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Area forecast discussion 
National Weather Service San Francisco Bay area
159 PM PDT Tuesday Oct 21 2014

Synopsis...dry weather conditions and seasonable temperatures will
persist trough Wednesday with precipitation possibly returning to
the North Bay on Thursday. Rain chances will then increase and
spread southward Friday night into Saturday as an upper level
system moves through the region.

&&

Discussion...as of 01:54 PM PDT Tuesday...quiet weather conditions
along with near seasonal temperatures will persist over the
region through Wednesday as zonal flow continues across the
region. Meanwhile...the forecast models continue to indicate a
deeper pool of moisture that will advect into the Pacific
northwest/northern California during the latter half of the week.
Weak disturbances are also forecast to ride along the mean
flow...interacting with the abundant moisture to bring rainfall
to locations north of our forecast area. By Thursday
morning...the models suggest the deeper moisture will drop
southward over the North Bay and bring increased chances for
rainfall. However...there will likely be a very distinct cut-off
from areas that will receive rainfall and those that will remain
dry. At this time...the best chance Thursday through Friday will
stay north of the Golden Gate. With this...temperatures will be a
bit cooler over northern areas while inland portions of the
central coast will warm slightly through Friday.

There is general agreement with the forecast models that a
middle/upper level trough will develop off the Pacific northwest
coast...pushing inland Friday night into Saturday. This would
drive a frontal boundary through the forecast area and increase
chance for precipitation across the entire region. While the
models agree on the trough pushing inland...they continue to
differ on the amount of available moisture as this transition
occurs. The latest European model (ecmwf) keeps precipitation amounts light as
southwest flow develops ahead of the approaching trough...drying
the airmass out ahead of the frontal boundary. For now...will
keep the best chances for precipitation and highest rainfall
amounts over the North Bay Friday night into Saturday. Will keep
a slight chance of precipitation into Saturday given timing
differences with the models and likelihood of lingering moisture
across the region.

Conditions dry out Sunday and Monday as the middle/upper level
trough shifts eastward toward The Rockies and northwest flow
develops aloft. Also expecting a slight warming trend for early
next week with zonal flow over the region. Another middle/upper
level system may bring rain chances back to the area by the
middle of next week. Something we will continue to monitor as the
forecast models come into better agreement...or diverge further.

&&

Aviation...as of 11:00 am PDT Tuesday...clear skies across the
area with VFR conditions presently ongoing. High pressure at the
surface is building in from the Pacific today and will continue to
do so through the taf period. This will bring light winds which
will gradually become west or northwest and pick up this
afternoon. As skies remain clear to start out tonight...the low
level moisture suggests that fog or low clouds will develop
towards 12z Wednesday. Will include MVFR conditions in prevailing groups
for now...but it may be more of a patchy fog situation given the
ridge position.

Vicinity of ksfo...VFR through 12z Wednesday. Mostly light winds this
morning...increasing to around 15 knots out of the west in the
afternoon. Lower ceilings or visibility with patchy fog is possible around
daybreak Wednesday. Low confidence.

Ksfo bridge approach...similar as ksfo.

Monterey Bay area terminals...VFR. Mostly light winds...offshore
in the morning before switching onshore in the afternoon. Low
clouds or fog will develop overnight to produce MVFR conditions.
Low-moderate confidence.

&&

Marine...as of 10:56 am PDT Tuesday...northwest winds over the
coastal waters will increase today as high pressure builds over
the eastern Pacific. The strongest winds will be near the coast
south of Point Sur. Large moderate period northwest swell will
continue to move through the waters today before decreasing
tonight.

&&

Mtr watches/warnings/advisories...
... Bar advisory for sf bar until 3 am
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm

&&

$$

Public forecast: rgass
aviation/marine: Johnson

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