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

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FXUS66 KMTR 291732

Area Forecast Discussion
National Weather Service San Francisco CA
1032 AM PDT Mon May 29 2023


Issued at 300 AM PDT Mon May 29 2023

Scattered showers over the ocean on Monday, with the slight
chance for coastal drizzle. Temperatures will remain below normal
with stratus clouds moving inland each night and back to the ocean
during the day. Slight chance for temperatures to have minor
warming creep toward seasonal normal values by next weekend.


(Today and tonight)
Issued at 300 AM PDT Mon May 29 2023

On look at satellite imagery and it is easy to spot the center of
weather system around the Bay are just along the coastline, west of
the Golden Gate Bridge. This counter-clockwise rotating cinnamon bun
swirl is moving relative slowly and has brought southwestward
moving convection along its northern periphery to the
Sonoma/Mendocino border, as well as northeast moving showers to
the south along the Big Sur coastline. While there have been
reports of moderate sized drops that have caused people to use
their windshield wipers, it has failed to register much in the way
of accumulations. Over the last 24 hours, only sporadic gauges
have tipped to record 0.01", and most others, like that in the
East Bay were a result of drizzle.

According to the latest ensemble runs, the closed low at 500 mb that
is driving the current weather will continue to move south through
Monday and not begin to hook eastward until it gets south of the
Monterey/San Luis Obispo until Tuesday. This will result in
consistent scattered showers over the ocean on Memorial Day. If the
low takes a course closer to the coastline, it would increase
probabilities of light sprinkles from San Mateo down to Monterey.
However, strongest confidence is for it to drift far enough away
from the coast to confine the scattered shower activity to the
ocean; though the chance for coastal drizzle will remain,
particularly along the Big Sur coast. The HRRR is interestingly
enough showing a small swath of moisture leave the Sierra and
wrap toward the low that brings a weak band of light rain to mid-
to- southern Monterey County on Monday afternoon. There is low
confidence in this materializing, but if it does, it should be
light rain and short duration. Cool air that is wrapping around
the center of the low will keep temperatures below normal.


(Tuesday through Sunday)
Issued at 300 AM PDT Mon May 29 2023

By Tuesday, ensembles are showing the low retain its shape but
weaken into more of short wave at 500 mb heading into Wednesday. A
slight southwest-to-northwest ridge axis will try to develop as the
low moves toward the Four Corners. However, the long wave trough
just west of British Columbia will slowly sink southward along the
Pacific Coastline. While rain chances diminish Tuesday through
Thursday, the weak vertical vorticity associated with the trough
will keep a healthy marine layer present, stratus clouds along the
coast and the risk of morning drizzle at least at the coast, in the
realm of possibility through the week.

Over the course of Thursday through Sunday, the long wave trough is
expected to weaken, allowing upper level riding to behind to
develop over the Eastern Pacific. Cluster analysis is still
uncertain on how the ridge will develop, both in strength and
magnitude. However, highest confidence is that it could result in
some light warming around the Bay Area. While it does not mean
heat, it has the potential to at least gradually crawl toward
temperatures back near seasonal normal values by next weekend. And
if what cluster analysis comes true and the omega block over the
CONUS redevelops late over the weekend, it will be a continued
cool start to the month of June.


(18Z TAFS)
Issued at 1030 AM PDT Mon May 29 2023

Current satellite shows closed low continuing to rotate off the
central CA coast bringing in a stout feed of moisture into the
region. DZ and SHRA activity expected to be much less likely today
as opposed to yesterday. As the low moves to the southwest of the
area, should see more widespread clearing of the stratus today.
In terms of cig height, marine layer has been holding steady at
about 2500 feet and a moist layer down to about 1200 feet per this
morning`s sounding from KOAK. Breezy onshore flow this afternoon
and a return of stratus this evening with higher-than-normal
uncertainty across southern portions of the region due to
interaction from the low.

Vicinity of SFO...Expect mostly VFR through the day today with some
interaction from mid-level clouds associated with moisture wrapping
around the top of the low. Breezy onshore this afternoon. Stratus
return with MVFR cigs tonight by midnight through Tuesday mid-

SFO Bridge Approach...Similar to SFO.

Monterey Bay...Slow to clear this morning with a robust marine layer
still intact up to 2500 feet. Breezy onshore flow this afternoon
with clearing to VFR and some lingering mid-to-high clouds.
Uncertain regarding timing the stratus return this evening due to
interaction with the low that will be moving into south-central CA.


(Today through Saturday)
Issued at 1030 AM PDT Mon May 29 2023

A 9 to 10 second northwest swell with a 18 to 20 second southerly
swell prevails. Winds are mainly light to breezy, as the stronger
winds are being measured farther out to sea. Winds remain mostly
southerly today as the low meanders about off the coast.
Northwesterly winds will begin across the seas starting by Tuesday
afternoon as the low moves ashore. Increasing winds through
midweek create steeper wind driven waves.





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