Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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832
FXUS66 KMTR 130603
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1103 PM PDT Sun May 12 2024

...New AVIATION, MARINE...

.SYNOPSIS...
Issued at 146 PM PDT Sun May 12 2024

Slight decrease in daytime highs today and tomorrow as a result of
weak troughing. Temperatures warm little into the mid week as
ridging resumes. Coastal stratus will continue to bring overcast
skies for coast and bay shorelines in the early portions of the
day, then clearing in the afternoons. Hints at a cooling trend in
the late week.

&&

.UPDATE...
Issued at 752 PM PDT Sun May 12 2024

Stratus remains widespread over the Pacific this evening and is
pushing inland through the Petaluma Gap, through the Golden Gate,
across much of San Francisco, and southern/eastern portions of
Monterey Bay region at this hour. This is as the marine layer
continues to deepen per the Fort Ord profiler and is now between
1,400-1,600 feet in depth. As such, look for stratus to continue to
penetrate inland into the North Bay, East Bay, South Bay, and
Salinas valleys overnight. Stratus will gradually retreat back to
the coast by mid-to-late morning on Monday. The forecast remains on
track and no updates are anticipated at this time.

&&

.SHORT TERM...
(This evening through Monday)
Issued at 146 PM PDT Sun May 12 2024

Its a quiet day, with a relatively quiet forecast in store, so lets
have some fun with this morning`s discussion and learn a little bit
about why we saw the cloud cover we did near the coast.

The 12Z sounding from KOAK this morning revealed a strong inversion,
a condition in which air temperature rises with increasing height
instead of falls, as well as a deeply saturated layer reaching just
shy of 950mb beneath this inversion layer. What is 950mb you ask?
Well, as you go higher in the atmosphere, pressure begins to fall.
950mb simply describes in the atmosphere a certain pressure level.
However, the actual altitude at which particular pressures are
found is always changing as a result of the atmosphere
continuously evolving as troughs and ridges build and decay. But
enough about that, lets move on.

If you were to glance at satellite imagery this morning, you would
have seen abundant cloud cover near the coast, as well as it
penetrating further inland into coastal valleys and bay communities
than in mornings previous. This cloud cover is represented by the
saturated layer within the sounding where the temperature and
dewpoint lines meet, with the inversion acting as a lid or a cap on
top, limiting the altitude to which these coastal clouds could
reach. A glance at the Ft. Ord profiler which examines the depth and
altitude of the marine layer shows the maximum altitude of the
marine layer reaching only to about 1400` approximately. Therefore,
if you live at a higher elevation near the coast, say 1500` and
above, you did not see overcast skies this morning from the marine
stratus, but rather, probably looked down on a blanket of grey and
white below. Meanwhile, coastal communities lying below 1400` feet
or so were enveloped in clouds, leading to grey skies. However, at
this time, coastal cloud cover has begun to recede as daytime solar
heating helps to warm air near the surface and the clouds. As this
warmer air rises, it will help to sap moisture, or humidity, from
the saturated layer, aka the clouds, and effectively evaporate them
until clear skies are achieved.

So what does this all mean in short? We saw extensive cloudy and
grey conditions near the coast, bays, and even reaching into coastal
valleys this morning as a result of total saturation being reached
in the lower levels of the atmosphere. These clouds reached their
maximum altitude at about 1400` before they hit a cap and could not
lift any higher. As the sun rose and began to warm the Earth, clouds
began to dissipate as warmer air was mixed into them and helped to
evaporate or rid the lower levels of moisture. As this warming
continues, cloud cover will begin to recede, leading to clear skies
for much of the region.

So there you have it! Perhaps you just wanted to hear what the
temperatures were going to be today, or what the *potential* aurora
viewing conditions might be tonight. Instead you got an unsolicited
meteorology lesson. Well, don`t fret, we`ll cover those other things
too.

Warm temperatures continue today, though will be just a few degrees
cooler than what was seen yesterday as weak troughing aloft begins
to build into the region. Expect daytime highs in the mid to lower
80s today for interior regions, and upper 50s to low 60s along the
coast. Tomorrow, those daytime highs drop to upper 70s to very low
80s, and mid to upper 50s, respectively. As this troughing regime
begins, this should allow the marine layer to deepen and for
coastal stratus to push a bit further inland tonight as air rises
and is able to surpass coastal mountains and elevated terrain
easier. That being said, if you`re looking to do some aurora
scouting, you`re going to need to go higher up in elevation, and
further inland. For more on geomagnetic storms and aurora
forecasts, please visit the Space Weather Prediction Center`s
website at spaceweather.gov. Or visit their Twitter/X account
@NWSSWPC.

&&

.LONG TERM...
(Monday night through next Saturday)
Issued at 146 PM PDT Sun May 12 2024

High pressure, or ridging, will begin to build towards
Tuesday, allowing temperatures to begin to warm up once again.
However, with the main ridge axis staying out over the northern
Pacific ocean, we are not likely to see temperatures warm
significantly, and likely not become as warm as we saw them Friday
and Saturday of this past week. For interior regions of our area,
daytime highs will likely be in the low 80s, topping out into the
mid 80s. Coastlines and bay shores will see high temps in the upper
50s to low 60s as onshore winds help to keep things cooler at the
coast. Minor heat risk for the most sensitive populations will be
present through much of the week. Stratus is expected to bring
cloudy conditions at the coast most mornings, then generally
clearing into the afternoons each day.

Towards the late week and next weekend, the end of the
forecast period, it appears the ridge begins to break down, and
troughing begins with decent agreement, though not perfect, amongst
models, hinting at a bit of a cool off for most locations. All
for now.

&&

.AVIATION...
(06Z TAFS)
Issued at 1059 PM PDT Sun May 12 2024

Currently seeing a mix of VFR-LIFR at the terminals. The marine
layer continues to deepen thanks to a coastal trough with coastal
profilers putting the depth of the marine layer somewhere in the
1,400-1,600 foot range. As this deeper marine layer is
subsequently allowed to penetrate further inland, all terminals
will be impacted by a reduction in flight category overnight.
Widespread VFR and breezy onshore flow will prevail by tomorrow
afternoon.

Vicinity of SFO...Currently VFR with breezy westerly flow. IFR-MVFR
stratus will impact the terminal overnight before scattering out by
late-morning tomorrow. Winds will remain breezy out of the west
through the TAF period. MVFR stratus looks to return in some form
tomorrow night; however, confidence was not high enough to include
it as a definitive ceiling at this time.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently IFR with westerly flow at MRY and
IFR with northwesterly flow at SNS. IFR conditions and light and
variable winds will prevail overnight. Conditions briefly improve to
VFR by late morning with a quick reduction right back down to IFR by
tomorrow evening.

&&

.MARINE...
(Tonight through next Saturday)
Issued at 1059 PM PDT Sun May 12 2024

Fair weather and stratus continues for the waters of Central
California through the week as high pressure builds over the
Eastern  Pacific. Fresh northwest to northerly winds continue
through mid- week before winds diminish heading into the weekend.
Across the  northern outer waters, fresh to strong gusts will be
possible  through Wednesday. Seas build to 12 to 15 feet by
Tuesday before  gradually abating through the remainder of the
week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 3 AM PDT Monday for Pt Pinos to Pt
     Piedras Blancas 0-10 nm.

     Small Craft Advisory from 3 PM to 9 PM PDT Monday for Pt Pinos
     to Pt Piedras Blancas 0-10 nm.

     Small Craft Advisory until 9 AM PDT Tuesday for Pt Arena to Pt
     Reyes 10-60 NM.

     Small Craft Advisory from 9 PM Monday to 9 AM PDT Tuesday for
     Pigeon Pt to Pt Pinos 10-60 NM.

&&

$$

SHORT TERM...AC
LONG TERM....AC
AVIATION...Sarment
MARINE...Kennedy

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