Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1047 PM PDT Thursday Oct 23 2014
Synopsis...a frontal boundary draped across northern California
will lift to the north tonight and Friday...thereby keeping all
rain north of our forecast area. Rain chances will develop region-wide
Friday night through Saturday morning as a stronger system pushes
a cold front into northern and central California. Dry weather
will return by Sunday.
Discussion...as of 8:50 PM PDT Thursday...a stationary frontal
boundary remains draped across northern California this evening.
According to latest radar and rain gage data...all precipitation with this
boundary is to the north of our forecast area. And...based on the
00z NAM and other model data...the frontal boundary across
northern California is expected to begin lifting northward overnight as
the trough offshore deepens. This will further diminish rain
chances in the North Bay. A forecast update was recently completed to
reduce North Bay rain chances tonight...essentially limiting
slight chance probability of precipitation to areas north of Santa Rosa. Another change
made to the short-term forecast was to add patchy fog to coastal
areas tonight as there has already been reports of fog at Half
Moon Bay. Fog was also added to the North Bay valleys later
tonight through early tomorrow morning. There has been enough
clearing to result in radiational fog in the typical foggy North Bay
valley locations. Finally...cloud cover was reduced in the
forecast for this evening and for some inland areas on Friday
Friday is shaping up to be a mostly sunny day...after patchy
morning fog and low clouds clear...along with slightly above
Attention then turns to robust Pacific weather system currently
deepening offshore along 140w. The 00z NAM is consistent with
previous model runs in spreading a cold-frontal rain band across
most of the forecast area late Friday night into Saturday
morning. Bands of showers are then forecast to follow the front
through the remainder of Saturday. There is also expected to be
enough instability in the North Bay to justify at least a slight
chance of thunderstorms Saturday afternoon as the cold upper
trough presses inland. In fact...based on the 00z NAM...an
argument could be made to add slight thunderstorm chances for
Saturday afternoon as far south as the sf peninsula and East Bay.
Showers will then taper off on Saturday evening...with precipitation
expected to end by Sunday. Rainfall amounts through Saturday
evening are projected to range from 0.25 to 0.75 in the North Bay.
Locations south of the Golden Gate are expected to see less than a
quarter of an inch except perhaps locally up to a half inch in the
Santa Cruz Mountains. Some portions of interior Monterey and San
Benito counties may not see any rain from the Saturday system.
Sunday should be a partly cloudy and dry day with cool
northwesterly winds. The weather pattern is looking dry from
Monday through Friday of next week as an upper ridge over
California diverts all Pacific storms/precipitation to our north.
Aviation...as of 10:47 PM PDT Thursday...stalled cold front and
associated rainfall across northern California expected to remain
just north of all terminals tonight and then lift farther to the
north on Friday. MVFR to IFR ceilings expected overnight across all
terminals as low clouds advect inland or develop over terminals.
Clearing is expected in most areas by 18z Friday. Winds will be
light tonight...turning light southerly by overnight...then veering
to the southwest and increasing somewhat on Friday.
Vicinity of ksfo...VFR conditions expected through late tonight
with MVFR ceilings likely by 07-08z...continuing until about 18z
Friday. Light winds through late tonight...turning light
southerly overnight and then to the southwest on Friday afternoon.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR conditions expected through
late tonight with IFR ceilings likely between 10-12z...continuing until
about 18z Friday. Light winds will continue through late
tonight...turning light southeasterly overnight and then to the
west southwest on Friday afternoon. Moderate confidence.
Marine...as of 10:36 PM PDT Thursday...a weak ridge of high
pressure continues over Southern California coast while low
pressure will strengthen to our west over the next few days. This
is bringing light winds to the area today which will become
southerly and increase Friday and Saturday as the low pressure
system strengthens off the Oregon coast. High pressure will build
off the California coast Sunday and Monday after the low moves
Public forecast: dykema
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