Small Craft Advisory 39, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
426 PM PST Wed Mar 2 2022

.SYNOPSIS...Low clouds return tonight. Unsettled weather will
impact the area Thursday into Saturday, though most precipitation
is expected to arrive during the day tomorrow, with hit or miss
showers Friday and Saturday.

&&

.DISCUSSION...as of 02:00 PM PST Wednesday...A change in the
weather pattern is underway across the region today as a 580dm
500mb high pressure ridge (now centered over SoCal/Desert
Southwest) is gradually replaced by an approaching 552dm Gulf of
Alaska trough. This change was evident as early as sunrise this
morning as scattered to broken high clouds saturated the region
and a thick fog bank enveloped the San Francisco Bayshore and
nearshore valleys. As of noon, temperatures are down a few to
several degrees where the fog bank developed across the Bay Area
while observations from the Central Coast are similar, if not
slightly higher, than yesterday's readings at the same time given
their proximity to the departing ridge. For the rest of the day,
expect minor additional warming to push nearshore locations into
the 60s, interior locations into the 70s, and a the warmest
southern locations into the low 80s, or roughly 8 to 16 degrees
above normal.

For tonight, HREF indicates a 80% probability of the return of
low clouds associated with marine stratus, this time also
including the entire coast to the Monterey Bay. The local WRF
backs this up giving increased confidence. Given a transition
towards lower pressure it is likely to see cloud bases lift at
least a few hundred feet to 800-1200 feet versus the fog bank that
developed this morning.

There have been some changes to the forecast regarding the
upcoming storm system. First, ensemble model systems and
deterministic hi- res models continue to slow down the arrival of
this storm system; it is now expected to arrive early Thursday
morning in the North Bay rather than tonight. Second, the
trajectory of the storm system has changed enough to drastically
alter the forecast precipitation amounts (more so offshore and
pivots in an unfavorable location). Finally, breezy to gusty
northerly winds of 25 to 40 mph are expected to arrive in the post
frontal environment Friday afternoon. Consequently, the afternoon
forecast package has been adjusted to account for these changes.

So, with these changes, how do the next few days look?

The low pressure trough descending from the Gulf of Alaska will be
driven by two discrete vorticity waves that will coincide with two
periods of wetter weather between tomorrow and Saturday.

The first of these vort pulses is driving the trough to dig
southward from the Gulf of Alaska on the heels of an atmospheric
river currently impacting the Pacific Northwest. As this trough
descends from the Gulf of Alaska it will advect the remnant
0.85 - 1.15" TPW atmospheric river precipitable water plume
southward towards the broader Bay Area between tonight and
tomorrow morning. The surface cold front out ahead of this feature
will then move into the coastal North Bay through Thursday
morning and bring light to locally moderate precipitation there
before stalling out through the tail end of tomorrow morning.
This stall occurs as a result of the vort max descends into the
base of the trough, causing the trough to reorganize into a quasi
cut off low and weaken it briefly. Areas of the interior North Bay
and East Bay look to suffer the most in response to the
reorganization of the trough as latest data suggestions
precipitation amounts falling to as low as 0.01 to 0.05" for these
areas.  At this point, the cold front will begin to fracture and
dissipate as the the trough axis pivots ashore and move into the
South Bay/Central Coast area tomorrow afternoon/evening. Not all
is lost as the trough becomes negatively tilted (enhanced
dynamics) as this occurs, increasing likelihood of rain showers
Thursday night into Friday morning. Northerly winds will
strengthen in the post frontal environment through the day on
Friday and result in breezy to gusty winds across the region,
roughly on the order of 25 mph to 40 mph, though locally windier
spots, such as KSFO and the higher terrain could see gusts to 50
mph or higher. A secondary, cooler vort wave will drive a second
pulse of showers later Friday into early Saturday that could lower
snow levels to 2500 ft and bring light accumulations to higher
peaks.

New storm total rain amounts are roughly as follows:
Bay Area Coastal ranges:     0.25 - 0.50"
Interior North Bay/East Bay: 0.01 - 0.10"
San Francisco Bayshore:      0.10 - 0.25"
Central Coast Shoreline:     0.15 - 0.35"
Interior Central Coast:      0.10 - 0.20"
Santa Lucias/Big Sur:        0.35 - 0.80"

&&

.AVIATION...as of 4:26 PM PST Wednesday...For the 00Z TAFs. There
are a couple of areas of fog /VLIFR-IFR/ along the immediate coastal
Bay Area otherwise it's VFR. Gradual cooling develops tonight and
Thursday since a low pressure trough will arrive from the west. The
lower level temperature inversion will weaken and lift overnight into
Thursday, showers developing Thursday.

Vicinity of KSFO...Mainly VFR, showers developing Thursday. West wind
near 10 knots, decreasing to light and variable wind tonight. West to
northwest wind 10 to 15 knots Thursday.

KSFO Bridge Approach...

Monterey Bay...VFR, light onshore winds becoming variable tonight
and Thursday morning. Showers developing Thursday.

&&

.MARINE...as of 4:13 PM PST Wednesday...A cold front will cross
the waters on Thursday with near gale to gale conditions in its
wake. Look for steeps seas Thursday through Friday. Meanwhile a
moderate period swell will slowly subside in the waters, while a
longer period swell will build late in the week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...NONE.

&&

PUBLIC FORECAST: DRP
AVIATION: Canepa
MARINE: DK/BFG

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