Mount Hermon, California Weather Forecast Discussion

960
FXUS66 KMTR 281914
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1214 PM PDT Sun Sep 28 2025
...New AVIATION...
.KEY MESSAGES...
Updated at 209 PM PDT Sat Sep 27 2025
- Rain and thunderstorm chances return Monday
- Rain lingers through Wednesday
&&
.SHORT TERM...
Issued at 212 AM PDT Sun Sep 28 2025
(Today and tonight)
Impressively deep marine layer blanketing the Bay Area and Central
Coast. The depth is in the 3,000-3,500 foot plus range, which is
definitely on the upper end of marine layer depth before mixing
out. Given the depth, ample low level moisture, and persistent
onshore/southerly flow drizzle has been added to the forecast for
Sunday AM. Areas most likely to see drizzle will be the coast and
terrain upslope areas like the Santa Cruz Mts, Marin Headlands,
and Berkeley Hills. Already seeing a few bucket tips over the last
six hours in response to the drizzle. So what about marine layer
clearing today? It will be slow going and some coastal areas may
not clear at all. Monterey will be one of those places that will
struggle to see the sun. Given the slow clearing and lower 500mb
heights aloft temperatures today will be below seasonal averages
with highs in the 60s and 70s with a few isolated low 80s far
interior locations.
Other item of note for early today will be wildfire smoke. The
Moon Complex in OR produced smoke and the upper air pattern
filtered it down into the Bay Area. Morning air quality sensors
shows moderate air quality, which is likely due to the wildfire
smoke. Thankfully, latest guidance shows the smoke lifting
northward today due to southerly flow. For more info on air
quality in the Bay Area visit: baaqmd.gov
The much talked about pattern change begins to take shape Sunday
night into Monday. Do expect another marine layer push ahead of
an approaching storm system.
&&
.LONG TERM...
Issued at 317 AM PDT Sun Sep 28 2025
(Monday through Saturday)
Active weather returns for the long term as two system will bring
rain and thunderstorms back to the region. A few weather model
trends noted yesterday persist on tonight`s runs. Slower arrival
of precip, less in the way of precip overall, chances for
thunderstorms have gone down, and the large difference in precip
amounts over the North Bay to interior Central Coast.
System one will bring rain back the region early Monday morning.
Steady/stratiform rain will overspread the region early Monday as
the warm sector arrives. Rainfall will initially be over the
North Bay before slowly spreading S and E through the day. Once
the cold front arrives later in the afternoon there are still some
convective dynamics at play, but they`re looking less. Therefore,
will still have a mention of thunderstorms, but over the northern
waters only. The cold front pushes through Monday night into
Tuesday. Post frontal showers will linger through the day on
Tuesday, mainly north of San Jose. Let`s not forget that this
system still has a decent moisture tap with a plume of moisture
extending back to HI. As such, the North Bay could see a period of
moderate to locally heavy rain, especially if it goes more
convective. On the flip side, interior Central Coast may note see
any precip at all.
System two will quickly follow system one with its fropa during
the day Wednesday. This system will have no real convective threat
with it. However, just like system one it will have some decent
moisture. Some of the moisture will be the remnants of Typhoon
Neoguri. Therefore, a period of moderate to locally heavy rain
will be possible, especially the North Bay. Atmospheric River
guidance continues to advertise a period of solid IVT streaming
over the North Bay, wavering south to SF and then  shifting back
north. That means most of the precip will be north of Monterey Bay
region. Once again, the Central Coast will be on the drier side
of things. All precip chances ultimately come to an end on
Thursday afternoon as ridging begins to build in from the west.
Give the southwesterly flow with both systems rain shadowing will
occur in inland valleys like Santa Clara. On the opposite end
coastal areas that favor upslope flow could see higher precip
amounts like Mt Tam and Santa Cruz Mts.
Storm total rainfall Monday through Thursday continues to trend
down. 0.25-1" for the North Bay, 0.01-0.5" for the Bay Area and a
few hundredths to nothing for the Central Coast.
&&
.AVIATION...
(18Z TAFS)
Issued at 1214 PM PDT Sun Sep 28 2025
Tricky forecast this morning as a lot of model guidance did not
anticipate the MVFR cigs hanging around this long. Opted to trend
with the HRRR for many TAF sites, which was one of the most
pessimistic models. This would keep MVFR cigs at most sites through
the afternoon, with some hope of VFR conditions for a few hours.
Will need to monitor sites closely and tweak forecasts as needed,
especially if the MVFR cigs linger rather than lift. Tonight MVFR
cigs should return to all sites, with some guidance hinting at IFR
along the coast. Cigs should lift mid to late morning, as a weak
front arrives. Showers might be possible prior to 18Z, but
confidence remained to add VCSH or PROB30 to most sites.
Vicinity of SFO...Very tricky forecast here as satellite shows a
healthy surge of stratus flowing in from the south. This has kept
much of the SF Bay area terminals with MVFR cigs. Most guidance,
including ensembles shows a brief period of VFR cigs.
Unfortunately, if these were right, it would have happened
already. Trended towards the HRRR for the update which kept MVFR
conditions through 22Z. Confidence is medium to low on the stratus
breaking out or clearing for a bit this afternoon, with the
possibility of SFO seeing MVFR cigs through the tomorrow morning.
Will reassess at the 21Z update to see if conditions favor the
pessimistic forecast and satellite trends.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...The Monterey Bay region has a mix of MVFR
and VFR conditions being reported. Around KMRY and KSNS cigs remain
just above VFR status, though there has been some hints of MVFR cigs
hugging the terminals. Opted to go optimistic with the TAFs here, as
most models favored the current satellite trends giving us several
hours of VFR conditions. MVFR to possible IFR conditions return
tonight.
&&
.MARINE...
(Today through Friday)
Issued at 455 AM PDT Sun Sep 28 2025
Gentle to moderate southerly winds continue across the majority of
the coastal waters through late this week. An upper level trough
and  surface cold front will bring rain showers to the coastal
waters  Monday into early Tuesday with embedded thunderstorms
possible  across the northern waters. A second system will bring
additional  rain showers to the waters late Tuesday into
Wednesday. Seas become  moderate rough and build to 10 to 12 feet
mid to late week with  winds expected to strengthen again next
weekend.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
SHORT TERM...MM
LONG TERM....MM
AVIATION...Kennedy
MARINE...KR
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NWS MTR Office Area Forecast Discussion