Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
256 am PDT Sat Oct 25 2014
Synopsis...a Pacific storm will continue to impact the area on
Saturday before lifting quickly northeast out of the area by
Sunday morning. It will push a cold front towards the area. Southerly
winds will increase ahead of the front and become locally strong
and gusty near the coast and in the hills Saturday morning. At the same
time...rain will spread across much of the region. The front will
pass through quickly on Saturday morning...but cooler unstable air
will bring scattered showers to much of the area Saturday
afternoon and evening. A few thunderstorms may develop as
well...especially from the Santa Cruz Mountains northward. Drier
and cooler weather is expected Sunday with a warming trend
beginning Monday as high pressure rebuilds along the coast.
Discussion...as of 02:28 am PDT Saturday...
the forecast focus today is on rain chances and probability of
thunderstorms and any severe weather potential.
As of 09z...a low pressure centered near 40n/130w is well evident
on infrared satellite imagery. This low is expected to lift
northeastward today...and into southwest Washington by 00z sun.
Current radar shows an initial band of rain showers moving into
the coastal North Bay presently. Model consensus continues to be
for light rain to push quickly through the area this morning.
Timing is fairly consistent from previous runs which indicates
that showers will move into the immediate San Francisco Bay area
by 12z and the Monterey Bay area around 15z. Hrrr indicates just a
quick shot...about 3-5 hours...of light rain with rapid moving
frontal bands. After these bands clear there will be a few hours
of dry weather as the atmosphere destabilizes with cold advection
noted. This will provide the basic ingredients needed for
scattered showers and thunderstorms this afternoon especially over
the North Bay. An analysis of forecast soundings showed a
moderately unstable airmass with some surface based instability.
The profile remains moist enough to support convection as well.
There will be a vigorous wind field with 0-6km shear in excess of
30kts and noted low level shear as well. In addition...a jet
streak will pass through northern California and leave the North
Bay in the favorable right entrance region this afternoon. The
surface based instability will be somewhat limited however noting
500 mb height falls associated with a shortwave at that level.
This will be sufficient lift and instability to support scattered
storms this afternoon and evening in the North Bay
especially...with isolated storms possible even as far south as
the Santa Cruz Mountains. Agree with Storm Prediction Center day 1 outlook of marginal
severe threat in the North Bay...and their forecast of potential
low topped supercell structure is reasonable given low freezing
heights and ample shear. Noted favorable hodograph as well to
support this forecast. Low freezing height will support small hail
potential. Shear becomes more unidirectional after 21z...and noted
storm motion will be quick from the southwest. 0-6km mean wind of
35-40kts by 21z to also indicate potential for gusty winds in
Other factor of note today is Wind Advisory which has been
extended a few hours in the elevated terrain. Marginal event will
have potential to produce greater impact given that it is the first
wind of the season...and drought conditions have produced more dead
or dying trees and vegetation this year. Models continue to suggest
gusts near 50 miles per hour or higher in the high terrain and coast. Current
gusts already near 50 miles per hour in some areas so this looks reasonable.
Heights will begin to rise overnight as the low exits the area.
Showers will taper off overnight and drying conditions are
expected. Ridging will gradually build back into the area and
produce warming temperatures. But cool nighttime lows expected in
the area Sunday night before the warming trend. Likely to see many
upper 30s in the favored cool interior valleys. Especially in the
North Bay and also interior Monterey County.
Aviation...as of 10:51 PM PDT Friday...for 06z tafs. An
approaching low pressure system and associated cold front
situated off the northern California coast will bring increasing
southeasterly winds overnight and wet runways by morning. T-storms
remain a concert for Saturday afternoon.
Vicinity of ksfo...VFR conditions expected through late tonight. Light
south winds will turn southeasterly and strengthen overnight. -Ra
expected around 11z tonight. By Saturday morning moderate and
gusty southeast winds and wet runways will impact the morning
traffic. Winds expected to veer southwesterly Saturday afternoon.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR conditions expected through
late tonight. Light winds will turn southeasterly overnight.
-Shra expected to develop around 12z-15z Saturday. By Saturday
morning southeast winds and wet runways will impact area
terminals. Winds expected to veer southwesterly Saturday
Marine...as of 02:28 am PDT Saturday...low pressure situated west
of Cape Mendocino will continue to deepen tonight resulting in
strong southerly winds across the coastal waters. Gale force winds
will develop overnight across the northern waters with strong
small craft winds developing elsewhere. A line of rain and showers
will move through the waters early Saturday morning with
thunderstorms possible north of the Golden Gate Saturday
afternoon. Winds will ease and showers will end by Saturday night
with lighter winds and moderate seas Sunday into Monday.
... Advisory...North Bay coast and mountains
Wind Advisory...San Mateo and Santa Cruz County coasts
Wind Advisory...Santa Cruz Mountains
Small Craft Advisory...sf Bay north of the Bay Bridge until 3 PM
Small Craft Advisory...sf Bay south of the Bay Bridge until 3 PM
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm until 3 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm until 3 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm until 3 PM
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm from 2 PM
glw...pt Arena to pt Reyes 0-10 nm until 2 PM
glw...pt Arena to pt Reyes 10-60 nm until 2 PM
glw...pt Reyes to Pigeon pt 10-60 nm until 2 PM
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm until 3 PM
Small Craft Advisory...Pigeon pt to pt pinos 10-60 nm until 3 PM
Small Craft Advisory...rough bar advisory for sf bar
Public forecast: Johnson
aviation/marine: west pi
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