Pacific Valley California Weather Forecast Discussion

606
FXUS66 KMTR 131146
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
445 AM PDT Mon Oct 13 2025
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 205 AM PDT Mon Oct 13 2025
- Widespread rainfall and thunderstorm chances today
- Post-frontal rain showers and thunderstorms tomorrow
- Warming and drying trend Wednesday through Saturday, unsettled
   weather returns Sunday
&&
.SHORT TERM...
Issued at 205 AM PDT Mon Oct 13 2025
(Today and Tuesday)
A developing gale force low pressure system off the
Oregon/Washington border will quickly dive down the West Coast today
with the help of a dip in the Polar Jet Stream. Rain showers are
underway north of our area with lightning remaining confined to
Oregon for the time being. During the beginning of this event, rain
showers are going to have a hard time making it to the surface. The
00Z October 13th KOAK sounding observed a precipitable water value
of 0.45 inches with a stout dry slot in the 900-750mb layer. The
latest aircraft sounding out of SJC observed a 0.593 inch
precipitable water value - this is likely due to mid-to-high level
clouds that have begun to stream into the region. Nonetheless, it`ll
likely be a virga show early this morning with perhaps an extra
beautiful sunrise. The morning commute should not be impacted by
rain or thunderstorms - the same cannot be said for the evening
commute. Clouds will lower through the morning with rainfall
increasing in coverage and intensity through the afternoon and into
the evening. Ahead of the cold front, strong southwesterly winds can
be expected with gusts up to 35 mph in the higher terrain of the Bay
Area and gusts of 35-45 mph in the higher terrain of the Central
Coast. Rainfall totals will be highly dependent upon where rain
showers/thunderstorms develop. Speaking of thunderstorms, the
critical ingredients of lift, instability, and moisture will all be
present. The low CAPE (500 Joules/kilogram with conditionally
unstable lapse rates of 7 degrees Celsius/kilometer) environment
will be capable of producing thunderstorms that will pose the
threats of lightning, nuisance flooding (particularly in urban areas
which are considered hydrophobic), small hail, erratic/gusty winds,
and waterspouts. Most of the rainfall expected with this system and
the best thunderstorm chances will be today. The Weather Prediction
Center has highlighted our area for a marginal risk (at least 5%) of
rainfall exceeding flash flood guidance which means that isolated
flash floods are possible, but that they would be localized and
primarily affect places that can experience rapid runoff with heavy
rainfall. As the surface low pressure system and it`s accompanying
cold front eject to the southeast late tonight, post-frontal rain
showers and thunderstorms will remain possible Tuesday. This system
is also going to be a cold one with the 850 millibar temperature
daily minimum record in jeopardy on Tuesday. At the surface, SJC is
forecast to jeopardize their daily low maximum temperature - the
official forecast is 63 degrees Fahrenheit with the record of 62
degrees Fahrenheit dating back to 1948, the period of record is
1893. Cold, clear, and calm conditions Tuesday night will yield two
hazards: near-freezing temperatures for far interior Monterey and
San Benito Counties east of Highway 101 and widespread fog. With
rainfall preceding this, conditions can be expected to feel colder
than what the thermometer reads - please take care of people, pets,
and plants. Additionally, patchy frost and black ice will be
possible within sheltered interior areas as well. Outside of these
areas, it will still be cold, so it may not be a bad idea to pull
the winter linens out.
&&
.LONG TERM...
Issued at 205 AM PDT Mon Oct 13 2025
(Wednesday through Sunday)
The upper-level low will slowly get nudged east by upper-level
shortwave ridging nosing in from the Eastern Pacific Ocean, this
will result in a brief warming and drying trend Wednesday through
Saturday. ECMWF and GFS are in agreement that longwave upper-level
troughing and rain will return Sunday. There`s a significant amount
of spread between their ensembles, but it is still worth mentioning.
&&
.AVIATION...
(12Z TAFS)
Issued at 445 AM PDT Mon Oct 13 2025
Expect quickly deteriorating weather conditions due to a potent,
deepening low pressure system from the north today. VFR-MVFR this
morning will lower to MVFR-IFR in heavy showers including a chance
of thunderstorms today. Winds strongest over the coastal waters
however gusty winds will also develop over land with a southward
moving cold front during the day, the surface front reaching the
north Central Coast early this evening. There`s plenty of cold air
aloft in contrast to sea surface temps in the upper 50s to lower 60s
resulting in plenty of destabilization and downpours, with low
pressure system dynamics and showers moving onshore; usually
northerly arriving systems (winter season) have ~ 50% precipitable
water the current incoming system has, in other words the system
today has a decent water vapor supply for a northerly system. Post
frontal showers and a cooler air mass may result in a few patches of
fog /IFR-LIFR/ otherwise low ceilings /IFR/ continue tonight and
Tuesday morning.
Vicinity of SFO...VFR then conditions lower to MVFR (possibly IFR)
at times in heavy showers. Not advertised in the 12z TAF however
will monitor radar and satellite for potential thunderstorm(s).
Southwest wind ahead of the cold front today shifts to westerly
23z-24z today with the cold front based on recent high resolution
model output. Lingering showers tonight and Tuesday morning.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR then conditions lower to MVFR-IFR in
heavy showers and a chance of thunderstorms later this morning and
through the afternoon. The cold front approaches the area by early
evening, downpours and thunderstorms chances continue up to and
during cold frontal passage. Southwest to south winds increasing
gusty today into the afternoon with a cold frontal passage resulting
in a wind shift to westerly by 02z-03z. Showers /MVFR-IFR/ continue
tonight and Tuesday morning due to cold air aloft and an unstable
atmosphere.
&&
.MARINE...
(Today through Saturday)
Issued at 412 AM PDT Mon Oct 13 2025
A deepening low pressure system with a surface cold front will
move southward across the coastal waters producing strengthening
winds as high as gales around frontal passage later today and
early this evening. By late morning, this system will also bring
heavy showers and a chance of thunderstorms. Thunderstorms could
bring localized gales, steep wind waves, reduced visibility and a
slight chance for waterspouts. Showers will persist Tuesday, but
will become more isolated while the winds decrease. Rough seas to
10 feet will persist into Wednesday.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 AM PDT this morning for SF Bay N of
     Bay Bridge.
Small Craft Advisory from 9 AM this morning to 3 PM PDT this
     afternoon for Mry Bay.
Gale Warning from 3 PM this afternoon to 9 PM PDT this evening
     for Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm.
Small Craft Advisory until 3 PM PDT this afternoon for Pigeon Pt
     to Pt Pinos 0-10 nm-Pt Arena to Pt Reyes 0-10 nm-Pt Reyes
     to Pigeon Pt 0-10 nm.
Small Craft Advisory from 3 PM this afternoon to 9 PM PDT this
     evening for Pt Pinos to Pt Piedras Blancas 0-10 nm.
Small Craft Advisory until 3 PM PDT Tuesday for Pigeon Pt to Pt
     Pinos 10-60 NM-Pt Arena to Pt Reyes 10-60 NM.
&&
$$
SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...Canepa
MARINE...Canepa
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NWS MTR Office Area Forecast Discussion