Flash Flooding and Debris Flows in California, February 20 to 24, 2017

Area Forecast Discussion
National Weather Service San Francisco Bay Area
334 AM PST Mon Feb 20 2017

.SYNOPSIS...High impact event unfolding across the region this
Monday morning and will likely persist through the afternoon hours
as an atmospheric river takes aim at the central California coast.
Periods of heavy rainfall are expected, especially along the coastal
ranges, along with strong and gusty southerly winds through this
evening. These conditions will likely result in widespread flooding,
downed trees and power lines resulting in isolated to widespread
power outages. Flooding of area creeks, streams and low lying,
poorly drained areas can also be expected as well as rises on main-
stem rivers. The threat for rock/mud slides, shallow landslides and
debris flows in recent burn scar areas will also increase during the
next 12 to 24 hours. Rainfall and winds will taper off late tonight
into Tuesday. Cool and somewhat drier weather conditions are
expected during the second half of the work week.

&&

.DISCUSSION...as of 02:34 AM PST Monday...While the arrival of a
much anticipated atmospheric (AR) river was delayed slightly,
widespread rainfall now spreading across the entire region this
morning with satellite derived PWAT values of 1.5" approaching the
Central Coast. Thus, over the past few hours moderate to locally
heavy rainfall has spread inland across the region with even a few
lightning strikes offshore and across the North Bay. At times,
rainfall rates have reached between 0.33" to 0.50"/hr with this
trend likely to continue through 4 AM PST. The latest forecast
models continue to indicate a band of heavy rainfall will develop
and become more narrow in focus somewhere between San Francisco
and the Big Sur coast. At this point, rainfall rates in excess of
0.50"/hr and potentially as much as 1.00"/hr will be possible,
especially along the southwest facing slopes, which will greatly
increased the threat for flooding and flash flooding where/when
this boundary stalls. Current thinking is that the San Mateo and
Santa Cruz coast and points inland may be hardest hit through the
morning hours. However, given the rapidly developing nature of
this system, a jog north or south remains possible and thus
current trends will need to be closely monitored through the
morning. For additional information, feel free to read the latest
Mesoscale Precipitation Discussion from the WPC:
http://origin.wpc.ncep.noaa.gov/metwatch/metwatch_mpd_multi.php?md=00
60&yr=2017

Southerly winds out ahead of this system have also increased early
this morning with gusts in excess of 45 MPH over the Pacific and in
the highest peaks across the region. Expecting these winds to
further intensify through the afternoon and evening as surface low
pressure deepens offshore and races toward northern California/
southwest Oregon. Wind gusts of 60 MPH or greater will be possible
this afternoon and evening, mainly in elevations above 1000 feet and
along the coastal ranges. Downed trees and power lines will likely
result in isolated to widespread power outages through the day.
Thus, a High Wind Warning and Wind Advisory remain in effect through
late tonight.

A Flood Watch also remains in effect for the entire region through
tonight given the intense rainfall forecast over already saturated
soils. The combination of heavy rainfall and strong winds will also
result in an increased threat for rock/mud slides, shallow
landslides and debris flows in recent burn scar areas through this
afternoon. Through early Tuesday morning, storm total rainfall
amounts of 1.50-2.00" will be possible for inland San Benito and
Monterey County with amounts less than 1.00" possible in the rain
shadowed valley locations. Santa Clara Valley, also rain shadowed
can expect between 1.25-3.00" (higher amounts closer to the
foothills and lesser possible in downtown San Jose). Inland East
Bay, San Francisco Bay Shoreline, and North Bay Valley areas can
expect 2.25-3.50", locally higher if the heavy band of precipitation
makes a shift to the north. Big Sur and East Bay peaks and ranges,
4.00-7.00", Coastal North Bay 3.00-5.00", Coastal Santa Cruz and San
Mateo ridges, 5.00-10.00", again locally higher depending on
if/when/where the band of heaviest precipitation stalls.

The AR and frontal boundary will then shift southward this afternoon
as the aforementioned surface low off of the coast shifts inland
well to our north and the main mid/upper level low deepens off of
the Pacific Northwest coast.  In wake of the frontal passage, light
to occasionally moderate precipitation will persist along the
southwest facing slopes while diminishing over inland valley
locations. With the arrive of mid/upper level support and a colder
air mass aloft, isolated thunderstorms will also be possible from
late tonight into Tuesday morning. While organized, heavy rainfall
will tapper off on Tuesday during the day, lingering showers will
remain possible into Wednesday morning as the trough axis shifts
inland across the region.

Cooler and drier conditions are forecast from Wednesday into early
Friday as a weak ridge builds in behind the exiting trough. Late in
the week and into the upcoming weekend, forecast models continue to
differ yet show the potential for cool, unsettled weather
conditions. However, widespread, heavy rainfall is not currently
anticipated with this colder system given the lack of a deep
moisture tap. Stay tuned for the latest!

&&

.AVIATION...As of 3:34 AM PST Sunday...Strong southerly winds
spreading from the coastal waters to over land early this morning
with the passage of a strong warm front from the eastern Pacific.
Area metars indicating mostly de-coupled surface winds while winds
aloft are southerly at 30-40 knots. Once the warm front clears the
area this morning a newly re-generating front will set up WSW-ENE
across the Bay Area this morning eventually coupling with a slowly
approaching cold front from the west causing winds to steepen
through the day. WRF model forecasts up to 60 knots SW wind at the
925 mb level (approx 2500 feet) this afternoon and evening over
the Bay Area. Thus, potential for long duration low level wind
shear at least this morning, quite likely redeveloping again this
afternoon and evening. Otherwise MVFR cigs/vsbys and areas IFR in
downpours.

Vicinity of KSFO...MVFR cig/vsby, heavy rain and wind today through
this evening. IFR possible at times in downpours.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...MVFR cigs/vsbys, heavy rain and wind
today through this evening. IFR possible at times in downpours.

&&

.MARINE...as of 2:55 AM PST Monday...A strong warm front with heavy
rain...wind and possible additional widely scattered thunderstorms
is moving rapidly eastward across the coastal waters and bays early
this morning. Extensively moist air originating from the subtropics
will become firmly entrenched across the entire area today. A cold
front over the eastern Pacific will move slowly in from the west
passing through the Bay Area this evening and the North Central
Coast by midnight. Gales are expected today with possible storm
force gusts over the bays later today. Marginally large swell
today over the coastal waters will result in hazardous seas.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...High Wind Warning...Higher terrain Bay Area to Monterey
			 and San Benito counties.
             Flood Watch...Entire forecast area.
             Wind Advisory...Entire forecast area.
             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 Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Rough Bar Advisory for SF Bar
             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: RGass
AVIATION/MARINE: Canepa

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Special Weather Statement
National Weather Service San Francisco Bay Area
319 AM PST Mon Feb 20 2017

CAZ006-505>513-516>518-528>530-202300-
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-
Including the cities of...San Francisco...Tamalpais-Homestead...
Sausalito...Santa Rosa...South Santa Rosa...Napa...San Rafael...
Petaluma...Novato...Rohnert Park...Angwin...
Lagunitas-Forest Knolls...Woodacre...Oakland...Fremont...
Hayward...Daly City...Berkeley...Pacifica...Concord...Antioch...
Livermore...Walnut Creek...Pleasanton...Pittsburg...San Ramon...
Blackhawk...Scotts Valley...Boulder Creek...Day Valley...
San Jose...Greenfield...King City...Salinas...Santa Cruz...
Watsonville...Live Oak...Seaside...Monterey...Marina
319 AM PST Mon Feb 20 2017

...POTENT STORM SYSTEM AND ATMOSPHERIC RIVER LIKELY TO RESULT IN
WIDESPREAD FLOODING, STRONG WINDS AND INCREASED THREAT FOR
ROCK/MUD SLIDES, SHALLOW LANDSLIDES AND POTENTIAL DEBRIS FLOWS IN
HILLY TERRAIN AND RECENT BURN SCAR AREAS THROUGH TODAY...

Rainfall and winds continue to increase in coverage and intensity
early this Monday morning and will continue to do so through this
afternoon. Rainfall will be heavy at times, especially in the
North Bay region and along the southwest facing coastal slopes
resulting in the potential for life-threatening flooding. If you
have experienced flooding at any time this winter season, either
near your home or on your daily commute, the potential exists for
additional flooding in these areas during the next 12 to 18
hours.

In addition, southerly winds will increase this afternoon and
result in a heightened threat for downed trees and power lines
with the potential for isolated to scattered power outages. The
combination of heavy rainfall and strong winds will also give way
to widespread rock/mud slides, shallow landslides and potential
debris flows in hilly terrain and recent burn scar areas.

Residents are urged to remain vigilant and encouraged to stay up-
to-date with the lastest weather information throughout the day.
Those taking to area roadways should also check the latest
information from the California Highway Patrol and California
Department of Transportation for potential road closures.
Remember, "Turn Around, Don't Drown" when encountering flooded
roadways. It is NEVER safe to drive or walk into flood waters.

RGass