Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service San Francisco Bay area
1109 am PDT Friday Mar 27 2015

Synopsis...a return of coastal fog and light onshore flow will
mean cooler temperatures today...especially near the coast.
Onshore flow will weaken on Saturday... and result in mostly clear
skies and warming temperatures for Sunday. Cooler temperatures are
then expected beginning on Monday. Dry weather to persist through
the forecast period.

&&

Discussion...as of 08:59 am PDT Friday...short term concerns this
morning will be fog. Increased onshore flow and low level moisture
has led to a shallow marine layer. Profilers and sodars indicate
a marine layer depth of a few hundred feet. The shallow nature has
also produced some patchy dense fog...especially along the
immediate coastline and locally inland to both bays. A handful of
sites are currently reporting visibilities less than one quarter
of a mile.

Burn off of the clouds/fog this morning will be gradual despite
the shallow nature thanks to increased onshore flow (sfo-SAC
gradient is up).

That being said...some patchy low clouds may linger along the
coast through the day...but much of the Bay area will be mostly
sunny today. Despite ample sunshine the cooler air and increased
onshore flow will lead to a noticeable cool down today. The 24
hour temperature change will be as much as 12 degrees colder than
yesterday. However...despite the cool down it should still be a
pleasant day with highs in the 70s and 80s inland and 60s along
the coast.

Forecast is on track and no update is needed.

&&

Previous discussion...as of 3:03 am PDT Friday...Saturday should
see similar temperatures with 60s and 70s most areas...and up to
the middle 80s far inland areas. A weak ridge of high pressure will
then build over the West Coast on Sunday for slight warming. By
early next week...however...a trough is prognosticated to brush across
the West Coast including northern California. This will bring
cooler temperatures to the district by Tuesday and
Wednesday...with highs expected to be in the 60s and 70s. Rain
chances with this trough will remain well to the north. Zonal flow
aloft to maintain across the West Coast through the remainder of
the forecast period.

&&

Aviation...as of 11:10 am PDT Friday... a shallow stratus deck
pushed onshore overnight which led to pockets of LIFR visbys and
ceilings along the coast and through inland valleys through this
morning. That initial overnight push inland has now retreated this
morning.

Onshore flow is expected to strengthen and become breezy/gusty
through the day as a weak system passes to a north and pressure
gradients tighten. Peak wind gusts 20-30kt posbl. GOES-west cloud
thickness imagery shows the stratus deck along cali shoreline
thickening from 300-500ft to over 2000ft in the vicinity of pt
Reyes and the Bay area. Latest visible satellite imagery shows
this deepening layer may make another push onshore with stronger
onshore flow aloft... at the same time that the shallower level
layer is retreating. Thus... can not rule out MVFR ceilings returning
into the Bay area prior to 00z.

Latest WRF suggests an early return of stratus...around 00z...in a
Summer like scenario. Confidence on this timing is low to
moderate. However expect stratus to return no later than 04z for a
majority of taf locations (excluding klvk and posbly ksjc).

Vicinity of ksfo... breezy onshore flow today with peak gusts
25-30 knots. VFR ceilings through 00z... then deteriorating ceilings. Slight
chance of early stratus return prior to 00z.

Confidence is low to moderate.

Sfo bridge approach...similar to ksfo.

Monterey Bay area terminals... onshore flow with VFR conds into
the afternoon... then deteriorating ceilings beginning around 00z.
Slight chance of early stratus return prior to 00z.

Confidence is low to moderate.

&&

Marine...as of 10:42 am PDT Friday...northwest winds are forecast
to increase significantly over the coastal waters today as a dry
system passes to the north. Strong north winds are expected to
generate steep fresh swell through the weekend and possibly into
early next week. Mixed swell through the period.

&&

Mtr watches/warnings/advisories...
... Arena to pt Reyes 10-60 nm from 2 PM
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm from 2 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm from 5 PM
Small Craft Advisory...Pigeon pt to pt pinos 10-60 nm from 5 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm from 5 PM
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm from 2 PM
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm from 5 PM
Small Craft Advisory...sf Bay north of the Bay Bridge from 2 PM
Small Craft Advisory...sf Bay south of the Bay Bridge from 2 PM

&&

$$

Public forecast: mm
aviation/marine: drp

Visit US at weather.Gov/sanfrancisco

Follow US on facebook, twitter, and youtube at:
www.Facebook.Com/nwsbayarea
www.Twitter.Com/nwsbayarea
www.Youtube.Com/nwsbayarea

National Weather Service Glossary of Abbreviations