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

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FXUS66 KMTR 250319

Area Forecast Discussion
National Weather Service San Francisco Bay Area
819 PM PDT Wed May 24 2017

.SYNOPSIS...A cooling trend can be expected through Friday as
high pressure weakens and the marine influence increases. Minor
warming is then expected during the upcoming weekend into early
next week as high pressure returns to the West Coast.

&& of 08:09 PM PDT Wednesday...No big changes made
to this evening`s forecast. Satellite continues to show marine
stratus along the central California coast. The Fort Ord profiler
indicates the marine layer is just above 2000 ft. Afternoon highs
were distinctly cooler across inland locations with many stations
at least 5 to 15 degrees cooler than Tuesday. Santa Rosa observed
one of the most prominent day-to-day changes: the high on Tuesday
was 84 but today it was only 64. The cooler temperatures were a
response to the departing ridge that had been parked over the West
Coast earlier this week, as well as a weak upper level low off
the California coast. The persistent onshore flow and overcast
skies helped to minimize the change in afternoon highs along the

Expect additional cooling through the end of the work week with
below normal temperatures for many locations. Coastal areas will
continue to see low clouds and experience highs ranging from the
middle 50s to 60s. Warmer weather will return for the holiday
weekend as another ridge builds over the West Coast with 90s
returning to the southern Salinas Valley as early as Sunday.


.PREV of 01:33 PM PDT Wednesday...Only minor changes
to the ongoing forecast as unseasonably cool weather prevails into
the weekend with a gradual warm up next week.

Synoptically speaking an upper low remains off the coast spreading
mid and high level clouds over the region. At the surface, onshore
flow continues to keep low clouds and patchy fog along the coast.
The upper low and onshore flow led to much cooler temps around
the region today. 24 hour trend shows some impressive 24 hour
drops, 10 to 15 degrees colder in some spots. At 1 PM Santa Rosa
is 13 degrees colder today than Tuesday.

Only minor tweaks were made to previous forecast. Night and
morning clouds will continue with a marine layer 1,500 to 2,000
feet into the weekend. Patchy fog and drizzle will be possible,
especially along the immediate coastline. Milder temps will be
possible above the marine layer the next few nights, but not a
very strong thermal belt. As for daytime highs, temperatures will
continue to cool through Friday. Interior locations will drop to
very cool temps for late May with highs in the 70s to lower 80s.
Friday`s forecast compare to climo is roughly 5-10 degrees below

Temperatures will begin to moderate and eventually warm over the
upcoming holiday weekend as high pressure builds over the region.
The warmest of the three days will be Memorial Day, but mainly
over the interior. Persistent low clouds/fog will keep
coastal/bay shoreline area cool. Highs on Monday will be 60s to
70s coast and 80s to lower 90s interior. Temperatures level off
next Tuesday and Wednesday.

Outside of some night/morning drizzle not forecasting any
noteworthy rain through the end of the month.


.AVIATION...As of 5:45 PM PDT Wednesday...It`s a chaotic stratus
and wind pattern presently with a steadily deepening marine layer
responding to ongoing lower level cooling on southerly flow along
the immediate coast. Much cooler Pacific air modified by coastal
SSTs in the 50s includes modified cooler air arriving from the
mid-high latitudes wrapping around a lower level circulation off
the CA coast. A mid level 500 mb height low approx 400 miles SW of
the Bay Area advances to the coast coinciding with a newly
developing weak surface low pressure center remaining nearly
stationary just west of the Bay Area Thu-Fri. As a result spotty
coastal drizzle or very light rain is possible, none indicated in
00z tafs presently, but will amend as needed.

The marine layer becomes more and more elevated tonight and Thursday.
The primary forecast challenge will be coverage and duration of
stratus ceilings. 925 mb and 850 mb cooling may reach the point of
mixing out the marine layer inversion almost entirely Thursday and
Thursday night.

Vicinity of KSFO...S-SW winds 10-15 knots til 06z this evening then
probably shifting back toward W-SW by later evening, timing is low
confidence. Coordinated with CWSU and we decided to delay the onset
of forecast stratus ceiling based on present local stratus coverage
and wind pattern. SFO-SAC pressure gradient is a healthy 3.6 mb thus
the stratus could suddenly shift inland, but the forecast strongly
hedges on the south wind component holding longer; tempo MVFR cig
06z-08z the MVFR prevailing into Thursday morning. VFR forecast from
17z Thursday through 06z Thursday evening.

SFO Bridge Approach...Similar to KSFO. Wind direction forecast is
low confidence from the Santa Clara Valley to SMB approach due to
recently strong 2 mb to 3 mb southerly gradient KSNS-KSJC; 5 min
metar obs show NW wind 10 knots at KSJC. 00z taf KSJC forecasts SE
wind developing by 03z.

Monterey Bay Terminals...Tempo MVFR cigs 00z-04z this evening then
MVFR prevailing tonight and Thursday morning. At least partial clearing
is likely by late Thursday morning depending if the marine layer
inversion persists.

&& of 08:09 PM PDT Wednesday...Low pressure off the
central California coast will bring southerly winds to the area
through Thursday. Local gusty winds can be expected Thursday
along the Big Sur and Monterey Bay coastline. Surface high
pressure further out in the Eastern Pacific is bringing northerly
winds offshore north of Point Reyes. This high will eventually
rebuild off the coast resulting in increasing northerly winds by
late this weekend.


     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm from 3 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 3 AM
             SCA...SF Bay until 9 PM



MARINE: Canepa

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