Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1001 am PST Friday Nov 28 2014
Synopsis...a frontal system will bring rain to the North Bay
later today and spread southward across the San Francisco and
Monterey Bay areas through Saturday. Unsettled conditions will
persist through early next week in advance of a stronger upper
level low forecast to impact the region by the middle of next
Discussion...as of 9:35 am PST Friday...patchy fog persists this
morning over the San Francisco Bay and locally into the North Bay
valleys and Santa Clara valleys. Around sunrise visibilities were
close to zero at several observing sites...but they have improved
with only one location (petaluma) reporting 1/4 mile as of 9 am.
Expect the rest of the fog to clear this morning...although
locally hazy conditions are expected to persist around the San
Francisco Bay through the afternoon. After a chilly start to the
day...highs today are expected to be mild and range from around 60
to the 70s in the warmest areas across the southern zones.
The main weather story is the impending series of rain events
expected to impact the area for the next 5 or 6 days. The first in
a series of rain events is currently bearing down on the Pacific
northwest and extreme northern California coast. This system is
expected to bring rain to the North Bay beginning this
afternoon...with rain spreading into the greater San Francisco Bay
area this evening and overnight...and finally reaching the
Monterey Bay and central coast by Saturday morning. Another
frontal band is prognosticated to move across Saturday night into Sunday
spreading additional rainfall across the area...although this
second band is forecast to focus more on areas from Santa Cruz
County south through the Santa lucias. The 00z run of the European model (ecmwf)
brings up to 2 additional inches to the coastal ranges from Santa
Cruz County south through midday Sunday with this second system.
All models agree on high pressure briefly rebuilding along the
coast on Monday with shower activity easing up. By Tuesday and
Wednesday another significant batch of rain is expected across the
district as an upper level low approaches the coast from the west.
There remain differences in solutions amongst the medium range
models regarding where the main focus of heavy rain will fall with
this impulse. The 00z runs of the European model (ecmwf) as well as the Gem focus the
heavier rains at our County Warning Area while the 12z operational run of the GFS
now nails the Los Angeles area with most of the heavy precipitation
through Wednesday. Also...the GFS ends precipitation by Thursday while the
non-domestic forecast models keep unsettled weather going through
the workweek. For now our forecasts have shower chances going through
Aviation...as of 10:00 am PST Friday...short term concerns
continues to be the fog sloshing around sf Bay. Both ksfo and
koak continue to show some visby restrictions and will through the
morning. Ksjc is finally showing some signs of improvement.
Monterey Bay is VFR. Next up will be a cold front currently moving
toward the North Bay. Ceilings and rain will develop from north to S overnight
with biggest impact early Saturday morning with moderate to heavy
rain. Wet runways and southeast winds seem likely Saturday morning.
Overall conf is medium.
Vicinity of ksfo...fog will linger around ksfo and koak through
1830-19z today. Expecting VFR this afternoon...but enough haze may
be present for low end MVFR. Front is forecast to lower ceilings
tonight with solid rain by 10-12z Saturday.
Ksfo bridge approach...ceilings at or below 500 through 1830z as late as 19z
with reduced visby too. Latest satellite does show some signs of
improvement. VFR this afternoon with some lingering haze issues.
Monterey Bay area terminals...VFR this afternoon/evening. MVFR
late in the forecast with precipitation developing late.
Marine...as of 8:35 am PST Friday...southerly flow will
gradually develop over all of the coastal waters today as a cold
front approaches the region. Southerly flow will increase over the
week and early next week as a storm system intensifies off the
coast. In addition...the unsettled weather will bring periods of
rain....potentially heavy at times...to the region this weekend
and much of next week.
Public forecast: rgass
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