Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
FXUS66 KMTR 180503
Area Forecast Discussion
National Weather Service San Francisco Bay Area
903 PM PST Tue Jan 17 2017
.SYNOPSIS...After several days of dry conditions, an active and
unsettled weather pattern returns on Wednesday as a series of
storm systems impact the region through the upcoming weekend. The
first will move through during the day Wednesday followed by
another late Thursday into Friday and the final set to arrive late
Saturday into Sunday. Moderate to heavy rainfall we be likely at
times, especially along the coastal ranges as well as locally
strong to gusty winds associated with the passage of each system.
.DISCUSSION...as of 8:59 PM PST Tuesday...Doppler radar depicts a
line of warm advection showers moving in over the North Bay this
evening...likely falling as virga at this time. No gages have
tipped as of this hour but light rain is likely reaching the
higher elevation peaks and will likely reach to the valley floor
shortly as the night progresses. These warm advection showers
pushing out in front of the main system that is progged to
reached the coast Wednesday morning will continue to spread south
moistening up the boundary layer with only light showers expected
to reach the ground. Have increased POPs over the North Bay as
well as along the San Mateo and Santa Cruz Mountains to account
for possible light rain overnight. Mild temperatures will continue
overnight as increasing cloud cover blankets the region.
The general consensus among the models has rain spreading across
the North Bay beginning around sunrise Wednesday morning then
spreading south into the greater Bay Area by midday. Showers are
expected to reach the Monterey Bay Area in the afternoon. Along
with the rain this system will introduce gusty southerly winds
ahead of the frontal boundary. Sustained winds of 20 to 30 mph
with gusts to around 45 mph are expected across the region. Higher
elevations will see higher wind speeds as well as gusts over 50
mph along the ridges. A Wind Advisory remains in effect for the
North Bay beginning Wednesday morning and for area to the south
beginning Wednesday afternoon as the frontal boundary moves
through. (See SFONPWMTR for more detailed information.)
From previous discussion...Moderate to locally heavy rainfall
will be possible during the afternoon and evening hours across
much of the region. The greatest rainfall amounts associated with
this first system will likely occur over the North Bay (1.50" to
2.50") and down along the coastal ranges (2.00" to 4.50").
Meanwhile, most inland and urban areas can expect totals generally
less that 1.00" and potentially less than 0.50" in the typically
drier locations. In wake of the frontal passage, showers will
linger from Wednesday night into Thursday.
Our second system appears to be a bit weaker with lesser rainfall
amounts expected as it pushes through the region Thursday night
into Friday morning. Breezy to gusty winds will also accompany
this system, again mainly ahead of and along the frontal boundary.
Additional rainfall amounts will range between 0.25" to around
1.00" in the urban areas and inland valley locations with 1.50" to
2.50" amounts in the North Bay and along the coastal ranges. By
Friday afternoon, lingering showers are expected as the more
moderate band of precipitation shifts southward. Showers are then
expected to linger into Saturday and snow levels are also forecast
to fall below 3,500 feet by Friday night in wake of the first two
systems. Thus, some of the higher peaks across the region may see
snow begin to accumulate from Friday night into Saturday morning.
A third and final system remains on track to impact the region
from late Saturday into Sunday. This system has may be stronger
than the first two as deeper moisture and higher PWAT values
advect into the central California coast. Do have a few days to
monitor this third system and we will continue to update the
forecasts to reflect the latest model output and forecaster
confidence. Nonetheless, increased southerly winds along with wind
gusts in the 40 to 50 MPH range will be likely again as the front
sweeps through the region.
While each of these systems alone will likely not result in major
hydro issues, the combined rainfall could result in issues along
the smaller/quick response creeks by the weekend. By the time all
is said and done, rainfall amounts across the region could range
between 1.50" to 3.00" in most urban areas outside of the North
Bay where 3.00" to 6.00" will be possible. The heaviest rainfall
will occur in the North Bay Mountains and coastal ranges down
through Monterey County where 5.00" to 8.00" will be possible with
locally higher amounts upwards of 10.00". Given the already
saturated soils and stronger winds, downed trees are again likely
as well as mud/rock slides and potential for debris flows in the
recent burn scar areas with the passage of each of these systems.
Please stay up to date with the latest forecast information
through late week and into the upcoming weekend!
While lingering showers may persist into Monday, dry conditions
are forecast to return by late Tuesday and likely persist into at
least much of next week as high pressure builds over the West
.AVIATION...As of 4:03 PM PST Tuesday...Inherited forecasts look
good. High clouds and generally hazy conditions are persisting
around the San Francisco Bay with some reduced slant visibilities
possible near sunset. Clouds will increase over the area and
gradually lower tonight. Southeast winds will ramp up tomorrow
ahead of an approaching weather system. Rain will spread from
north to south through the day.
Vicinity of KSFO....VFR with some reduced slant visibility near
sunset. Light east winds becoming southeast and gusty Wednesday.
Rain developing near 18z Wednesday.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR. Light winds tonight becoming SE and
increasing on Wednesday.
.MARINE...as of 3:46 PM PST Tuesday...Southerly winds will
increase by Wednesday ahead of a frontal system. Winds will
decrease late Wednesday night as the low lifts northeast but will
increase again by Friday as another storm approaches. Very large
west swells will arrive later this week with swells building as
high as over 25 feet Friday and Saturday.
.Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm from 8 PM
SCA...Pt Arena to Pigeon Pt 10-60 nm from 8 PM
PUBLIC FORECAST: CW
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