Debris Flows and Flooding in California, February 25 to 28, 2019

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1014 AM PST Mon Feb 25 2019

.SYNOPSIS...An atmospheric river is set to take aim at the San
Francisco Bay Area today through early Wednesday morning. Periods
of moderate to heavy rainfall and strong, gusty winds will impact
the region with the potential for widespread flooding from the
Santa Cruz Mountains northward into the North Bay. Rain will then
spread over the entire region late Tuesday into Tuesday night as a
cold front pushes inland. Precipitation chances diminish on
Wednesday with the next chance of rainfall expected Saturday into
Sunday.

&&

.DISCUSSION...as of 09:50 AM PST Monday...Overnight temps were
generally in the 40s to lower 50s. These milder temps are due to
increasing moisture and cloud cover preventing strong overnight
cooling. Southeasterly winds in the lower elevations were fairly
light but are starting to increase. Winds in the hills are
increasing more rapidly with gusts between 30 and 50 mph already
being observed. Some light rain fell overnight in the North Bay
with KMUX radar indicating rain increasing across northern
portions of our area.

Forecast remains on track. An atmospheric river is forecast to
bring major impacts to the Bay Area through Wednesday. Rain will
continue to become more widespread and heavy across the North Bay
this morning and reach the Bay Area this afternoon. The rainfall
band with the atmospheric river will be narrow and is shown by the
hi-res models to stall out tonight, limiting its southward
progress. San Francisco and points north are forecast to receive
heavy rainfall this afternoon through the overnight hours tonight.
Points to the south of San Mateo and Alameda counties are more
likely to see light rain through tonight, with areas near the
Monterey Bay likely to stay dry. Rainfall for the Santa Cruz
Mountains through tonight is still highly uncertain. A slight
shift south in the atmospheric river would bring abundant
rainfall overnight, and some of the models disagree about exactly
how far south the band will get. The heaviest rainfall totals
through sunrise Tuesday will be focused on the North Bay with
valley locations receiving 2-3 inches and mountains 3-6 inches with
locally higher amounts.

The band will then shift a little further south and stall on
Tuesday morning with areas from Santa Cruz County north to Sonoma
County expected to receive rainfall throughout the day.
Additional rainfall will exacerbate flooding concerns throughout
the North Bay with flooding potential increasing across the
central and southern Bay. The Santa Clara Valley is likely to
receive much less rain than surrounding areas due to a strong rain
shadow. Flooding concerns will also increase in the Santa Cruz
Mountains on Tuesday. A shortwave trough will push through on
Tuesday night, finally pushing the front and heavy rain through by
Wednesday morning. Monterey and San Benito counties, which likely
will not receive nearly as much rainfall through Tuesday
afternoon, will see most of their rainfall overnight on Tuesday.
Rainfall totals through Wednesday morning will range from 6-12
inches in the North Bay with locally higher amounts, 6-10 inches
in the Santa Cruz Mountains, and 3-5 inches around the SF Bay
shoreline and coastal areas. Rain shadowed areas, such as the
Santa Clara Valley and portions of Monterey County, will likely
see less than an inch from the event. Heavy rain through much of
the area will lead to flooding and hydro concerns, primarily north
of the Monterey Bay.

Winds will become gusty at the coast and in the higher terrain
this afternoon and continue through Tuesday night until the front
pushes through. Wind speeds are likely to maximize Tuesday
afternoon and evening as the trough approaches and the front
pushes through. Gusts to 50 mph along the coast and in the hills
will be possible. Downed trees and power outages are possible with
these winds with saturated soils.

&&

.PREV DISCUSSION...as of 02:09 AM PST Monday...Generally tranquil
weather conditions across the region early this morning with
temperatures in the 40s and 50s and generally light winds.
However, an atmospheric river is taking aim at the northern
California coast with widespread rainfall occuring. The latest
forecast models show the deeper plume of moisture shifting
southward into the North Bay by late morning and early afternoon.
Widespread rainfall is then forecast to develop over the San
Francisco Bay Area this afternoon and then as far south as the
Santa Cruz Mountains late this evening and into tonight. As with
any atmospheric river, the forecast model solutions diverge from
one another on exactly where this boundary will stall during the
next 24 hours or so. The GFS and ECMWF keep the focus on the North
Bay while the NAM, local WRF and other short-range, high
resolution models point to a solution further to the south over
the Santa Cruz Mountains. With this said, rainfall will be
heaviest along the coastal ranges with rain shadowing likely in
the interior valley locations such as the Santa Clara Valley. With
IVT values at or above 500 kg/ms (moderate AR) forecast to advect
inland for a period of 24 to 36 hours, rainfall amounts in excess
of 6 to 10 inches will be likely in the coastal ranges from the
Santa Cruz Mountains northward today through Wednesday morning.
This will likely result in flooding of small creeks/streams, low
lying and poorly drained areas as well as on the main stem rivers.
Widespread rock/mud slides will also be likely in the region's
hilly terrain. Thus, this high impact weather event will need to
be monitored closely in the next 48 hours. Flash Flood/Flood
Watches have been posted from Santa Cruz and Santa Clara Counties
northward.

Ahead of the aforementioned boundary, southerly winds will increase
and become locally gusty by this afternoon over the region's higher
elevations as well as along the coastline. Thus, a Wind Advisory is
in effect through late tonight for south winds of 25 to 35 mph with
gusts up to 50 mph. Strong winds and already saturated soils will
likely result in downed trees and potential for isolated to
scattered power outages. The Wind Advisory in effect through early
Tuesday morning will likely need to be extended into late Tuesday
night for a portion of the region as another round of strong winds
is expected ahead of a cold front front that will push inland
Tuesday night into Wednesday morning. The local WRF model shows
another very strong wind event for areas such as Pebble Beach,
Pacific Grove and around Point Lobos (similar to our previous wind
events). Winds are forecast to diminish Wednesday morning in wake of
the frontal passage.

While the main atmospheric river event is likely to be focused on
the northern portion of the region, widespread rainfall is forecast
over the Central Coast Tuesday night into Wednesday morning as the
main cold front pushes inland. There will also be the potential for
a few thunderstorms with increased instability ahead of the frontal
passage. As mentioned above, rainfall totals will be greatest over
the North Bay and into the Santa Cruz Mountains with 6 to 10 inches
in the hills, coastal ranges and higher elevations. Will not be
surprised to see amounts upwards of 12 inches in the region's
wettest locations while areas such as San Jose may see less than 1
inch through the event. Urban areas from San Francisco northward and
into the East Bay Hills could see amounts of 3 to 5 inches. To the
south, the coastal ranges down the Big Sur coast could see up to 5
inches with only 0.25" to 0.50" around Monterey and Salinas. Again,
rainfall totals will vary widely.

Precipitation diminish Wednesday morning with only lingering showers
expected through the afternoon. Dry conditions develop by Thursday
and persist into Friday with cooler overnight temperatures. Another
storm system is forecast for the upcoming weekend with additional
light to moderate rainfall amounts. With all attention focused on
the next 48 hours or so...please stay up to date with the latest
forecast information in the coming days.

&&

.AVIATION...As of 10:13 AM PST Monday...For 18z tafs. Challenging
forecast over the next 24-36 hrs as incoming atmospheric river
will bring rain (moderate-heavy), gusty winds and reduced
visibility. One would expect another delay program to be put in
place once the event unfolds. Current thinking is rain north of
the Golden Gate through early this afternoon then spreading
southward impacting SFO/OAK/LVK. SJC will be initially rain
shadowed so VCSH is carried. MRY/SNS remain dry through early
tomorrow. The band of precip is forecast to drift north overnight,
which may give SFO/OAK a break. Once the boundary begins to
finally move southward on Tuesday all terminals will see period of
moderate to heavy rain. Rain will bring MVFR to IFR cigs with
visibility of 3-5 SM. In addition to rain, winds will increase
over the next 24 hours with gusty sfc winds, but rather strong
winds at 2500 feet. LLWS will likely be a concern.

Vicinity of KSFO...Breezy SE winds with MVFR to VFR cigs through this
afternoon. Will bring showers into SFO this afternoon with a brief
break possible overnight, before moderate to heavy rain tomorrow.
Winds will be gusty with wet runways on Tuesday, but as of now
winds look to below 35 kt and an AWW is not needed.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR with occasional MVFR cigs. Trend has
been to slow down precip onset for MRY and SNS until tomorrow.
Winds will increase through tomorrow as well with SNS being the
strongest with gusts to 35kt.

&&

.MARINE...as of 09:55 AM PST Monday...Southerly winds will
increase across all waters today a potent storm system impacts the
region. Strong winds will generate steep waves creating hazardous
conditions, especially for smaller  vessels. Windy and wet weather
will continue through Wednesday  morning before winds are forecast
to diminish. Light to moderate  northwest swell will persist
before a longer period northwest  swell arrives late this week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Wind Advisory...CAZ006-505-507-509-511-512-517-529-530
             Flash Flood Watch...CAZ006-508>513-529
             Flood Watch...North Bay
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 1 PM
             GLW...Pigeon Pt to Pt Pinos 0-10 nm from 1 PM
             GLW...Pt Reyes to Pigeon Pt 0-10 nm
             GLW...Pt Arena to Pt Reyes 0-10 nm
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 PM
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 3 PM
             SCA...SF Bay from 10 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 10 AM
             GLW...SF Bay from 4 PM
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm from 4 PM
             SCA...Mry Bay from 1 PM

&&

PUBLIC FORECAST: ST
AVIATION: MM
MARINE: MM

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