Lucas Valley, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

232
FXUS66 KMTR 152318
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
418 PM PDT Tue Apr 15 2025

...New AVIATION, MARINE...

.SYNOPSIS...
Issued at 146 PM PDT Tue Apr 15 2025
Cooler conditions will continue through Wednesday, with a 20%
chance for isolated showers and a thunderstorm or two. Thereafter,
a warming trend is anticipated through the weekend.

&&

.SHORT TERM...
Issued at 146 PM PDT Tue Apr 15 2025
(This evening through Wednesday Night)

Short Term Key Messages
*Cool conditions continue
*10 to 15% chance for very isolated dry storms
*Patchy Coastal Drizzle

A stubborn marine layer today continues to gradually erode as
southerly flow envelops the region. A veil of mid to upper level
clouds continues to lift northward ahead of a cutoff upper low
parked to our southwest. These intermittent clouds will translate
to partly sunny skies across most areas this afternoon, with
little in the way of fanfare anticipated this afternoon. Afternoon
ACARs data (soundings from aircraft) from across the Bay Area
indicate that moisture is bifurcated vertically and this pairs
well with the aforementioned satellite discussion. This is
important because moisture is currently not aligned with the
location of the steepest lapse rates (at least not yet), which is
needed for convection. I don`t anticipate this to change this
afternoon or tonight across our region and as a result, the worded
forecast for this time period remain void of any rain or storms.

It`s highly probable that we`ll contend with another round of
marine stratus late tonight and into Wednesday morning as 925mb
flow remains largely onshore. Similar to this morning, the very
gentle ascent through the column, thanks to the upper low to the
south, will promote the development of drizzle where the marine
layer is deepest/most moist and across areas of terrain. The
largely onshore flow will promote cool conditions across the area.

As it pertains to the potential for high based convection, the
opportunity remains around 20%. Most model guidance remains in
agreement that a plume of moisture, characterized by precipitable
water values of at or above 0.75" will lift northward. In
addition, 700-500mb lapse rates of 8 C/km are also advertised. The
combination of the moisture and cooler air aloft will support
MUCAPE values of a few hundred J/kg. While this matches conceptual
models of infamous lightning events across the region, it appears
unlikely (though not zero) that there will be alignment of
sufficient moisture and steeper lapse rates in the vertical to
warrant anything more than a 20 PoP. Furthermore, a consensus of
higher resolution NWP and other tools don`t appear to necessitate
the messaging of a higher end threat. For now, we`ll continue to
monitor trends in satellite, radar, and supplemental upper air
observations.

The most probable area for convection (high based showers and/or
dry thunderstorms) will be across extreme southern portions of the
South Bay and into the Central Coast through the day on Wednesday.
The primary hazards, as is typically the case with high based
convection, will be for isolated lightning strikes and
gusty/erratic outflow winds over a confined area. The good news is
that proxies for fuel/vegetation volatility suggest that while
lightning-initiated fire starts will be possible, they should not
exhibit much resistance to control. Furthermore, the extended
forecast does not offer much in the way of any *big* wind events
should the coverage of dry thunderstorms exceed expectations.

&&

.LONG TERM...
Issued at 146 PM PDT Tue Apr 15 2025
(Thursday through next Monday)

Longer Term Key Messages
*Warming trend ensues
*Little to no precipitation forecast

The long term forecast appears very tranquil, with really the
primary item to note being a warming trend. Medium to long range
ensemble cluster guidance indicates that the long term pattern
will be largely governed by slight ridging over the Gulf of
Alaska. Downstream from this feature, zonal to slight troughing is
anticipated.

Ahead of this ridging across the Gulf of AK will be a longwave
trough that will slide southward through the Great Basin on
Saturday. As this feature slides southward, it`ll tighten the
surface pressure gradient between building surface high pressure
across Nevada and remnant troughing along the coast. These
offshore winds don`t appear terribly strong, but the 925mb winds
average between 20 and 25 knots and the tail end of our local WRF
does support some of this momentum mixing down toward the surface.
We`ll need to be mindful of any changes to the winds here. Otherwise,
it appears rain-free for the foreseeable future and as such, the
current extended forecast offers no meaningful PoPs.

&&

.AVIATION...
(00Z TAFS)
Issued at 418 PM PDT Tue Apr 15 2025

Generally VFR with mid- to high level clouds across the Bay Area and
interior Central Coast, with MVFR stratus persisting in the North
Bay and parts of the Monterey Bay region including at SNS. A robust
stratus feed from the ocean will likely keep the North Bay socked in
through the rest of the day, while high clouds over the Central
Coast inhibit stratus clearing at SNS. For the rest of the region,
MVFR-IFR stratus will start to build into the region overnight
through Wednesday morning, with moderate confidence that most
terminals will clear out later in the morning, save the western
North Bay and the western San Mateo Peninsula where MVFR stratus
persists through the end of the TAF period. Breezy southwesterly
winds will diminish through the evening, before resuming Wednesday
afternoon.

Vicinity of SFO... VFR with mid- to high level clouds through the
evening, before MVFR stratus returns overnight into Wednesday
morning. Moderate confidence on the clearing time on Wednesday
morning, but VFR conditions should return by 20Z. Breezy west-
southwest winds diminishing through the evening hours, before
resuming Wednesday afternoon.

SFO Bridge Approach... Somewhat lower confidence of MVFR
stratus over the approach path. Otherwise similar to SFO.

Monterey Bay Terminals... A difficult short-term forecast for the
MVFR stratus over SNS which the models aren`t capturing well, with
high clouds inhibiting any daytime clearing and satellite
observations of the same. Have opted to keep the ceilings over the
terminal for another couple of hours, but this is quite low
confidence and may need to be amended to reflect observed
conditions. For the rest of the region, VFR with mid- to high level
clouds persists through the evening, with stratus returning somewhat
early overnight. Moderate confidence in clearing by late Wednesday
morning. Breezy onshore flow (southwesterly at MRY, northwesterly at
SNS) diminishes this evening before resuming Wednesday afternoon.

&&

.MARINE...
(Tonight through next Sunday)
Issued at 418 PM PDT Tue Apr 15 2025

Southerly flow with light to moderate winds will remain through
early Wednesday. Northerly winds will develop north of Point
Reyes Wednesday, but southerly winds persist elsewhere. Northerly
winds will finally take over all areas by the weekend. Small to
moderate seas persist into the weekend.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&

$$

SHORT TERM...Bain
LONG TERM....Bain
AVIATION...DialH
MARINE...MM

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