Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service San Francisco Bay area
1047 PM PDT sun Sep 21 2014

Synopsis...two more days of near normal temperatures and coastal
low clouds in store for the forecast area. A Pacific storm system
will bring rain to our area by midweek.

&& of 8:32 PM Sunday...the final low light visible
satellite image was showing low clouds banked along the coast and
already pushing into the in land valleys. According to The Fort
Ord profiler the marine layer is running right around 2500 feet
deep which supports the 0000z koak sounding also indicating a 2500
foot deep marine layer. Steady onshore flow will allow low clouds
to spread deep inland overnight tonight. Have updated the forecast
to show slightly more cloud cover and fog coverage. The deep
marine layer and associated low clouds should maintain mild
overnight temperatures in the middle 50s to low 60s in most

The 1200z European model (ecmwf) and 1800z gfs40 have initialized well with the
current synoptic pattern. The two models remain in fairly good
agreement through the up coming work week. Both advertising an
unseasonable deep Pacific storm system reaching the central coast
by middle week and sticking around into at least Friday morning. Over
the past few days the forecast models have been very consistence
with this system and our official forecast reflects that. At this
point no further updates are needed on the current forecast
package. For additional information on this approaching storm
system please see the previous discussion below.

Previous of 3:00 PM PDT Sunday...another round of
sunny skies inland with widespread clouds along the coast. In
fact, many coastal spots have stayed cloudy all day. Even a good
portion of San Francisco have not had any breaks from the clouds.
Marine layer stands over 2000 feet with the onshore gradient
around 2.5 mb from the west. This will help to bring clouds
quickly back into our County Warning Area overnight.

Quiet weather can be expected for Monday and Tuesday as a long wave
trough slowly advances from the Pacific while the ridge continues to
progress to the east. Surface flow should remain generally
westerly along with the marine layer sticking close to 2,000 feet.
Highs will vary little each day from today's readings. Generally
middle 60s to middle 70s at the coast with middle 70s to middle 80s inland.

Things get more interesting for Wednesday and Thursday as a
Pacific storm system and associated cold front move into the pacnw
down to our region. Model guidance remains fairly consistent with
the timing and brings the leading edge of rain into the North Bay
by Wednesday afternoon and then into sf Bay region to around Santa
Cruz Wednesday night into early Thursday. One encouraging sign is
model trends have been a bit wetter compared to solutions from the
past few days, so probability of precipitation were increased for almost all spots north
of Monterey County. Some instability is also noted Wednesday night
into Thursday, so at this time thunder cannot be ruled out. The
good news is any storms that produce lightning should be wet which
will help to lessen the fire weather risk. Thunder was left out of
the grids for now, however once the higher resolution runs come in,
it may be added in. Still a little early to come up with specifics
for rainfall amounts, however based off both the operational runs
plus the ensembles, looks like parts of the North Bay could easily
pick up more than 1/2" of rain. For urban spots around the Bay
right now closer to 1/10"-1/4" looks more likely. For Santa Cruz
County southward generally less than 1/10" can be expected
although a stray shower or two could bring locally higher amounts
in the Santa Cruz Mountains. To give you an idea how much rain we
would need to catch up to normal, San Francisco is nearly 22"
behind where it should be looking back to the start of 2013!

Although rain chances will be on the decrease for the second half
of Thursday, a 120 knots jet stream is forecast to be overhead while
the long wave trough axis is still to our west. Due to that kept
showers going a bit longer than quantitative precipitation forecast is suggesting in case we get
some Post frontal activity into Thursday night and maybe even
Friday. Similar to Wednesday night into Thursday, the best chance
for rainfall will be for the northern third of the County Warning Area.

For the weekend into next week, there is less agreement between
the models with the progressive nature of the trough. Some of them
have the trough advancing to the east over the weekend with a ridge
of high pressure building in while others have an associated upper
level low becoming nearly stationary near our County Warning Area. In the case of
the second scenario, moisture would wrap around the low and
possibly move back into our County Warning Area by Sunday. Due to the large
disagreement, for now will keep a generally dry weekend/early next
week forecast going with seasonable temperatures.

&& of 10:30 PM PDT Sunday...deep marine layer allowing
good inland penetration of stratus. Unlike today there should be
clearing Monday afternoon at sfo and Oak as 500 mb heights rise
leading to a compression of the marine layer.

Vicinity of ksfo...MVFR ceilings through 18z.

Ksfo bridge approach...same as ksfo.

Monterey Bay area terminals...MVFR ceilings through 18-19z.


Climate...autumn officially begins (autumnal equinox) at 7:29 PM
PDT on Monday September 22.

&& of 08:11 PM PDT Sunday...weak high pressure over the
Great Basin will keep through Monday. A low pressure system
approaching the Pacific northwest will bring increasing southerly
winds Tuesday and Wednesday. A large west to northwest swell is
forecast to impact our area starting on Wednesday.


Mtr watches/warnings/advisories...


Public forecast: Larry/Bell
aviation/marine: west pi

Visit US at weather.Gov/sanfrancisco

Follow US on facebook, twitter, and youtube at:

National Weather Service Glossary of Abbreviations