable
to return after 16Z. There are some indications that marine
stratus will make another run at KMRY and KSNS after 00Z Tuesday,
but have opted for a more optimistic given uncertainty in stratus
re-developing.
&&
.MARINE...
(Tonight through next Saturday)
Issued at 920 PM PDT Sun Sep 21 2025
Hazardous boating conditions are anticipated to persist through
the pre-dawn hours on Monday due to northwesterly wind gusts near
25 knots. This will translate to steep seas, especially for
waters near and north of Point Reyes. Northwesterly swell will
continue through the entire period with wave heights gradually
easing. Waters south of Point Pinos will be prone to 12 to 15
second period southerly swell, though wave heights will average
between 2 and 3 feet.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 3 AM PDT Monday for Pt Arena to Pt
     Reyes 0-10 nm.
Small Craft Advisory until 3 PM PDT Monday for Pt Arena to Pt
     Reyes 10-60 NM.
&&
$$
SHORT TERM...RGass
LONG TERM....RGass
AVIATION...Bain
MARINE...Bain
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NWS MTR Office Area Forecast Discussion