Flash Flooding and Debris Flows in California, February 6 to 8, 2017

Area Forecast Discussion
National Weather Service San Francisco Bay Area
439 PM PST Mon Feb 6 2017

.SYNOPSIS...An atmospheric river is forecast to impact the region
from tonight through Tuesday afternoon and will result in
widespread rainfall and locally strong winds. Given already
saturated soils, flood concerns will increase as well as potential
for rock/mud slides across the region. Breezy to gusty winds are
also expected to result in downed trees and isolated power
outages. Precipitation will then taper off from Tuesday night into
Wednesday before another frontal system sweeps through on
Thursday. Drier weather conditions are then forecast to return by
the upcoming weekend.

&&

.DISCUSSION...as of 02:21 PM PST Monday...The latest forecast
models continue to show an atmospheric river with PWAT values
exceeding 1.25" aimed at our region from late this evening into
Tuesday afternoon. Light rain has continued through much of the
day across the North Bay and along the coastal ranges and this
will continue through into the night. As the deeper moisture
advection reaches the coast later tonight into Tuesday morning,
rainfall will increase in coverage and intensity. The models also
show an up tick in rainfall rates from around 4 AM through 10 AM
PST Tuesday as an impulse approaches the coast and interacts with
the deep moisture and ongoing upslope. Southerly winds ahead of
this system will also increase late this evening and persist into
Tuesday afternoon. A Wind Advisory remains in effect for much of
the region as gusts of 40 to 45 MPH will be possible along the
coast and in the lower elevations with gust greater than 50 MPH
possible in the higher elevations. Precipitation may linger
through the afternoon along Big Sur coast before tapering off
Tuesday night region-wide. Rainfall amounts through Tuesday night
will range from 2.00"-2.50" in the North Bay with 3.00"-6"00 along
the coastal ranges. Locally higher rainfall amounts will be
possible. Outside of the North Bay, rainfall totals in the urban
areas will range from 0.50" to 1.75" with lesser amounts possible
in the Santa Clara and Salinas Valleys due to downsloping.

Light precipitation may continue into Wednesday, yet many areas
will see a brief break in rainfall. Further drying is forecast
into Wednesday night before another frontal system impacts the
region on Thursday/Thursday night. The latest models show this
system to be quick moving, yet stronger mid/upper level support
will help to produce widespread rainfall across the region once
again. Winds will also increase and become gusty along and ahead
of the frontal passage. Rainfall amounts remain difficult to nail
down at this point, yet another 0.25" to 0.50" will be likely in
many urban areas with an additional 1.00" to 3.00" possible along
the coastal ranges. Showers are then forecast to linger in wake of
this system into Friday and potentially Friday night.

Dry conditions are then forecast by the upcoming weekend as high
pressure begins to build over the Eastern Pacific. This ridge is
forecast to shift inland and result in a slight warming trend as
well region-wide into early next week.

&&

.AVIATION...As of 4:15 AM PST Monday...For 00z tafs. Showers will
continue across area terminals through much of forecast period
with periods of moderate to heavy rain expected tonight into
Tuesday as a system moves into the area. Southerly winds will
strengthen and turn increasingly to the southeast by this
evening. Strong winds aloft will mix down to the surface tonight
resulting in strong and gusty winds at area terminals tonight
into Tuesday. Monterey bay terminals will be much drier with the
southeast winds keeping rain and reduced vsbys in check. Precip
should turn showery for the Bay Area on Tuesday afternoon as the
band of heavier rain shifts southward and impact Monterey bay
terminals.

Vicinity of KSFO...Southerly winds will strengthen and become
gusty this evening. Winds will also turn increasingly
southeasterly through tonight as the next boundary approaches.An
Airport Weather Warning for winds is in effect from 06z till 20z
Tuesday. Periods of moderate to heavy rain with reduced vsbys
expected after 04z. MVFR cigs expected to prevail.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...South to southeasterly winds will
strengthen and become gusty after 06z tonight. Light showers
expected through this evening with periods of moderate rain with
reduced vsbys expected after 05z. MVFR to VFR cigs expected.
Showers will continues Tuesday morning but then more organized
rain band shifts southward after 18z Tuesday.

&&

.MARINE...as of 04:21 PM PST Monday...Southerly winds will
increase tonight as the next weather system moves over the region
with periods of rain accompanied by some gale force wind gusts
along the sonoma coast and inside the northern san francisco bay.
southerly winds continue on tuesday behind the system but will
ease slightly. southerly winds and moderate seas continue through
midweek then another cold front moves through by thursday with
winds turning nw later thursday into friday and saturday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Wind Advisory...CAZ006-505-507>513-517-518-528>530
             Flash Flood Watch...CAZ505>507-509-512-517-529
             SCA...Mry Bay from 9 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 9 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 9 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 9 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm from 3 AM
             GLW...Pt Arena to Pt Reyes 0-10 nm from 6 PM until 3 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm from 9 PM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm from 2 AM
             GLW...Pt Reyes to Pigeon Pt 0-10 nm from 8 PM until 2 AM
             GLW...SF Bay from 9 PM

&&

PUBLIC FORECAST: RGass
AVIATION: CW
MARINE: RWW

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HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
357 AM PST MON FEB 6 2017

CAZ006-505>513-516>518-528>530-071200-
SAN FRANCISCO-
COASTAL NORTH BAY INCLUDING POINT REYES NATIONAL SEASHORE-
NORTH BAY INTERIOR VALLEYS-NORTH BAY MOUNTAINS-
SAN FRANCISCO BAY SHORELINE-SAN FRANCISCO PENINSULA COAST-
EAST BAY INTERIOR VALLEYS-EAST BAY HILLS AND THE DIABLO RANGE-
SANTA CRUZ MOUNTAINS-SANTA CLARA VALLEY INCLUDING SAN JOSE-
SOUTHERN SALINAS VALLEY/ARROYO SECO AND LAKE SAN ANTONIO-
SANTA LUCIA MOUNTAINS AND LOS PADRES NATIONAL FOREST-
MOUNTAINS OF SAN BENITO COUNTY AND INTERIOR MONTEREY COUNTY
INCLUDING PINNACLES NATIONAL PARK-
NORTHERN SALINAS VALLEY/HOLLISTER VALLEY AND CARMEL VALLEY-
NORTHERN MONTEREY BAY-SOUTHERN MONTEREY BAY AND BIG SUR COAST-
357 AM PST MON FEB 6 2017

THIS HAZARDOUS WEATHER OUTLOOK IS FOR THE SAN FRANCISCO AND
MONTEREY BAY REGION.

ANOTHER ROUND OF WIDESPREAD RAIN AND GUSTY SOUTHERLY WINDS WILL
MOVE THROUGH OUR REGION THIS EVENING THROUGH AT LEAST TUESDAY
MORNING. AT THIS TIME STREAMS AND RIVERS ARE EXPECTED TO RISE
RAPIDLY BUT SHOULD STAY UNDER FLOOD LEVELS. RAINFALL COULD BE
HEAVY AT TIMES AND WINDS WILL LOCALLY GUST TO OVER 35 MPH. A WIND
ADVISORY IS IN EFFECT FOR TONIGHT THROUGH TUESDAY MORNING.

SHOWERS WILL SWITCH BACK TO WIDESPREAD RAINFALL ON THURSDAY WITH
MORE SHOWERS LIKELY ON FRIDAY. DRIER WEATHER WILL RETURN ON
SATURDAY.