Major storm tonight through Friday

Weather spotters should self-activate starting tonight.

Hazardous Weather Conditions

From MTR AFD:

.SYNOPSIS...
Issued at 111 AM PST Wed Feb 12 2025

The precursor rain event continues through the morning south of the
Golden Gate. A more intense system brings the risk for flooding and
damaging winds through Thursday. A Flood Watch for the coastal
counties and a High Wind Watch for the entire region come into
effect at 10 PM tonight. The storm system clears out late Thursday
and Friday with drier conditions going into the weekend.

&&

.SHORT TERM...
(Today and Thursday)
Issued at 111 AM PST Wed Feb 12 2025

KEY POINTS

* Precursor rainfall event continues early this morning, more
  impactful event Wed PM/Thu AM
* Flood Watch in effect 10 PM Wed to 10 PM Sat (coastal counties)
* High Wind Watch in effect 10 PM Wed to 10 AM Fri (entire CWA)
* Soils saturated after initial rain Wednesday, widespread shallow
  landslides, downed trees
* Highest impact Santa Cruz Mtns, Big Sur Coast
* High Surf Warning in effect 3 PM Thu to 3 AM Sat (Pacific Coast)

Radar returns and satellite imagery reveal widespread light
rainfall, with some regions of heavier rain rates, impacting the
region south of Marin County and the Carquinez Strait down through
the Central Coast. Rainfall totals have remained relatively modest,
with accumulations of a tenth to a quarter of an inch across the
coastal regions and a few hundredths of an inch inland. Up to half
an inch of additional rainfall is expected across the Santa Lucias,
with generally around a tenth to a quarter inch of additional
rainfall expected in the Monterey Bay and Salinas Valley regions.
This additional rainfall is expected to fall before sunrise across
most of the region, with the southern half of Monterey County
remaining rather showery into the late morning hours. As the first
hit of rain moves out, today should be rather dry but cloudy, with
high temperatures reaching the middle to upper 50s for most of the
lowlands (except the North Bay coast where highs reach the low 50s).

The next storm system begins to impact the region Wednesday night,
as a deep upper level low rapidly develops off the Pacific Northwest
while a surface low strengthens to around 984 mb off Northern
California. (Peak intensification is currently forecast for around
16 mb in 24 hours, around 2-3 mb short of the bomb cyclone criteria
for a cyclone at 42N latitude, which is where the cyclone is
expected to develop.) An intense rain band is expected to develop in
the North Bay late tonight into early Thursday morning, and spread
into the Central Coast by sunrise. Given ample upper level support
from the left exit region of a 150 kt jet streak, which favors
strong rising motion in the atmosphere, rainfall rates in this
intense band will rise to 0.8 to 1.2 inches per hour. The greatest
impacts from these intense rain rates are currently expected in the
Santa Cruz and Santa Lucia ranges, but we can`t rule out brief
intense rain along the North Bay coastal ranges and San Francisco.

A Flood Watch has been issued for the coastal counties of
Sonoma, Marin, San Francisco, San Mateo, Santa Cruz, and Monterey,
in effect from 10 PM Wednesday to 10 PM Saturday. When all is said
and done, most inland locations can expect 1.5 to 2.5 inches, less
in rainshadowed valleys, and 3 to 6 inches in the Santa Cruz Mtns
and Big Sur coastal range. In addition, soils around the Bay Area
will be sufficiently saturated after tonight`s rainfall to support
widespread shallow landslides Thursday morning in areas that see the
higher end rain rates. Use caution on roadways and near steep
slopes.  Current areas of most concerns for river flooding are the
Russian River near Guerneville and Mark West Creek, as well as the
San Lorenzo River in Santa Cruz county. Otherwise, urban and small
stream flooding will almost certainly be an issue elsewhere
throughout the Flood Watch area.

In addition to the rainfall, damaging winds are expected with the
system, particularly along the coast and the ridgetops, with a High
Wind Watch issued for the entire forecast region from 10 PM
Wednesday to 10 AM Friday. Widespread gusts up to 60 mph range are
possible, with potential gusts in the 70-80 mph range across the
immediate coastline and the ridgetops on the coastal ranges. The
combination of saturated soils and high winds will result in the
increased possibility of downed trees and powerlines, leading to
further damage and/or power outages. There is also a slight chance
for thunderstorms Thursday afternoon as colder air moves in aloft
and some clearing allows for more surface heating and consequently a
more unstable atmosphere. Outside of locally heavy rainfall and
gusty winds, as well a few lightning strikes, no other severe
hazards are expected. Nearly straight hodographs with little low
level directional shear don`t support much in the way of rotating
updrafts. An exception would be along the coast near Point Reyes and
the Sonoma coast where the lower level shear profile is a bit more
favorable to support a brief waterspout or two.

&&

.LONG TERM...
(Thursday night through Tuesday)
Issued at 111 AM PST Wed Feb 12 2025

The surface low moves onshore late Thursday night, bringing an end
to the widespread rain and wind. Friday and Saturday look to be
mostly dry with a few lingering showers possible early on Friday. A
dissipating shortwave comes into the Pacific Northwest on Sunday,
bringing slight rain chances to the North Bay. CPC outlooks show a
lean towards temperatures above seasonal averages and precipitation
below seasonal averages for the second half of February.

&&