Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
FXUS66 KMTR 172150
Area Forecast Discussion
National Weather Service San Francisco Bay Area
150 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 01:43 PM PST Tuesday...Dry weather conditions
along with seasonable temperatures persist over the region today
and will continue into the overnight hours. These conditions as
well as the passing high clouds are in advance of our next
approaching storm system set to arrive on Wednesday. Given the
increase in clouds and lower level moisture, temperatures
overnight/Wednesday morning will be slightly warmer than previous
mornings, generally in the low to upper 40s region- wide.
The forecast models are in good agreement that rain will begin to
spread into the North Bay generally around or slightly after
sunrise and then spread into the San Francisco Bay Area through
the afternoon. Rain will also begin to develop along the coastal
ranges further south ahead of the approaching cold front.
Southerly winds out ahead of the boundary will also increase ans
become gusty at times. With wind gusts of 40 to 45 MPH in the
lower elevations and upwards of 50 MPH in the higher elevations, a
Wind Advisory has been posted for a good portion of the region
tomorrow for winds ahead of and along the frontal boundary.
Precipitation will continue to spread southward through the
afternoon hours down into the Central Coast and Monterey Bay
region. 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 9:40 AM PST Tuesday...Areas of low clouds and
fog around the North Bay and also edging eastward from the
Central Valley into the East Bay. Hazy conditions are occurring
around the San Francisco Bay with some slant visibilities to 5
miles. Otherwise, high clouds are increasing over the area and
will gradually lower tonight. Southeast winds will ramp up
tomorrow ahead of an approaching weather system.
Vicinity of KSFO....Mainly VFR today with some slant vsby down to
5 miles in haze. Light east winds becoming southeast by Wednesday
morning and increasing.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR.
.MARINE...as of 08:54 AM PST Tuesday...High pressure off the
central California coast will weaken today as a low pressure
system approaches the Pacific Northwest. Southerly winds will
increase 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 20 to 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: RGass
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