Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service San Francisco Bay area
1038 PM PDT Wednesday may 6 2015

Synopsis...an upper level low over Oregon will reach the Bay area
late tonight and Thursday resulting in showers and isolated
thunderstorms over the southern half of the region. A few showers
may linger into Friday. Dry weather conditions and slightly warmer
temperatures return for the weekend as the system shifts inland
and high pressure builds over the region.

&&

Discussion...as of 9:30 PM PDT Wednesday...compared to Tuesday
daytime highs today were either the same or a couple degrees
warmer over the Bay area and it was just the exact opposite over
the northern central coast. Low clouds tended to hang on the
longest over the northern central coast acting as a brake on the
diurnal warming. Strong and gusty northwest winds are reported out at the
coastal buoys and some of the gusty northwest-west wind have overlapped the
immediate coast. The North Bay counties have also reported some
gusty northwest winds today. Almost could call it April-like weather
still lingering into early may...and temperatures were mostly near
average area-wide. An early sign of thursday's weather arrived
late in the afternoon over northern Napa County when moderate
convective clouds led to a few showers seen on the kmux radar beam
up to approx 15 thousand feet. Isolated light measurable rain was
possible under the shower activity earlier but otherwise gauges in
the area reported no rain. The rest of the area was mostly sunny
to sunny today.

Rapidly falling 500 mb heights late tonight and Thursday will
accompany a closed low pressure area from Oregon as it moves south
over the Bay area Thursday. Falling heights equates to colder middle
level temperatures and a destabilizing atmosphere which will generate
some shower activity and possibly a few isolated T-storms. 500 mb
level temperatures fall to near -28c Thursday which is a fairly
decent cold core low for the time of year. Inspection of modeled
forecast soundings do show some moderate to locally strong middle
level convective instability arriving over the North Bay as soon
as 11 PM - 2 am tonight then quickly moving south to the East Bay
early Thursday morning. Total totals are upwards of 46 briefly in
the vicinity of Napa County then settle back to the middle 30s as the
area of highest total totals expands and moves south over the Bay
area very early Thursday. Much of this hinges on the presence of
middle level moisture as well as upward vertical motion at the defined
levels centered near 700 mb and by inspection of current satellite
it looks devoid of moisture at the moment. Updated earlier to
include isolated showers for Napa County...but not sure if this
will be enough...meaning would not be surprised if the shower
and/or isolated T-storm activity redevelops. Went with isolated
showers as a compromise. On Thursday surface warming will lead to
increasing surface based cape values increasing to 400-750 j/kg
mostly focused over the southern half and interior sections.
There's a bit of negative buoyancy around as well...but main
message here is expect scattered shower and isolated T-storms
Thursday. Areas of gusty winds are more likely in the afternoon
during time of maximum heating.

A few showers may linger into Friday...but by this time the center
of the low will be over southern Nevada. Showers should end no
late than Friday afternoon. High pressure will build in from the
epac for Saturday then flatten out and move east of the area by
Sunday. Dry weather is expected over the weekend. The next closed
low from the epac arrives early next week which the 12z GFS run
shows the north central coast picking up briefly light rain late
Monday into Tuesday morning...otherwise it looks dry for the remainder
of the area. Another upper level ridge moves in by the middle of
next week followed by another trough Friday.

&&

Aviation...as of 10:38 PM PDT Wednesday...the northerly gradient
remains strong around 6 mb from acv to sfo this evening as a upper
level low approaches from the north. Breezy onshore flow is
anticipated at times tonight as the gradient tightens in response
to this system. Low clouds are anticipated to return late tonight
across a select terminals.

Vicinity of ksfo...VFR conditions will persist through late
tonight. MVFR ceilings are expected to return around 13z. Light to
moderate onshore flow is expected through tonight. Winds will
pick up Thursday afternoon with sustained winds around 15 to 20 knots
with gusts around 25 knots.

Ksfo bridge approach...similar to ksfo.

Monterey Bay area terminals...MVFR ceilings are expected through the
forecast period. Light southwesterly winds will prevail trough the
night. Winds will picking up in the afternoon with sustained
winds of 10 to 15 knots with gusts to around 20 knots.

&&

Marine...as of 8:51 PM PDT Wednesday...high pressure over the
eastern Pacific coupled with a low dropping down from the Pacific
northwest will maintain strong northerly flow across the coastal
waters into through tonight. Gale force winds will continue across
the coastal waters north of Pigeon Point. Winds will ease a bit
Thursday but remain within Small Craft Advisory criteria. The
strong winds will produce hazardous conditions with steep wind
waves and fresh swell.

&&

Mtr watches/warnings/advisories...
... Pt to pt pinos 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm from 3 am
glw...pt Arena to pt Reyes 0-10 nm until 3 am
glw...pt Arena to pt Reyes 10-60 nm until 3 am
glw...pt Reyes to Pigeon pt 10-60 nm until 3 am
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm from 3 am
Small Craft Advisory...Pigeon pt to pt pinos 10-60 nm
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm from 3 am
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...rough bar advisory for sf bar
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm

&&

$$

Public forecast: canepa
aviation/marine: CW

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