Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
920 am PDT Sat Oct 25 2014
Synopsis...a Pacific storm will continue to impact the area today
before lifting quickly northeast out of the area by Sunday morning.
A cold front will move east of the area this afternoon. Gusty
southerly winds along the coast and over the mountains should
gradually decrease this afternoon and evening. Behind the cold front
and associated rain bands... cooler unstable air will bring
scattered showers to much of the area this 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 08:35 am PDT Saturday...fine tuning details in
the forecast for today and this evening. Frontal band is
progressing eastward and has marched into East Bay and extends
south into Monterey County. Good break in precipitation behind the
front with sf Bay looking for a period of dry weather before
cooler more unstable airmass moves onshore. Backed off
precipitation and cloud cover somewhat this morning central and
Broad brushed much of area with scattered showers and even
numerous showers North Bay this afternoon as breaks in clouds and
solar heating work on the moist destabilizing airmass. Isolated
thunderstorms in the cards as well from about Santa Cruz Mountains
northward this afternoon. Best chance of any threat of thunder
into the Bay area would be middle afternoon through early evening. North Bay
could easily see thunder anytime this afternoon. Would not rule
out small hail with some of the storms given relatively high cape
values. NAM forecast sounding for sts this afternoon shows convective available potential energy
over 500j which is pretty impressive for this area...but helicity
is much lower than was forecast yesterday. Wind shear still pretty
strong...but directional shear breaks down about the time
convection gets going.
Wind Advisory is marginal given current wind speeds and trend in
wind will be downward this afternoon. Given potential impacts with
drought stressed trees and lack of significant winds for some time
now...will leave the advisory ride for now with gusts over 35 miles per hour
still likely in elevated terrain.
Previous 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 4:45 am PDT Saturday...rain and MVFR ceilings moving
into the area with strong southeast winds blowing ahead of the
front. Frontal passage expected in the sfo Bay area around 16z
with a wind shift to the south to southwest. Low pressure off the
northern California coast will keep a threat of showers through
the afternoon. There is a chance of thunderstorms in sts. Winds
will switch to the west after 02z in most areas.
Vicinity of ksfo...MVFR through 21z. Light rain turning to showers
after 16z. Ceilings rising into VFR range after 21z. Showers ending
with a wind shift to the west after 02z.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...ceilings could lower to MVFR range after
14z as rain showers move into the mry Bay area. Strongest
southeast winds will be in the Salinas valleys with winds to 20
knots. Winds expected to switch to southwesterly Saturday afternoon.
Marine...as of 09:10 am PDT Saturday...a strong surface low
pressure centered off the Oregon coast will continue to lift
northeastward today...resulting in strong and gusty southerly
winds. Winds will then diminish and switch to the northwest
through the afternoon in wake of a frontal passage. This system
will bring showers to the region today with the possibility of
thunderstorms over the northern waters this afternoon and evening.
Northwest flow will increase early next week as high pressure
builds off the California coast.
... 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 south of the Bay Bridge until 3 PM
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm 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...Pigeon pt to pt pinos 10-60 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 0-10 nm from 2 PM
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm from 2 PM
Small Craft Advisory...pt Reyes to Pigeon pt 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...rough bar advisory for sf bar
Small Craft Advisory...sf Bay north of the Bay Bridge
Public forecast: kbb
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