Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
848 PM PDT Thursday Oct 30 2014
Synopsis...a cold front will spread rain across the entire region
late tonight into Friday. Showers and possible thunderstorms are
forecast from late Friday afternoon through Friday night.
Scattered showers will continue into Saturday. Dry weather will
return by Sunday and continue through much of next week as high
Discussion...as of 8:45 PM PDT Thursday...incoming Pacific
weather system continues to look promising on satellite with a
cold upper trough digging southeast offshore. Precipitable water values
as high as 1.2 inches are feeding in from the SW along the cold-
frontal rain band which is approaching the northern California coast this
evening. Models have lately been trending wetter with this system
and that seems like a good bet given relatively strong dynamics
and a good moisture source. Another model trend has been to focus
the heavier precipitation across the southern portion of our forecast
Timing of precipitation onset remains consistent. Rain will likely begin
in the North Bay late tonight and spread into San Francisco by
around sunrise. Looks like much of the sf Bay area will have a wet
morning commute...particularly the North Bay...San Francisco
peninsula and East Bay. Rain probably won't reach the South Bay
until late morning and then the Monterey Bay area around midday or
early afternoon. The southeastward progression of the frontal rain
band is expected to slow considerably around early afternoon as a
130 knot upper jet digs into the base of the upper trough offshore.
Also...the NAM forecasts a wave to develop along the front just
west of Monterey Bay late Friday morning....which will slow the
front down and enhance low level convergence and precipitation intensity.
For these reasons it looks like the largest rainfall totals will
be over areas from the Santa Cruz Mountains southward. In
fact...the 00z NAM forecasts over 2 inches of rain in the Santa
Cruz Mountains vs. Only about a tenth in the interior of Sonoma
County. Other models show a more uniform quantitative precipitation forecast distribution...but
all of the models agree that heavier amounts will be across the
Precipitation will begin to taper off in the North Bay by midday...across
the rest of the sf Bay area by middle afternoon...but not until middle
evening from Monterey Bay south.
The airmass will become increasing unstable during the late
afternoon and evening hours of Friday as cold advection occurs
behind the front. Isolated thunderstorms are possible from late
afternoon Friday through Friday night. Best chances will be over
the ocean and across the southern half of the County Warning Area.
Shower activity is expected to gradually diminish Friday
night...but then become more widespread by late Saturday morning
as a shortwave drops in on the backside of the departing longwave
trough. Showers will then taper off Saturday evening and end by
late Saturday night as the upper trough shifts well to our east.
Rain totals across the north will mostly range from a quarter to a
half inch. Farther south rain totals are generally expected to range
from a half inch to an inch...except from one to two inches in the
Santa Cruz and Santa Lucia mountain ranges.
This system is not expected to produce strong winds. Local wind
gusts of up to about 25 miles per hour are possible...but probably not much
Temperatures will be much cooler over the weekend...with highs
mainly in the 60s...as much as 20 degrees cooler than what we've
Dry weather is expected from Sunday through most of next week as
an upper level ridge builds over California. Temperatures will gradually
warm to levels above seasonal norms by midweek. A weak system may
bring rain to the northern portion of our area late in the week.
Aviation...as of 5:05 PM PDT Thursday...for 00z tafs. Difficult
forecast this afternoon as high clouds streaming across the region
are obstructing The View of the clouds below that will move in
this evening and tonight. MVFR ceilings will return over area terminals
this evening as they have already over kmry however confidence is
low with respect to timing. Winds have also been tricky with many
terminals already experiencing southerly winds ahead of the
approaching frontal system. Rain is forecast to begin over the
North Bay around 12z tonight then is expected to spread southward
Friday. Bay area should anticipate rain to begin around 14z with
Monterey area terminals holding out till around 18z. Wet runways
and south plan expected for the morning rush Friday.
Vicinity of ksfo...VFR conditions expected through this afternoon
and evening with light southerly flow and high clouds. The
concern persists for a patchy cloud deck to move into ksfo by
03-05z time frame but for now will keep things VFR through the
evening rush. Southeast winds develop overnight with light rain
likely by around 14z Friday. Would expect to start the day with
southeast winds and wet runways. Wind shift to SW could occur as
early as 18z.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR conditions expected through
this afternoon and evening with light southerly flow and high
clouds. Difficult to pinpoint but expect low clouds to spread
inland this evening ahead of the cold front. Southeast winds will
develop overnight...strongest Salinas valley with light rain
approaching the area by about 16-18z Friday.
Marine...as of 01:58 PM PDT Thursday...light wind and seas today
ahead of an approaching cold front. Winds will turn southerly
tonight as the front approaches with rain developing over the
waters tonight into Friday. Winds will turn northwest by Friday afternoon
but in general light winds are forecast with the frontal passage.
Northwest winds will increase later Friday night into Saturday along with
a slight chance of thunderstorms and building northwest seas.
... Pt to pt pinos 0-10 nm from 6 am
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm from 3 am
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm from 3 am
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm from 3 am
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm from 6 am
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm from 6 am
Public forecast: dykema
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