Debris Flows and Flooding in California, January 26 to 29, 2021

Area Forecast Discussion
National Weather Service San Francisco Bay Area
936 AM PST Tue Jan 26 2021

.SYNOPSIS...Its the calm before the storm. Rain will develop in
the North Bay this afternoon then spread southward through the Bay
Area overnight with heavy rain and damaging southerly winds. High
Wind Warnings and Flash Flood Watches are in effect throughout
the region. The Atmospheric River is forecast to stall over
Monterey county on Wednesday bringing copious rain to coastal
Monterey county while numerous rain showers will persist
throughout the Bay Area into Thursday. Shower activity should end
by Friday morning with dry weather for much of Friday. A weak
front will bring more light rain Saturday with another strong
storm possible by early next week.

&&

.DISCUSSION...as of 9:36 AM PST Tuesday...No significant short
term changes to ongoing forecast. Freeze Warning was allowed to
expire at 8 am. Cold temperatures persist across the region ahead
of the incoming AR. This is the type of set-up where we could see
some lower elevation snow at the onset of precipitation in the
North Bay. However, warm frontal structure of incoming boundary
will allow snow levels to rise fairly quickly above the hills of
the North Bay.

In terms of the incoming AR. Rain to develop in the North Bay this
afternoon, initially steady but light to moderate in the warm
sector. Rain will overspread the Bay Area creating a messy evening
commute. All attention on the cold front associated with the AR
and frontal boundary. Expect heavy rain into the North Bay by
02-04z. First concern will be North Bay 2020 burn areas. Short
term rain-rates could approach debris-flow thresholds. In addition
local flooding of roads, etc will be quick to occur. The main
front will pass through the Bay area around midnight. The next
focus will then be on the CZU burn area. Short term rain rates
approaching 0.75 will be possible as the cold-frontal boundary
passes through. The daylight hours of today will be your last
chance to make potentially life saving decisions. The heavy rain
occurring during the darkness of night along with winds likely to
produce power outages in the Santa Cruz mtns will only add to the
confusion of the upcoming night.

The one bit of good news with the latest model trends is the
boundary should pass through Santa Cruz county fairly quickly with
the heaviest rain ending by around sunrise Weds for the CZU burn
area. Daylight will reveal what massive logs, debris and mud do
when encountered by heavy rain-rates.

The AR is then progged to stall over Monterey County. The hills
above Big Sur will be the perfect lifting mechanism to squeeze out
moisture as the boundary will stall for upwards of 24 hours. The
Dolan, Carmel and River burn areas from 2020 will be of great
concern for mud and debris flow issues. Even without those this
is the type of event that usually causes numerous rock and
mudslides along Highway 1. Residents of Big Sur should be prepared
for extended periods of isolation. The 12z nam is showing amounts
from 10-14 inches for the Santa Lucia range which is line with our
current forecasts. Our local in-house WRF model is showing
extensive storm totals in the Big Sur hills in excess of 20
inches with a bullseye amount in excess of 31 inches. Not
forecasting that but sharing what type of data we are looking at
as the moisture boundary stalls with upwards of 24 plus hours of 1
inch rates possible in the wettest southwest facing slopes of the
Santa Lucia range. While the AR is stalled on the Central Coast
the Bay Area will see reinforcing waves of rain through the day
Weds into Thursday but rain-rates and amounts during this time
frame will be more nuisance than flood-causing. Nonetheless
unnecessary travel should be avoided throughout the region to
reduce car related accidents tonight through Thursday. Expect
power outages and downed trees/limbs throughout the region,
especially in the hills.

Shower activity should end by sunrise Friday. Another weak system
to bring some light rain Saturday with another moderate to strong
system possible by around Monday but this one may be more focused
over the North Bay in terms of heaviest precip.

&&

.PREV DISCUSSION...as of 4:00 AM PST Tuesday...Now for the much
talked above Atmospheric River and its impacts to the region. The
longwave pattern shows a trough ridge trough pattern over the
EPac. The moisture plume associated with the AR is riding up and
over the ridge and is now being focus eastward again toward the
West Coast. The final eastward push of moisture is being helped by
a low pressure system and associated cold front approaching the
PacNW this morning. In the big picture, the overall forecast has
not changed that much. This AR is still on track to be hazardous
to a large portion of NorCal with heavy rains, gusty winds, and a
Sierra snow dump. Locally, pre-frontal rains will first develop
over the North Bay mid-afternoon before spreading southward to Big
Sur by early evening. The initial push of showers will generally
be light to moderate. Given the lower snow levels currently over
the area the leading edge of precip may actually fall as snow over
the higher peaks of the North and East Bay, Santa Cruz Mts, and
peaks of Monterey/San Benito. This is a low confidence part of the
forecast. If timing is just right, some minor accumulations would
be possible.

Rainfall coverage and intensity is forecast to increase rapidly
with the cold front once it arrives. The lastest timing for the
cold front will be early evening for the North Bay - the heart of
SF Bay close to midnight and finally down to Santa
Cruz/Monterey/Big Sur early Wednesday morning. Hi- res models are
still showing ample dynamics (strong UL divergence and strong LL
convergence) with front leading to increased confidence for a
NCFR. This scenario definitely increases the likelihood of rain
rate thresholds being met for debris flow concerns on 2020 burn
areas. NBM/HRRR/HREF all suggest hourly rates >0.5"/hr with peak
intensity >0.75"/hr. A Flash Flood Watch is currently in play to
cover this potential for debris flow. If you have not already
heeded local county advice don't wait any longer. This morning is
the time to act and prepare if you're near a burn area.

Once the front/AR makes it to Big Sur models have been
advertising a stall with a slight pivot northward late Wednesday
into Thursday. Earlier runs did suggest a northward drift back to
near Santa Cruz, but that is looking less likely now, which is
good for Santa Cruz. Less rainfall totals, but Santa Cruz will
still have to deal with the fropa rain rates. Big Sur on the other
hand is starting to look like it could get dumped on if the AR
does stall. Ensemble guidance and AR model guidance show this
scenario with well above normal PWAT with a moderate AR category. The
stall/pivot will be key to overall rainfall accumulations. The
other wrinkle in the forecast that could impact rain rates with
the fropa or overall rainfall accumulations will be thunderstorms.
A slight chance of thunder will be possible with fropa and
possibly lingering into Thursday.

Rainfall amounts from Tuesday through Thursday haven't changed too
much: 2-3" for most lower elevation/urban locations, locally up
to 4" in southwest facing locations such as Santa Cruz. Some rain
shadowing due to the coastal ranges will occur, but be less
pronounced than usual given the stronger surface dynamics.
3-5" will be possible in the North Bay and interior
hills/mountains of the South and East Bay. 6-8" in the Santa Cruz
Mountains. 7-10" Mts of Monterey and San Benito Counties.
Isolated totals in excess of 10.00" cannot be ruled out across the
southwest facing coastal slopes along Big Sur. WPC has highlighted much
of the forecast area for excessive rainfall - Day 1 has most of
the region in a slgt risk for flash flooding and Day 2 slgt risk
over Santa Cruz and mdt risk over Big Sur for flash flooding. One
other item of note that could lead to increased potential for
flash flooding is the snowpack over the Santa Lucia Mts. The
snowpack isn't that big, but the highest peaks likely have 5-10
inches with ample rime. Snow melt will enhance runoff to the
watersheds in Big Sur.

The front will also pack a dangerous punch in the way of southerly
winds. Southerly winds will increase later today and be strongest
over the coastal waters, immediate coast, and higher peaks with
gusts 50-70 mph possible. A High Wind Warning remains in effect
from Tuesday evening through Wednesday. Other areas will also be
windy with gusts 35-50 mph with a Wind Advisory in effect.

The AR will finally move southward Thursday morning/afternoon
trending diminishing precip from N to S. A few lingering showers
will be possible through Friday morning.

An additional rain chances will be possible Friday night into
Saturday.

&&

.AVIATION...as of 04:19 AM PST Tuesday...For the 12Z TAFs. Mostly
clear, with lingering showers south of Monterey Bay as the bay
itself continues to generate its own showers. Winds have calmed
across the region and cold temperatures allowing a slight chance
for freezing fog in the North Bay Valleys. Otherwise VFR this
morning, also known as the calm before the storm.

Late Tuesday and into the afternoon brings about the main show: an
atmospheric river with strong southerly wind gusts along with it.
This system will bring heavy rain and a slight chance for the the
occasional thunder. As the system moves in Tuesday morning, expect
cigs to lower and vis ability to decrease, particularly during
periods of heaviest rainfall dropping conditions to MVFR/IFR
conditions as rainfall intensifies. Winds will shift to the
southwest, with strongest winds Tuesday evening with gusts 30 to 40
kts, and isolated terminals seeing gusts up to 50 kts.
Additionally, LLWS is in the TAF considering the winds aloft are
expected to be stronger with periods of time reaching between
60-70 kts aloft. Winds and rain can be expect to slowly subside
toward the end of the TAF period, but not really cease until after
12z.

Vicinity of KSFO...Currently VFR with light winds and a few clouds
passing by. As the next system moves in winds will shift to the
southwest and increase through the day and reach close to 50 kt
Tuesday night. An Airport Weather Warning will be issued for strong
wind gusts between 03z and 09z. Models are consistently depicting
strong winds aloft with enough differential from the surface for
strong LLWS to be added to much of the TAF. In addition to the wind,
as the leading edge of the system approaches the terminal cigs will
lower and moderate to heavy rainfall is expected with lower vis
possible for MVFR/IFR conditions.

SFO Bridge Approach...Similar to KSFO, lower visibility upon
approach during periods of heaviest rain.

Monterey Bay Terminals...Differential heating and constant NW
flow over Monterey Bay continues to develop its own showers near
the peninsula terminals. Conditions are expected to remain
MVFR/VFR through Tuesday morning. By Tuesday afternoon, moderate
to heavy rain is expected with strong southerly winds as a part of
the potent cold front and atmospheric river headed our way. Cigs
and vis will decrease as the rain approaches. LLWS is also in the
TAF as winds aloft are expected to strong, even with strong gusts
expected at the terminal.

&&

.MARINE...as of 08:58 AM PST Tuesday...Northwest winds have
diminished, but continue with stronger gusts this morning with
squared seas around 11 to 13 feet at 11 to 13  seconds. A large
storm classified as an atmospheric river  approaches the Bay Area
that will drastically shift winds to the  south to southwest later
this morning along its leading front  before storm force winds
develop Tuesday afternoon across the  northern waters then expand
southward by late Tuesday. Heavy rain is expected and occasional
thunder is possible with the atmospheric river. Strong south to
southwest winds and very steep southerly seas at 7 to 9 seconds
will result in hazardous conditions across the coastal waters
through mid- week. Extreme caution is advised.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Flash Flood Watch...CAZ505>507-509-511-512-517
             SCA...Mry Bay until 9 AM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 9 AM
             SCA...Pt Arena to Pt Reyes 0-10 nm until 3 PM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 PM
             SRW...SF Bay from 3 PM
             SRW...Pt Arena to Pt Reyes 0-10 nm from 3 PM
             SRW...Pt Reyes to Pigeon Pt 0-10 nm from 3 PM
             SRW...Pt Arena to Pigeon Pt 10-60 nm from 3 PM
             SCA...SF Bay from 10 AM until 3 PM

&&

PUBLIC FORECAST: RWW/MM
AVIATION: DRP
MARINE: DRP

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