Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1046 PM PDT Friday Aug 28 2015
Synopsis...a Pacific weather system will bring a chance of light
rain to the northern portion of our district late tonight and
Saturday morning. Much cooler weather is forecast for this weekend.
Dry and seasonably cool weather is expected next week.
Discussion...as of 8:50 PM PDT Friday...a very warm airmass
persisted across our region longer than expected...and the
anticipated onshore push of cooling marine air held off until late
in the day. Consequently...temperatures today were warmer than
anticipated. But only one daily record high temperature was
set...in King City with a 107.
Evening observations show the marine push is underway as onshore
gradients rapidly increase and onshore breezes ramp up. 8 PM
temperatures are as much as 11 degrees cooler compared to the same
time last evening and relative humidity values are as much as 50
percent higher. Cool marine air will continue to push inland
overnight as a Pacific weather system approaches the West Coast.
This incoming system has plenty of moisture at its disposal...with
satellite data showing precipitable water values in excess of 1.5
inches just ahead of the approaching cold frontal boundary.
However...the system is lifting to the northeast as it approaches
the coast which means the bulk of the moisture and energy will
move inland well to our north. The models have been consistent in
bringing some light rain as far south as the North Bay late
tonight and Saturday morning...with the NAM producing some light
rain or sprinkles as far south as the San Francisco peninsula
around sunrise Saturday. The system is forecast to lift quickly to
the northeast and all rain chances should end by midday Saturday.
Rainfall amounts in the North Bay are forecast to be less than a
tenth of an inch.
A much cooler airmass will sweep across our region tonight and
Saturday. High temperatures on Saturday are forecast to be from 10
to 20 degrees cooler than today. For instance...the forecast high
in Livermore tomorrow is 83...19 degrees cooler than today's high
The medium range models have been consistent in keeping the
longwave trough position locked in place along the West Coast
through all of next week. Precipitation will remain well to our
north and we can expect dry and relatively cool conditions through
the end of the week and probably into the following weekend.
Aviation...as of 10:46 PM PDT Friday...low clouds have advanced
into the San Francisco Bay much faster than previously forecast.
However...low clouds have not yet impacted the Monterey
terminals...but are expected to shortly. There is a front moving
toward the coast tonight which will lift the low clouds to MVFR
Saturday morning. The clouds will mix out by late morning early
afternoon as the front passes to the east.
Vicinity of ksfo...IFR bases broken-ovc009 have arrive at the
terminals for the evening. These clouds will lift and become
cumulus type clouds beginning around 1600z as the front
approaches the terminals. The clouds will further lift and mix out
between 1800 and 2000z.
Confidence is moderate.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR is anticipated to give way to
MVFR broken-ovc015 by around 0800z further deteriorating to IFR broken-
ovc008 by around 1100z. Clouds are expected to lift and become
more cumulus type beginning around 1500z as the front approaches
the terminals. The clouds are anticipated to further lift and mix
out by around 1800z
Confidence is moderate.
Beaches...as of 11:30 am PDT Friday...a long period west swell
has arrived and will continue to advance on west facing beaches
through the weekend and into early next week. These long period
swells can produce hazardous rip currents that can quickly pull
swimmers out to sea. The highest risk of hazardous conditions will
be this weekend when onshore winds weaken and the period
increases from 15 to 18 seconds. Swimmers caught in a rip current
should swim parallel to the coast to escape the rip currents.
Marine...as of 02:15 PM PDT Friday...predominately light westerly
winds and seas will continue across the coastal waters. A long
period swell has begun to move into the area. A more energetic
westerly swell will impact the region this weekend.
... Fog advisory...mry Bay until 8 am
marine fog advisory...pt Arena to pt Reyes 0-10 nm until 8 am
marine fog advisory...pt Reyes to Pigeon pt 0-10 nm until 8 am
marine fog advisory...Pigeon pt to pt pinos 0-10 nm until 8 am
marine fog advisory...pt pinos to pt Piedras Blancas 0-10 nm
until 8 am
marine fog advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
until 8 am
Public forecast: dykema
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