Scotts Valley, California Weather Forecast Discussion

375
FXUS66 KMTR 010822
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
122 AM PDT Wed Oct 1 2025
...New SHORT TERM, LONG TERM...
.KEY MESSAGES...
Updated at 1213 AM PDT Wed Oct 1 2025
- Beneficial rain today
- Warming and drying trend starts Friday
&&
.SHORT TERM...
Issued at 1213 AM PDT Wed Oct 1 2025
(Today and tonight)
Light rain showers are ongoing across the Bay Area ahead of a cold
front located about 200 miles west of San Francisco. This front is
connected to a mature, cut off low pressure system way up in the
southern Gulf of Alaska. This separation from the parent low is
causing a very slow forward progression of the cold front. A
ribbon of moisture on the latest water vapor imagery is already
showing signs of stalling. As a result, the prefrontal rain will
continue through the day. Fortunately the rain intensity looks to
remain light outside of a mid day shower or two in the North Bay.
The West-WRF ensemble shows, within 1 standard deviation, the IVT
will remain between 250-500 kg of water vapor per meter per
second today. Thunderstorm chances are less than 10% today. The
lapse rates are pretty weak (around 5-6 C/km). With the upper
level low so far removed from the surface front, there is a
notable lack of cold air aloft needed to create the necessary
buoyancy. Based on the 00Z sounding, the 850-500 mb lapse rate is
5.0 C/km, which is actually in the bottom 10th percentile for
this time of year. The 850 temp isn`t the problem (9.35 C, close
the bottom 10th percentile), but the 500 mb temp (-11.75C) is
close to the median for this time of year. Furthermore, the slow
motion of the cold front doesn`t support robust updrafts. Finally,
most of the current showers are below the 12,500 foot freezing
level, limiting the amount of ice particles necessary for the
charge buildup in thunderstorms. When all is said and done we`re
expecting up to 1" in the coastal mountains of the North Bay with
up to 1/4" across the Bay Area and 1/10" across the Central Coast.
With the rain spread out over the day, this is mostly a
beneficial event and will help put a damper on fire season.
&&
.LONG TERM...
Issued at 1213 AM PDT Wed Oct 1 2025
(Thursday through Tuesday)
The prefrontal rain will finally taper off by Thursday morning as
the moisture plume slides south. The associated 500 mb trough
will swing through Thursday-Friday. This will cause winds to shift
from SW to NW, and may support some convection each afternoon,
though the lack of moisture will be a major limiting factor for
thunderstorm development. This trough will bring lower surface
pressure to southern California. Meanwhile higher surface
pressure will build over the northern Rockies through the weekend.
The gradient across the pressure field will support some
northerly, offshore wind from Saturday - Monday. Typically these
winds pose a fire weather concern. Fortunately the antecedent
conditions and wind strength are mitigating factors. The PGE-WRF
ensemble shows the SFO-WMC gradient bottoming out around -2 to -6
mb. Enough for offshore winds, but not a particularly strong
event.
&&
.AVIATION...
(06Z TAFS)
Issued at 954 PM PDT Tue Sep 30 2025
A mix of stratus offshore of the Central Coast and showers to the
north over the Bay Area are visible on satellite/radar. Shower
activity is expected to spread north to south overnight and reach
the Central Coast by tomorrow afternoon. If a shower passes
directly over an airport temporary reductions in visibility and
lower ceiling heights may be possible. Low to moderate confidence
that CIGs will stay above 3000 ft at most airports tonight
(excluding HAF, MRY, and SNS) with the most recent guidance
backing off of MVFR CIGs across the Bay Area tonight. Winds
initially will be out of the south but are expected to become more
west to northwest throughout the day. Initial guidance is
indicating stratus for HAF, MRY, and SNS by late tomorrow evening
but confidence is low at this time.
Vicinity of SFO...VFR through the TAF period with some potential for
MVFR CIGs to develop tomorrow morning as showers move over the Bay
Area. Confidence is moderate that CIGs will remain VFR / above 3000
ft through the entire TAF period with recent guidance having backed
off of MVFR CIGs at SFO. Initial southerly winds will shift more
west to northwest by the end of the TAF period with gusts up to 21
knots possible during the afternoon/evening.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...MVFR tonight becoming VFR by late tomorrow
morning. Current showers are located well to the north of MRY and
SNS with satellite showing stratus developing offshore of the
Monterey Peninsula. MVFR-IFR stratus is expected at both SNS and MRY
tonight and is expected to clear by mid to late tomorrow morning.
Shower potential increases by the afternoon/evening hours with low
to moderate confidence that showers will directly impact either
terminal. Winds will initially be more southerly and shift to more
west to northwest by the end of the TAF period.
&&
.MARINE...
(Tonight through next Monday)
Issued at 954 PM PDT Tue Sep 30 2025
An upper level trough and associated surface cold front will
result  in scattered showers across the coastal waters now through
late  Wednesday. A moderate to rough northwest swell associated
with this  system arrives Wednesday night and will bring waves
between 10 to 11  feet across the outer waters. Wave heights will
diminish below 10  feet Thursday into Friday before building again
late weekend into  next week. Winds are expected to strengthen to
fresh to strong on  Friday which will result in a continuation of
rough seas and  hazardous conditions for small crafts into the
weekend.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 9 AM this morning 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...Flynn
LONG TERM....Flynn
AVIATION...Kennedy
MARINE...Kennedy
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NWS MTR Office Area Forecast Discussion