Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
506 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 will develop by the weekend.
Dry and seasonably cool weather is expected next week.
Discussion...as of 3:00 PM PDT Friday...a bit of a surprise today
as temperatures again became very warm to hot today. Quick scan of
current readings shows widespread 70s and 80s at the coast with
90s to lower 100s for many inland spots. Had expected temperatures to
actually be a few degrees cooler today due to a slight onshore
push plus flattening of the ridge. However...current analysis
shows that did not come to fruition.
Still looks like cooler weather will return to our region over the
weekend as the ridge progresses to the east while a longwave trough
advances to the West Coast. At the same time a cold front will
push across norcal/pacnw overnight into Saturday which will help
to bring rainfall (mostly north of our area) plus cooler weather.
Models remain in good agreement that the southern extent of the
moisture will move into our northern outer waters water this
evening and eastward to the North Bay plus sf Bay area overnight
into Saturday. Amounts will likely be very light with the greatest
amounts over the North Bay mountains. Still looks like just a
hundredth or so for most urban spots. Up to a tenth may fall
locally for higher elevation spots in Napa/Sonoma. Humidity values
are mostly in the 20 to 30 percent range although values should
recover enough overnight to allow some of the rain to reach the
ground. Precipitation is not expected for the southern half of our
area. Highs will be noticeably cooler with many inland spots 15 or
more degrees cooler over the weekend.
Big picture view across the Continental U.S. For next week continues to show
lower than normal heights over the western Continental U.S. With above normal
heights to the east. This will lead to cooler than normal temperatures
especially for inland spots. The storm track should stay to the
north so no organized rainfall forecast.
Aviation...as of 5:05 PM PDT Friday....currently the satellite
water vapour image is showing a ridge of high pressure positioned
over southern and central California with a trough of low pressure
approaching northern California. The ridge of high pressure has
compressed the marine layer and allowed for some hot...for the San
Francisco and Monterey Bay area...temperatures today. With the hot
temperatures and compressed marine layer don't expect low clouds
to return to the terminals until later tonight early tomorrow
morning. The exception being koak which could see low clouds by
early evening. The trough of low pressure will lift any low clouds
tomorrow morning but should maintain MVFR through late
morning...cumulus type clouds.
Vicinity of ksfo...VFR is expected to prevail through sun rise
Saturday at which point expect a period of low clouds bkn-ovc008.
Clouds will lift and become cumulus type clouds as the front
approaches the terminals.
Confidence is moderate.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR through early Saturday morning.
IFR broken-ovc008 is anticipated to arrive early Saturday morning.
Clouds are expected to lift and become more cumulus type as the
front approaches the terminals.
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
marine fog advisory...pt Arena to pt Reyes 0-10 nm
marine fog advisory...pt Reyes to Pigeon pt 0-10 nm
marine fog advisory...Pigeon pt to pt pinos 0-10 nm
marine fog advisory...pt pinos to pt Piedras Blancas 0-10 nm
marine fog advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...sf Bay
Public forecast: Bell
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