Debris Flows and Flooding in California, December 1 to 5, 2019

Area Forecast Discussion
National Weather Service San Francisco Bay Area
955 PM PST Sat Nov 30 2019

.SYNOPSIS....The first atmospheric river of the season has
arrived. Rain will steadily increase overnight, and winds will
remain strong, as the storm draws closer to the coast and then
stalls. Peak storm intensity is expected from late this evening
through Sunday evening. Winds and rain will subside by Monday,
but periods of rain will likely continue through Tuesday, or even
into Wednesday morning, especially across the southern part of
the forecast area. Another strong storm is forecast to impact the
area on Thursday night or Friday.

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.DISCUSSION...As of 8:35 PM PST Sunday...A plume of moist air
(Atmospheric River) currently extends from near Hawaii all the
way to the California coast, with satellite detecting
precipitable water values as high as 1.5 inches within this
plume. Precipitation today has mostly been light to moderate in
intensity as forcing with this system has not yet been all that
strong given how far offshore it remains. Also, orographic
enhancement has been limited so far due to the low level jet being
out of the southeast and thus parallel to the coastal ranges.
Rainfall totals since this morning are mostly under a half inch,
with some totals as high as 1.25" in Marin County and also in the
Santa Lucia Mountains. Winds have been locally strong and gusty
from the east and southeast, mainly in the hills and near the
coast. A weather station at 1200 feet of elevation in the Santa
Lucias near the Big Sur Coast has been reporting wind gusts as
high as 64 mph this evening.

Both the 00Z NAM and 00Z GFS indicate increasing intensity in
rain rates overnight as a wave travels along the nearly
stationary frontal boundary offshore and approaches the coast.
Also, orographic precipitation enhancement will increase in the
coastal hills overnight as the low level jet veers from southeast
to south and the flow becomes more orthogonal to the terrain.
There is a fair amount of uncertainty as to where the heaviest
rain will occur overnight as the models struggle to agree on the
specifics. However, the best bet for heavy rain rates tonight
appears to be the Santa Cruz Mountains and Santa Lucia Mountains.
Heavy rain rates are also possible in the North Bay Hills
overnight. A Flash Flood Watch remains in effect for the Kincade
Burn Scar and areas downstream from the scar.

Periods of heavy rain will continue through Sunday evening and
the models are in better agreement that the focus of heaviest rain
on Sunday and Sunday evening will be from the Santa Cruz
Mountains southward.

According to the last local WRF model, wind speeds will be
strongest from about now through Sunday morning and then gradually
subside from Sunday afternoon through Sunday night. The areas most
under the gun for strong and potentially damaging winds are
coastal areas from San Mateo County southward, especially along
the northern Monterey Co. Coast from Carmel south to Point Sur.
Strong and gusty winds will also continue in the hills and locally
in the valleys, especially the Salinas Valley. High Wind Warnings
(coast) and Wind Advisories (elsewhere) remain in effect through
at least Noon Sunday, with some wind warnings/advisories
continuing into Sunday night.

Rainfall totals through Sunday night are expected to range from 1
to 3 inches at the coast and in the valleys (Lowest amounts in
the rain-shadowed inland valleys such as the Salinas Valley,
northern Santa Clara Valley, and some East Bay Valleys. Highest
amounts near the coast). In the hills/mountains, rain totals
through Sunday night are projected to range from 3 to 5 inches in
general, except 5 to 8 inches in the Santa Cruz and Santa Lucia
Mountains. The local WRF model forecasts locally up to 10 inches
in the Santa Lucias and up to 13 inches in the Santa Cruz
Mountains through Sunday night. There is very little concern about
mainstem river flooding given how low the rivers are currently
running and due to the fact that the soil is nowhere near
saturated at this early stage of the wet season. Aside from flash
flood potential in or near the Kincade Burn Scar, the primary
flooding potential will be along smaller creeks and streams,
especially in the Santa Cruz Mountains if the excessive rainfall
amounts projected by the WRF model were to come to fruition. Also,
minor urban flooding in low lying and poorly drained areas is
likely later tonight and through the day on Sunday.

The nearly stationary trough offshore is forecast to shift to the
southwest starting late Sunday night, which will result in
rainfall gradually tapering off, as well as decreasing winds, from
late Sunday night through Tuesday. The trough is then forecast to
track eastward and move inland over south-central California late
Tuesday night into Wednesday. This may result in another burst of
heavier precipitation across the southern portion of our forecast
area, mainly the Santa Lucias, on Tuesday night and early
Wednesday. But it looks like the bulk of the precipitation during
this time frame will be over southern CA.

Our area should see a dry break from late Wednesday through at
least midday Thursday before a strong system approaches from the
northwest and brings another round of potential heavy rain and
strong winds sometime between Thursday night (GFS) or Friday
(ECMWF). Showers may linger into Saturday with drier weather by
next Sunday.

From Previous Discussion...Poor travel conditions will be the
main impact from the current storm system as a large number of
people make their return trips home to the region after the
Thanksgiving holiday. Expect delayed or cancelled flights through
the weekend if flying. If driving, expect road delays or closures
(especially on I5 near Mt Shasta, I80/Highway 50 over the Sierra
Nevada, and along Highway 1 along Big Sur, among others), ponding
of water / debris on roadways. Travel is not advised over the
Sierra given the strong storm impacting the area but if you must
head towards the mountains, ensure your car is fully fueled, your
phone fully charged, you have chains for your tires, and you have
enough food and warm clothes in case of emergency. Otherwise, the
main impact for those non travelling will be downed trees and
power lines which may lead to power outages as well as nuisance
flooding in low lying or poorly drained urban areas.

&&

.AVIATION...as of 9:53 PM PST Saturday...Thus far only light rain
with VFR cigs except for MVFR cigs in the North Bay. Models
showing rain increasing after about 09Z through as a wave travels
along the frontal boundary. Rain will be light to moderate in
intensity as the heaviest rain will be confined to the coastal
mountains. Light rain continues through Sunday night. Strong
southeast winds will continue through about 18Z then slowly
decrease. Above the surface winds could be as high as 50-55 kt
causing LLWS.

Vicinity of KSFO...Light to moderate rain expected through most
of the period. Cigs around 3500 ft lowering to 2500 ft after 09Z.
East to southeast winds to 20-25 kt gusting to 30-35 kt slowly
decreasing after 18Z. Low level wind shear possible with winds
above the surface as high as 50-55 kt.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Rain will be mainly light due to
rain-shadowing. Cigs lowering to MVFR after 10Z. Large contrast
of wind speeds between MRY and SNS. Southeast winds have been
gusting as high as 40 kt in the Salinas Valley and this should
continue through 18-20Z before decreasing. Lighter winds expected
at MRY with winds as high as 10-15 kt gusting to 20 kt after 10Z.
Winds above the surface however are near 50 kt causing possible
LLWS.

&&

.MARINE...as of 09:32 PM PST Saturday...A 993 mb low centered 450
miles west of Cape Mendocino will remain in place through Monday
before dropping south and weakening. As a result periods of gale
force winds can be expected over the  coastal waters and the bays
through Sunday night before  decreasing.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Wind Advisory...CAZ006-506>508-510>513-516>518-528-529
             High Wind Warning...CAZ509-530
             High Wind Warning...CAZ505
             Flash Flood Watch...CAZ506-507
             SRW...Pt Arena to Pt Reyes 0-10 nm
             SRW...Pt Reyes to Pigeon Pt 0-10 nm
             SRW...Pigeon Pt to Pt Pinos 0-10 nm
             SRW...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SRW...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SRW...Pt Arena to Pigeon Pt 10-60 nm
             GLW...Mry Bay
             GLW...SF Bay

&&

PUBLIC FORECAST: Dykema
AVIATION: W Pi
MARINE: W Pi

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
640 PM PST SAT NOV 30 2019

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0617 PM     NON-TSTM WND DMG 6 S DEL MONTE FOREST    36.50N 121.94W
11/30/2019                   MONTEREY           CA   LAW ENFORCEMENT

            TREE IN NORTHBOUND LANE OF HWY 1.

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
656 PM PST SAT NOV 30 2019

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0522 PM     NON-TSTM WND GST 1 SSW SAN FRANCISCO     37.76N 122.43W
11/30/2019  E36 MPH          SAN FRANCISCO      CA   TRAINED SPOTTER

            SPOTTER: POLANSKY.

&&