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

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753
FXUS66 KMTR 200044
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
444 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
brings widespread rainfall and strong winds to the region. Periods
of heavy rainfall are likely from late this evening through Monday
evening, potentially resulting in flooding across portions of the
region. In addition, southerly winds will increase late today and
be locally strong and gusty through Monday evening. Widespread
rainfall will then taper off late Monday night into Tuesday.

&&

.DISCUSSION...as of 03:00 PM PST Sunday... Latest water vapor
satellite imagery depicts a 1.30 - 1.70" TPW precipitable water
plume extending from Hawaii to central California early this
afternoon. This precipitable water plume qualifies as an
atmospheric river given that forecast integrated water vapor
transport values well exceed the accepted threshold values and
then some. Model IVT values exceed 500 kg/ms (twice the accepted
250kg/ms threshold), between 36-38N (roughly encompassing Big
Sur to the North Bay) which indicates the potential for excessive
rainfall for our forecast area from this plume. WPC (the NWS`
national forecast/modeling center) agrees with this assessment
and have placed our local forecast area under a MOD risk of
excessive rainfall, as well areas around Sacramento and the
northern Sierra Nevada.

As of now, only the lighter warm sector precipitation has advanced
onshore ahead of the main moisture surge. KMUX radar is sensing
light to moderate echoes of up to about 35dBz with these
overrunning rain showers, which have already brought 0.75-1.30"
to the coastal North Bay mountains. These rain showers initially
began over the North Bay, but have now spread in coverage from the
Oregon border in the north and Point Conception to the south.
Satellite and forecast data have continued to suggest that the
initial arrival of the main moisture surge would encompass a large
stretch of coastal California, which reflects what is now being
observed across the mosaic radar imagery, before narrowing into
more intense band of extremely heavy rainfall later in the event.

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 4:15 PM PST Sunday...MVFR cigs. Areas of light
rain will continue into early evening. Latest models show that
by 08Z a narrow plume of moderate to heavy rain will move into the
SFO Bay Area. Hard to pinpoint where this plume is aimed at but
all terminals should expect to see some moderate rains. Rain will
taper off late Monday morning as the plume moves inland but a
surface low will pass closest to the area Monday afternoon as it
lifts northeast. This will keep showers going through Monday
night. Southeast wind gusts increasing to 25-30 kt after 06Z as
the plume of moisture arrives. Winds shift to south to southwest
Monday morning then increase in gusts to 35-40 kt Monday afternoon
due to the close proximity of the low.

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.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR/MVFR. Light rain tapering off after
13Z as moisture plume shifts north. Southeast winds gusting to 25
kt in the Salinas Valley becoming more widespread after 06Z.
Low level wind shear (llws) developing during the evening.

&&

.MARINE...as of 01:40 PM PST Sunday...An approaching storm system
will bring gusty southerly winds to the coastal waters through
monday with gale force winds developing near the coast. marginally
large swell monday over the southern coastal waters will result in
hazardous seas.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Flood Watch...Entire San Francisco and Monterey Bay areas
             Wind Advisory...Entire San Francisco and Monterey Bay areas
             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...SF Bay
             SCA...Mry Bay
             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

&&

$$

PUBLIC FORECAST: DRP
AVIATION: W Pi
MARINE: Sims


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