Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
903 PM PST Friday Feb 27 2015
Synopsis...a storm system will bring showers and cooler
temperatures to the area starting through the Saturday night.
Another weak system will bring additional shower chances to the
area on Monday.
Discussion...as of 9:00 PM PST Friday...upper level low over
western Oregon continues moving south. Cooler air aloft filtering
over the area destabilized the atmosphere and skies were partly to
mostly clouds today with breezy northwest winds. Oak sounding
shows an inversion cap at 850 mb which prevented vertical
development of the clouds so only a few light showers were able to
fall mainly over the hills. Shower activity will continue to be
limited tonight with the loss of daytime heating.
Shower activity will increase Saturday morning as the upper low
moves over the district and part of the system moves over the
water and picks up extra moisture. Models in good agreement that
the southern part of the County Warning Area from the Santa Cruz Mountains south
will get most of the showers. There is a slight chance of
thunderstorms containing small hail. Rainfall amounts are
expected to be less than one-tenth of an inch for the sfo Bay area
with one-quarter to one-half inch to the south.
Shower chances will diminish over the sfo Bay area during the
night Saturday but chances will continue south of the sfo Bay
area along with possible thunderstorms and small hail. Snow
levels will lower to around 4000 feet so could see a dusting of
snow on the highest mountains.
Partly cloudy skies are forecast Sunday with still the chance for
an afternoon shower over the Santa Lucia and San Benito mountains.
A second upper level system will move into the area Monday. There
is still some uncertainty with where this system will track.
Latest models now tracking this system further west which would
suggest a better chance for showers but there is a possibility it
will track too far west so that the showers stay offshore. Any
rainfall from this system will be similar to or less than this the
current system. This system will quickly depart well to the south
by Tuesday. Temperatures will be remain cool through early next
A ridge of high pressure will build back to our region Wednesday
as the storm track moves back north of the Continental U.S.. temperatures will
rebound to above normal levels for the remainder of the week.
Aviation...as of 5:47 PM PST Friday...a broad area of gusty northwest
winds continues to sweep in cooler air into the region. Lots of
small to occasionally moderate cumulus clouds have been seen
developing through the day as well as isolated passing showers as
diurnal warming reached a maximum. Clouds are mostly capped at the
subsidence inversion based approx 4500 feet (agl) per 00z Oakland
upper air sounding. Land based cumulus clouds will disappear after
sunset then return Saturday with the return of the diurnal warming
cycle and steeper lower to middle level temperature lapse rates. 500 mb
temperatures are as cold as -26c to -28c Saturday. Scattered showers are
expected region-wide except showers are more likely central
coast...particularly over the higher terrain...on Saturday.
Greater instability associated with the cold core low pressure
area on Saturday increases the chances that small hail accompanies
any passing shower or isolated thundershower.
Vicinity of ksfo...the Airport weather warning for gusty west to
northwest winds continues until 04z Sat (8 PST this eve). West winds
will slowly subside tonight into Sat morning. Not confident west winds
will stay below 20 knots Saturday afternoon and early evening given
the present weather pattern. Will look over the area winds forecasted
for Saturday and adjust ksfo taf winds upwards if needed. It should
be less windy on Saturday compared to today but possibly a bit stronger
than currently advertised.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...MVFR/VFR tonight with generally an
increasing trend back toward low cloudiness moving in off the
Monterey Bay waters. A few showers are possible very late tonight
Marine...as of 3:00 PM PST Friday...gusty north and northwest
winds will continue along the central coast tonight and Saturday
as a storm system sweeps through the region. The gusty winds will
produce hazardous conditions with steep wind waves and fresh
swell. The storm system will also bring showers and possibly
thunderstorms to the coastal waters tonight through Saturday
evening. Winds will decrease and sea conditions will improve for a
majority of the waters by Saturday night as the storm exits the
... Bay until 1 am
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm until 1 am
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm until 1 am
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm until 1 am
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm
Small Craft Advisory...Pigeon pt to pt pinos 10-60 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm
Small Craft Advisory...sf Bay north of the Bay Bridge until 10 PM
Small Craft Advisory...sf Bay south of the Bay Bridge until 10 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm until 10 PM
Public forecast: west pi
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