Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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000
FXUS66 KMTR 200559
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
959 PM PST Sun Feb 19 2017

.SYNOPSIS...High impact event set to unfold during the next 24 to
36 hours as a potent Pacific storm system and atmospheric river
bring widespread rainfall and strong winds to the region. Periods
of heavy rainfall are likely from late Sunday Night through
Monday evening, potentially resulting in flooding across portions
of the region. In addition, strong and gusty southerly winds are
expected from tonight through Monday night, with strongest winds
anticipated on Monday afternoon and evening. Rainfall and winds will
taper off late Monday night into Tuesday. Cool and drier weather
are expected during the second half of the work week.

&&

.DISCUSSION...As of 9:30 PM PST Sunday...Atmospheric river (AR)
with precipitable water values as high as 1.5 inches is
approaching the central California coast this evening. Leading
edge of incoming rainfall has just recently been picked up by KMUX
radar and widespread precipitation should begin to spread inland
across coastal areas south of San Francisco over the next few
hours. Although the onset of precipitation with this system is
somewhat later than expected, there is still ample evidence to
suggest that this system will generate locally heavy rainfall
after midnight tonight, as well as increasing winds along the
coast and in the hills. Then, more widespread heavy rain and
stronger winds will develop on Monday. In addition, there are
convective elements within the incoming moist flow. Lightning
detection networks have been picking up several lightning strikes
about 200 miles west of Monterey County over the past few hours.
Have therefore added a slight chance of thunderstorms to the
forecast for tonight for the southern half of our forecast area.

Although forecast confidence is high that this incoming system
will be a high impact weather event with flooding likely,
predicting where the heaviest and most prolonged rainfall will
occur (and thus greatest flooding threat) is still problematic as
the models are having a tough time deciding where the most
persistent heavy rain will occur. The general idea is that
precipitation intensity will increase across western Monterey
County late this evening, but then quickly shift northward into
the San Francisco Bay Area after midnight tonight. On Monday, the
GFS and HRRR models indicate the frontal boundary will stall
across the central part of the Bay Area, resulting in prolonged
moderate to heavy rain rates from southern Marin County south
through Santa Cruz and Santa Clara Counties. The NAM and the WRF
models, on the other hand, focus more of the heavy rain across the
North Bay tomorrow, although they too show pockets of heavy rain
south of the Golden Gate. The upshot is that the entire San
Francisco Bay Area and portions of the Monterey Bay Area should be
prepared for excessive rainfall late tonight and through the day
on Monday. This rainfall will likely result in widespread flooding
as well as debris flows (mud and rock slides).

Some specific areas for flooding concerns are:

*  Russian River at Guerneville: May flood by Tuesday morning
*  Coyote Creek in Santa Clara County: Will likely flood by Monday
   evening
*  Carmel River in Monterey County: May flood by Monday Night
*  San Lorenzo River in Santa Cruz County: May flood by late
   Monday morning.

In addition, numerous other small rivers and streams will likely
reach or exceed bankfull over the next 24 to 36 hours, and all
rivers will see significant rises.

Southerly winds have already picked up over the coastal waters
where they currently gusting as high as 40 mph. Expect winds to
increase over land through the night, but mainly near the coast
and in the hills. The models have been consistent in forecasting
the strongest winds to occur Monday afternoon and evening. A wind
advisory is currently in effect for our entire forecast area
through 1 pm Monday. Then, beginning 1 pm Monday, a High Wind
Warning goes into effect for most coastal areas as well as the
hills above 1000 feet. This wind advisory/high wind warning will
remain in effect until late Monday night when winds are finally
expected to diminish. Note: The North Bay Valleys have been
removed from the High Wind Warning but a wind advisory remains in
effect for that zone.

From Previous Discussion...Forecast models have continued to
struggle pinpointing the axis of heaviest precipitation with this
particular atmospheric river, with various runs and models
highlighting every location from Big Sur to the North Bay with
intense rainfall. Model trends begin to emerge amongst the noise,
which have given higher confidence than any single model alone
could do. These trends suggest that for late this afternoon/this
evening, rainfall will initially be advected inward on an east to
west axis and widespread along the coastal ranges from Big Sur to
the North Bay with no clear stand out. Next, the plume will begin
to narrow and intensify between 10pm and 4am overnight as it
interacts with a descending low to the north and ridge to the
south. As this band intensifies, the axis will shift from an east
to west setup to southwest to northeast axis. This is important
because it will likely enhance the orographics of this event and
further increase anticipated rainfall. This narrow, intense band
has been trending towards a landfall somewhere between the coastal
Santa Cruz mountains and North Bay, with a mean landfall zone over
the I80 corridor (Golden Gate Bridge to Sacramento to Northern
Sierra Nevada). Confidence is moderate to high that the heaviest
rain will fall somewhere in this zone.

Forecast precipitation values have been wildly varying run to run
and model to model, so confidence is not as high on exact storm
total precipitation values. That said, the afternoon forecast
package has been built around the most likely scenario previously
discussed. Given that scenario, here are how the forecast values
have played out, from least to most. Inland San Benito/Monterey
county will be rain shadowed by the Big Sur range, 1.0-2.5". Santa
Clara Valley, partially rain shadowed, 1.5-3.0". Inland East Bay,
2.5-4.0". Inland North Bay and San Francisco Bay Shorelines,
3.0-5.0", locally higher if main intense rainfall band stalls
aloft here. Big Sur and East Bay peaks and ranges, 4.0-6.0".
Coastal North Bay 5.0-9.0". Coastal Santa Cruz and San Mateo
ridges, 5.0-10.0, locally higher if main intense rainfall band
stalls aloft here. Depending on how far north of south the main
intense rainfall band wobbles over the next 24-36 hours, one or
more of these locations could see locally higher or lower amounts.
A flood watch is in effect for all areas in anticipated of this
excessive rainfall through the coming days.

In addition to these heavy rains, this system will also generate
strong southerly winds through the duration of the event. The
strongest winds are anticipated to arrive Monday afternoon into
evening, before tapering off overnight into Tuesday. A wind
advisory is effect for all areas tonight into early Monday, with a
high wind warning in effect for most coastal and higher elevation
locations by Monday afternoon. The strongest winds from this
system are NOT expected to be as strong as our last windy system
on Friday.

Given the wet antecedent conditions from weeks of wet weather,
and the anticipated heavy rainfall and accompany wind forecast,
except to see widespread impacts in various forms. Excessive
rainfall falling on already saturated soils will runoff moreso
than be absorbed, which will lead to widespread areal flooding
from pooling of water in low lying areas, and rapid rises on
creeks, streams, and rivers. These saturated soils will also
respond to the additional rain in the form of rock, mud, or
landslides, especially in steeper terrain. These slides could
block, damage, or destroy roadways. Furthermore, even if the winds
are not as strong as the last system, they will be sufficient to
knock over additional weakened trees who are rooted in saturated
soils, which could lead to power outages, blocked roadways, or
blocked drainages. Be sure to plan ahead if travel is necessary
and check the latest road conditions available from CalTrans
websites. Turn around, don`t drown, flooded areas can be
deceptively deep.

The core of the upper low associated with this feature will shift
inland early next week, leading to an increased chance of
thunderstorms, primarily for the North Bay. Drier weather is then
anticipated for later Wednesday, Thursday, and into early Friday.
Model solutions diverge late in the week into early next week, but
there it seems more likely than not another system will roll
through our area by next weekend.

&&

.AVIATION...As of 10:00 PM PST Sunday...MVFR cigs. Areas of light
rain will continue into early evening. Radar indicating more
widespread rain approaching the area. Latest models show
a narrow plume of moderate to heavy rain will move into the SFO
Bay Area after 09Z. Hard to pinpoint where this plume is aimed at
but all terminals should expect to see a period of moderate
to locally heavy rain. Rain will turn to showers Monday night
as the plume moves inland but a surface low will pass closest to
the area Monday night as it lifts northeast. Southeast wind gusts
increasing to 25-30 kt as the plume of moisture arrives. Winds
will increase even more to 35-40 kt gusts Monday evening due to
the close proximity of the low. Winds slowly decreasing late
Monday night.

Vicinity of KSFO...MVFR. Light rain today becoming moderate
after 09Z. Southeast wind gusts to 25 kt after 06Z increasing to
30-35 kt after 18Z and 35-40 kt Monday evening.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR/MVFR with periods of light rain
through Monday. Rain may intensify Monday night as the moisture
plume is finally expected to reach the MRY Bay Area.
Southeast winds gusting to 25 kt in the Salinas Valley increasing
to 30-35 kt Monday afternoon and evening. Low level wind shear
(llws) developing during the evening.

&&

.MARINE...as of 10:00 PM PST Sunday...An approaching storm system will
bring gusty southerly winds to the coastal waters through Tuesday with
gale force winds developing in some areas. Marginally large swell Monday over the
coastal waters will result in hazardous seas.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...High Wind Warning...CAZ006-505-507-509>512-517-518-529-530
             Flood Watch...CAZ006-505>513-516>518-528>530
             Wind Advisory...CAZ006-505-507-509>512-517-518-529-530
             Wind Advisory...CAZ506-508-513-516-528
             GLW...Pt Reyes to Pigeon Pt 0-10 nm
             GLW...Pigeon Pt to Pt Pinos 0-10 nm
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Rough Bar Advisory for SF Bar until 3 PM
             SCA...Mry Bay until 1 PM
             GLW...Mry Bay from 1 PM
             SCA...SF Bay until 7 AM
             GLW...SF Bay from 7 AM

&&

$$

PUBLIC FORECAST: Dykema/DRP
AVIATION: W Pi
MARINE: W Pi


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