000
FXUS66 KMTR 120248
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
748 PM PDT Thu Apr 11 2024

...New UPDATE...

.SYNOPSIS...
Issued at 103 AM PDT Thu Apr 11 2024

Another warm day ahead today with temps trending cooler into the
weekend. Much cooler on Saturday with rain chances throughout the
region. Slight thunderstorm chances for coastal areas around Santa
Cruz and Monterey. Warming trend after the weekend into next week.

&&

.UPDATE...
Issued at 656 PM PDT Thu Apr 11 2024

Clear skies prevail as the sun sets this evening. Enhanced
radiational cooling and a reforming marine layer will bring low
clouds and some patchy fog early Friday morning. As the day
progresses, winds will shift to SW and increase to a strong
breeze, signaling the approaching weather system on Saturday. This
will be a pretty drastic swing from the warm weather we`ve had
this week. As an example, the high temperature at Walnut Creek was
87 degrees today and is expected to drop to 56 degrees by
Saturday.

The forecast remains unchanged. Some consideration was given to a
wind advisory for the upcoming storm. The mountains of San Benito
County and Big Sur coast are most at risk of reaching criteria,
with the Santa Cruz and East Bay mountains next in line. Given the
SW direction, the NW to SE oriented valleys (North Bay, Santa
Clara, and Salinas) will likely be sheltered from the strongest
gusts. Ensemble based probabilities are greater than 90% to
exceed 45 mph in the San Benito mountains late Friday into early
Saturday morning. The hang-up is low elevation wind, which isn`t
quite meeting criteria at this time.

&&

.SHORT TERM...
(This afternoon through Sunday Evening)
Issued at 1120 AM PDT Thu Apr 11 2024

Our main weather highlight will be in the short term as the next
storm system is forecast to slowly meander southward along the
California coast. After a week of above normal warmth, this
weekend will be cooler, wetter, and windier.

A pleasant afternoon will be on tap for the area with mostly sunny
skies, except along the immediate coastline where marine
stratus/fog persist. Temperatures this afternoon will range
between the 50s along the coast to the low to mid 80s in interior
sections of the Bay Area and Central Coast. The fog forecast is a
little tricky tonight, but at this time, guidance suggests more in
the way of a stratus intrusion for the coast and inland areas of
the North Bay, East Bay, and Monterey Bay regions. Visibility may
still fall down below 5 mile in spots, with chances of this
happening around 20-30%.

Friday will be largely rain-free for the area, with the exception
being perhaps some pockets of drizzle or spotty rain showers as
clouds build with more in the way of onshore flow. The clouds and
onshore flow will translate to temperatures being anywhere from 5
to 15 degrees lower than today (Thursday). Where clouds arrive
after peak heating. Highs will range between the low to mid 50s
along the coast to the 60s and low 70s elsewhere. With the
approach of the upper low, the surface pressure/low level height
gradients will strengthen. This will support a wind field
characterized by 15 to 25 mph southerly flow, with gusts ranging
between 30 and 35 mph. Gusts closer to 35 mph are more probable
across coastal and mountainous areas. For areas between Big Sur
and the Monterey Peninsula, there stands a chance (around 35%)
that a coastal jet capable of wind gusts of over 40 mph will
exist.

As the upper low slowly eases into the region, forcing for ascent
will support an extensive precipitation band that will invade from
the northwest. Model solutions continue to come into better
agreement regarding the timeline in which areas can anticipate
a HIGH probability (>70%) of the onset of rain showers at their
location.

-North Bay (Bodega Bay and Inverness Ridge)- 6-9pm Fri.
-North Bay SF Peninsula/western SC County - Midnight-3am Sat
-Western Monterey County/Big Sur Coast - Midnight-3am Sat
-East and South Bay and San Benito Counties- 3am-6am Sat

Overall, rainfall amounts with this system remain consistent with
this morning`s forecast package with rain amounts largely around
1/2 inch to 1 1/2 inches for most locales. Higher terrain across
Big Sur Country, the Santa Cruz Mountains, and some of the
Mayacamas will average closer to 2 inches. A few spots across the
Santa Lucia Range may get closer to 3 inches when all is said and
done, however, the probabilities of exceeding this remain under
20%. As such, there will be a continued risk for ponding, minor
flooding (especially across poor drainage and low-lying areas).
The threat for slides will also continue, especially down across
the Big Sur coastline where percent of normal rainfall over the
past couple of weeks remains in excess of 150%. Caution is urged
particularly across this area. In addition to the potential for
heavy rainfall, cooler air aloft associated with the main upper
low will support a risk for thunderstorms, especially south of the
San Francisco and East Bay regions where the chances for surface
based CAPE or fuel for storms have increased. The probabilities
for CAPE in excess of 100 J/kg now range between 50-60% (compared
to 20-30%) over the land areas as the upper low may be farther
east during the daylight hours on Saturday. With lapse rates
projected to approach 8 C/km, there will be a risk for showers
and a few storms, capable of mostly copious amounts of small hail
(1/4 inch). Again, this threat is greatest south of SF Bay and
East Bay regions. We`ll keep an eye on the potential for
waterspouts as well, given the presence of a couple of frontal
boundaries aiding in low level vorticity generation.

Not to be outdone, there`s a potential for light snow
accumulations (around 2-3") across the southern end of the Santa
Lucia Range at the highest peaks. At this juncture, significant
impacts are not anticipated.

Temperatures will tumble 10 to almost 15 degrees (more in some
areas) Saturday compared to Friday. Highs on Saturday will
struggle to get out of the 50s and 60s thanks to the cloud cover
and widespread rainfall.

&&

.LONG TERM...
(Sunday night through next Thursday)
Issued at 1213 PM PDT Thu Apr 11 2024

Drying conditions are expected to return by Sunday afternoon as the
mid/upper level low pushes further inland and away from the region.
This too will kick off a warming trend from Monday through next
Thursday with high temperatures expected to reach the upper 50s to
mid 60s near the coast and lower 70s inland. However, by Tuesday
broad troughing is expected over the Great Basin that will result it
the potential for offshore flow across the interior. Overnight lows
will generally be in the 40s region-wide much of next week.

&&

.AVIATION...
(00Z TAFS)
Issued at 446 PM PDT Thu Apr 11 2024

Currently VFR at all terminals. All terminals except LVK are
forecast to receive stratus overnight. Confidence is highest in
MVFR stratus being the result; however, there is a potential for
marginal IFR ceilings at MRY, OAK, SFO, SNS, and STS. Winds will
be breezy to gusty out of the southwest tomorrow afternoon ahead
of the approaching cold front. Rain chances begin to increase
towards the end of the TAF period with no on station precipitation
forecast until the next TAF cycle.

Vicinity of SFO...Currently VFR with breezy westerly flow. MVFR
stratus will impact the terminal in the mid-morning hours before
ceilings lift to become VFR by the afternoon. Winds will be gusty
out of the southwest by tomorrow afternoon ahead of the approaching
cold front. Rain chances increase towards the end of the TAF period.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently VFR with breezy northwesterly
flow at MRY and VFR with breezy westerly flow at SNS. Winds will
become light overnight. Both terminals are slated to see MVFR
stratus overnight before ceilings lift by the afternoon.

&&

.MARINE...
(Tonight through next Tuesday)
Issued at 446 PM PDT Thu Apr 11 2024

Moderate northwest winds will persist tonight before diminishing
Friday morning. Southerly winds then increase ahead of an
approaching weather system that will bring rain, thunderstorms,
stronger winds, and steep fresh swell from Friday night into
Saturday. Moderate to strong northerly winds return in wake of the
passing system by Monday and Tuesday. A moderate northwesterly
swell will traverse the coastal waters too.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM Friday to 9 AM PDT Saturday for
     Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos
     10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm.

     Small Craft Advisory until 3 AM PDT Friday for Pigeon Pt to Pt
     Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt
     Reyes 10-60 NM-Pt Reyes to Pigeon Pt 0-10 nm.

     Small Craft Advisory from 9 PM Friday to 3 AM PDT Saturday for
     Pt Reyes to Pigeon Pt 0-10 nm.

     Small Craft Advisory until 9 PM PDT this evening for Pigeon Pt
     to Pt Pinos 0-10 nm.

     Small Craft Advisory until 9 AM PDT Friday for Pt Pinos to Pt
     Piedras Blancas 0-10 nm.

&&

$$

SHORT TERM...Bain
LONG TERM....RGass
AVIATION...Sarment
MARINE...Sarment

Visit us at www.weather.gov/sanfrancisco

Follow us on Facebook, Twitter, and YouTube at:
www.facebook.com/nwsbayarea
www.twitter.com/nwsbayarea
www.youtube.com/nwsbayarea