Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
915 am PDT Mon Oct 24 2016
Synopsis...light rain will develop in the North Bay today but be
mainly light with the rain band only slowly sagging southward
towards the Golden Gate by late this afternoon or evening. Some
light rain overnight into Tuesday morning for the San Francisco
peninsula and East Bay before the front slides back to the north.
Dry and seasonable conditions likely Tuesday night into
Wednesday. Next front arrives Thursday with a better chance of
soaking rains, especially for the coastal hills. Long range trends
remain unsettled with shower chances next weekend into Halloween.
Discussion...as of 09:11 am PDT Monday...light rain beginning to
be picked up by automated sites over the far northern portions of
the North Bay this morning. Meanwhile, further to the south away
from the approaching storm system, rain continues to fall over the
Santa Lucia mountains along the Big Sur coast as well. Rainfall
amounts range from around 0.10" to 0.30" over this area and is
mainly a result of orographic lifting. Have updated the forecast
to reflect these trends and even mention light rain over the Santa
Cruz Mountains where a few hundredths of an inch have fallen over
the past few hours. As mentioned in the previous forecast
discussion, the best chances for rainfall today will be over the
North Bay as a frontal boundary sags southward into the region.
Precipitation will then spread southward by this afternoon and
evening into the San Francisco Bay area down to around the Santa
Cruz region. This will likely cause minor issues in these areas
during the afternoon/evening commute. With that said, the boundary
is forecast to wash out and begin to retreat northward late tonight
with lesser chances of measurable precipitation south of Santa
Cruz. In addition, inland valley locations such as the Santa Clara
Valley will likely see very little to no rainfall through tonight.
Please see previous forecast discussion below for additional
Previous discussion...as of 3:35 am PDT Monday...lightning activity
overnight has been confined to the Sierra foothills east of our
district with radar showing rapidly developing cells continuing to
move northeast away from the Bay area. Our attention is now on the
frontal boundary approaching Cape Mendocino. This feature will
move into northern Sonoma County by mid-morning with some light
rain developing. In general it appears the trends are to slow and
weaken the system as it reaches the North Bay and especially
points south of the Golden Gate. The frontal boundary is becoming
occluded with the main upper low and cold advection staying well
north of the Bay area. So expect periods of rain, mainly light for
the North Bay increasing through the afternoon hours. Main commute
impacts should be for the afternoon and evening commute north of
the Golden Gate. Frontal boundary will try and sag south of the
Golden Gate overnight with some light rain for the sf peninsula
and the East Bay. However there are weak dynamics and no cold
advection, some favored southwest facing slopes may fair will due
to local orographics but dont believe the deep moisture and
sustained southwest flow will make it as far south as the Santa
Cruz mtns with this system leaving only some light rains for the
Santa Cruz mtns thru Tuesday morning.
Boundary will then shift northward on Tuesday and dissipate in
response to next system and digging offshore trough.
The trends seem to indicate that the heaviest rains with the late
week system will be focused somewhere between the Santa Cruz mtns
and Santa Barbara County putting the central coast (i.E. Big sur)
potentially in the bullseye for an incoming southwest flow system.
The ECMWF has been pretty consistent with this solution for days
now and the latest 06z GFS is more or less in line. Given the
trajectory of the incoming low would expect the heavier rains with
the late week system for the southern portions of the district
most notably the Santa Cruz mtns and Big Sur hills where 1-4
inches may occur with the late week system Thursday into early
Beyond that the pattern remains unsettled, especially for October
with shower chances continuing but no all day rain events
expected next weekend. Models do portend more storms potentially
lining up by around Halloween into next week.
Aviation...as of 5:00 am PDT Monday...main concern for the day is
the gusty southerly winds. Sfo gusting to 30 kts this morning and
gusts should continue in the 25-30 kt range through the day. Low
to mid cigs settling over the area as a frontal system approaches.
Bases are near 3000 feet. Clouds will lift into VFR range late
this morning except in the North Bay. Deeper moisture arrives
after 22z in the North Bay spreading into sfo and Oak after 03z.
Vicinity of ksfo...VFR with cigs at or above 3000 feet. S winds
gusting to 25-30 kt decreasing this evening. Light rain with wet
runways possible after 03z.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR with cigs remaining above 3000 feet.
Southeast winds increasing to 15-20 kt at sns after 16z.
Marine...as of 09:11 am PDT Monday...a frontal boundary is
currently draped just north of the regional coastal waters this
morning and will drift southward today. It is forecast to stall
and weaken over the San Francisco Bay area late tonight. Light to
moderate showers in the northern waters can be expected late this
morning...then will drift southward through the day. The frontal
boundary is producing strong southerly winds out ahead of it.
Occasional gale force wind gusts are possible. These strong winds
will likely persist through Wednesday. A weaker low develops off
the central California coast Thursday and Friday so southerly
winds will not be as strong.
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...mry Bay from 12 PM
Public forecast: rgass/rww
aviation: west pi
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