NWS Forecast Area Discussion - San Francisco Area

FXUS66 KMTR 292351

Area Forecast Discussion
National Weather Service San Francisco Bay Area
451 PM PDT Fri May 29 2020

.SYNOPSIS...A late season storm system will bring a chance of
showers and isolated thunderstorms later Friday through Saturday
afternoon. Weak wrap around showers possibly Sunday but otherwise
drier and seasonable through the next week after weak trough takes
over the coastal states.

&& of 02:30 PM PDT Friday... A compact but deep
late season storm system will navigate towards and then over the
greater San Francisco Bay area over the course of the next 24

A 565dm 500mb (and deepening) low pressure system has been looming
offshore just beyond the high pressure system that brought record
temperatures to the broader region earlier this week. This storm
system is making its final approach from offshore towards the San
Francisco Bay Area on a northnortheast trajectory early this
afternoon. Satellite imagery depicts several bands of altocumulus
spreading northeastward from the center of this storm system ahead
of the deeper moisture and instability expected to move ashore
later tonight and tomorrow. Numerous reports on social media also
confirm the presence of mid to high based altocumulus spreading
inland from the southwest today. These altocumulus bands (also
known as accus) are a sign of elevated moisture and instability
and typically serve as a precursor to deeper elevated convection
in the coming hours. KMUX radar spun into action in anticipation
of this even earlier today and has now begun to light up with the
first indications of moderately deeper convective development
approximately 60 to 150 miles off of the coastline from Marin
southward through Monterey. Furthermore, lightning detection
networks are now alerting us to the fact lightning is nearing,
with the nearest strike as of 145pm approximately 125 miles
westsouthwest of Half Moon Bay. For what its worth, this system
has been fairly active in terms of lightning both in its outer
band on its northern flank as well as near the core of the storm.
Otherwise, todays temperatures may feel abnormally warm given the
close proximity of a storm system to the area however it is not
unusual for mild conditions to prevail in the warm sector of a
storm arriving from the southwest.

A week ago, this storm system was the major wildcard looming after
the previous week`s heat up, with models struggling to understand
how it would impact our area. All that has changed since the end
of the heat wave, with every short, medium, and long term model
set in a high level of agreement of how this storm system will
play out.

First, increasing altocumulus bands manifest from the southwest
as elevated moisture and instability advect inland well ahead of
the storm... which has already occurred.

Next, southerly winds will pick up through the afternoon and into
this evening especially along the coast and higher elevations
with peak gusts from 35 to 50 mph possible... which is actively

Finally, the main phase of this storm system will come as a 1 2
punch, with the initial hit coming from moisture and energy
suspended at a higher level in the atmosphere late this afternoon
through late tonight, and the secondary hit coming from more mid
to surface level convection from sunrise Saturday through the mid
Saturday afternoon.  Forecast models suggest that the initial
elevated convective bands/showers set to arrive within the next
few to several hours will struggle to produce precipitation at
the surface and will instead generate predominately virga (though
some rain is possible) given the dry air mass in the mid levels
beneath the base of this storm. Due to this, there is a potential
for isolated dry lightning strikes to occur later today throughout
tonight. The good news though is that the recent marine layer
surge helped to resaturate some of the finer fuels such as
grasses after recent days of well above normal temperatures dried
them out. Furthermore, some of the medium to larger fuels are not
too terribly dry either after other late season storm systems
brought some needed rain/moisture to the region. The fuel factor
alone should help abate fire risk for some areas, but for other
areas deeper inland less exposed to the marine layer or recent
rains, dry lightning strikes may allow for some limited wildfire
activity to occur.

By early Saturday morning, the low and mid levels will begin to
saturate with moisture and potential energy as the core of the
upper low shifts directly over the Bay Area. This will kick off
surface based convection with a higher probability of producing
more scattered to widespread showers with wetting rains that reach
the surface. Short to mid term model guidance has consistently
indicated that the bulk of these showers will be aimed over the
North Bay and areas even farther northward (Eureka/Gold Coast) and
northeastward (Northern Sacramento Valley/Sierra Nevada). Those
areas could see 1-2" from convective precipitation while closer to
home, we are anticipating significantly less given we are where
many showers may spawn but not mature. The North Bay will see up
to around 0.25" while areas south of the Golden Gate will see a
few hundredths up to around a tenth of an inch. Inland portions of
the East Bay, South Bay, and Central Coast may be blanked out
from any precipitation at all as this system looks to focus on the
coast and northern portions of the San Francisco Bay Area.
Precipitation amounts will not be uniform given the hit or miss
nature of convective showers, meaning some parts of a community
could see little to no rain while others fall under brief heavy

Looking ahead, our region will transition into the silent sector
of the upper low by mid to late afternoon if not as late as early
evening. This will cause shower activity to dwindle rapidly as the
bulk of the activity shifts to the north and east of home. Wrap
around moisture could bring a few more stray, weak showers Sunday
but the impacts will be minimal.

A broad trough will then sag over the Pacific coastal states and
dictate the weather from Sunday through much of next week,
bringing generally seasonable weather to the region.  Some
uncertainty regarding temperatures mid week next week exists as the
longer term models are moving out of phase -- so something to keep
an eye on moving forward.

&& of 4:50 PM PDT Friday...For 00Z TAFs. Mostly VFR
conditions, but MVFR conditions are seen at select terminals due
to the movement of a low pressure system moving toward the Bay
Area from the SW. More widespread MVFR through the evening. Winds
remain out of the south- southwest ahead of the weather system,
and producing some strong gusts associated with it. Models
consistently showing shower activity after 06-08z and continuing
into the night. Isolated t-storms across the region are possible
early Saturday morning. Lingering showers are expected as the low
passes through for wet runways as cigs lift in the afternoon.
Winds will mostly remain out of the south, but slowly transition
to the northwest behind the system Saturday night into Sunday.

Vicinity of KSFO...Stronger south-southwest winds producing a few
stronger gusts at the terminal. Increasing mid and high clouds
continue to move in ahead of the next system. Showers expected
around and after 08z. Cigs are currently VFR, but with the influx
of moisture, expect MVFR overnight tonight. The strong low level
southerly winds should keep cloud bases above 1200 feet. Cigs lift
tomorrow while winds remain out of the south. Shower activity
through the morning could make for wet runways.

SFO Bridge Approach...Potential for reduced visibility Saturday
morning on approach if showers present.

Monterey Bay Terminals...VFR/MVFR as mid-high level clouds from
the low pressure system move onshore. MVFR conditions expected
overnight with showery activity present. Winds will remain out of
the south and ease slightly in most locations, but can remain
stronger in the Salinas Valley. Cloud cover remains on Saturday,
but lift to VFR in the afternoon with breezy southerly winds

&& of 1:51 PM PDT Friday...Low pressure west of Big Sur
is producing thunderstorms over the offshore waters. The threat
for showers and thunderstorms will increase tonight through early
Saturday. As the low moves towards the coast some southerly gale
force winds will occur near Pt Pinos and locally into Monterey
Bay. Gusty west winds from the Golden Gate out into the West Delta
will create rough seas inside the Bay through tonight. A long
period south swell is also passing through the waters and may
create dangerous surf conditions. The low will shift north of the
area by Saturday night and Sunday with a return of nw wind and


     .Tngt...SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 AM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 3 AM
             GLW...Mry Bay until 9 PM
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm until 9 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 9 PM until 3 AM
             SCA...SF Bay until 9 PM




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NWS MTR Office Area Forecast Discussion