Storm updates (Wed night)

This is shaping up to be a major storm with periods of heavy rain and high winds throughout the Bay Area. The worst of it is likely to be in the early morning hours Thursday, but all day Thurs will be rainy and windy. A flood watch and wind advisory are or will be in effect. See below and refer to sfoskywarn.org/alerts for up-to-date alerts. Due to the expected heavy rain and high winds, be on the lookout for flooding as well as downed trees and powerlines. Stay safe, everyone.

This shows the 4am Thursday forecast (GFS model, from Tropical Tidbits). Note the confluence of the AR and the intense low pressure system whose steep gradients means gusty winds.

Hazardous Weather Conditions

From MTR AFD:

.SYNOPSIS...
Issued at 330 PM PST Wed Feb 12 2025

Potentially major impacts from a strong storm system incoming
tonight and lasting into Thursday. Local flooding and widespread
damaging winds are the main threats through Thursday morning.
Slight chance for thunderstorms Thursday afternoon. Drier
conditions on Friday into the weekend. Potential showers for the
North Bay on Sunday.

&&

.SHORT TERM...
(This evening through Thursday)
Issued at 330 PM PST Wed Feb 12 2025

KEY POINTS
* Flood Watch in effect 10 PM Wed to 10 PM Sat (area-wide)
* High Wind Warning (coastal) and Wind Advisory (inland) in
  effect from 10 PM Wed to 10 AM Fri (coastal)
* Widespread shallow landslides possible Thursday morning
* Downed trees and power outages likely
* Highest impact areas include Santa Cruz Mtns and Big Sur Coast
* High Surf Warning in effect 3 PM Thu to 3 AM Sat

METEOROLOGY:

Major winter storm still on track to impact the region beginning
tonight. Beautiful visible satellite representation of the storm
currently centered about 1,000 miles to our northwest. One can
clearly make out features such as the nearly stationary warm front
draped along the CA coast, as well as the intense surface cold
front wrapping into the storm from the south and west. The storm
will continue translating southeastward guided by an intense 170
kt jet streak aloft. Latest advanced scatterometer pass over the
storm from about 1 PM today shows estimated sustained surface
winds of 40-45 kts, which is in line with latest forecast wind
intensities. Current surface pressure has been analyzed at 986 mb
as of 1 PM this afternoon. Expecting these winds to hold or
slightly increase their intensity as the storm nears the CA coast
and deepens to around 981 mb by late tonight. As the storm nears
the coast tonight, rainfall will likely start out as a gradual
warm frontal overrunning in which rain will be widespread, but
only light to moderate in intensity, lacking the robust cold
frontal processes and enhanced upslope moisture transport.
Progressing further after midnight tonight, the warm front moves
ashore and the cold front enters the North Bay, moving quickly
down the coast through the early morning hours. This will be the
period in which the heaviest rainfall rates will occur...on the
order of 1.0 to 1.5 in/hr, especially in the Santa Cruz Mtns and
Big Sur coast. In addition to the intense rainfall rates, damaging
winds will be a factor through the morning hours as well. Once
the mid-level trough axis comes ashore around midnight, the strong
mid-to-upper level flow begins to impinge on the CA coast. 850 mb
flow out of the southwest at 50 to 60 kts spreads across the
region from north to south from about midnight in the North Bay to
11 AM along the Big Sur coast. This will likely result in surface
wind gusts of 40 to 50 mph across the area, with coastal areas
and higher elevations seeing gusts in the 60 to 70 mph range.
Isolated areas along the coast and coastal mountain ridgetops may
end up seeing gusts closer to 80 mph as the more intense winds mix
down with the passage of the cold front. Behind the cold front,
the environment remains supportive for thunderstorms well into the
afternoon. Immediately after the cold frontal passage, any
thunderstorms that initiate will likely be over powered by robust
speed shear and limited instability. There will also be a bit of
dry air mixing into the storm top level per RAP forecast soundings
along the coast, so there is some limited downburst potential
with a few storms that are able to stay intact along the coast.
Beyond the morning hours, instability increases a bit as the
surface warms slightly, and speed shear decreases a bit as well
as the upper level jet start to move out of the region. This will
likely provide a window of time where storm ingredients are better
suited to lead to more prolonged updrafts and better potential
for rotation should any storm deviate to the right of the mean
storm motion. For more details on the severe threat, see
spc.noaa.gov, which places us in a general risk for thunderstorms
for Thursday.

IMPACTS:

General flooding impacts will begin in the North Bay as early as
tonight and spread across the rest of the region shortly after
midnight. It may appear as a slow start for those areas that start
to see rainfall first, as it will mostly be associated with a
warm/stationary front, thus not terribly heavy. However, after
midnight, rainfall rates are expected to increase substantially
with the passage of the main cold front. This will exacerbate
flooding concerns, especially in the Santa Cruz Mtns and Big Sur
coast. By the time the cold front moves through, soils will be
nearly saturated in the Santa Cruz Mtns, so we do expect to see
issues with quick runoff and shallow landsliding. Not only will
this alone be an issue, but winds are expected to gust up to 60
mph (as high as 80 mph along the ridge of the SC Mtns). Combining
the high winds with saturated soils is a recipe for downed trees
and power lines, leading to potential structure damage and power
outages. The Flood Watch has been expanded to cover the entire
region, and the High Wind Watch has been upgraded to a High Wind
Warning for coastal areas, and a Wind Advisory for inland areas.
We will likely see notable rises on many major rivers, but the
Russian River at Guerneville and the San Lorenzo River have the
highest chances of going into flood stage. The San Lorenzo perhaps
being one of the more worrisome ones given how flashy the terrain
is known to be in that region. Other coastal areas outside of the
Santa Cruz Mtns may not see the same degree of shallow landsliding,
but nonetheless will still have impacts due to downed trees and
powerlines, especially areas near the Marin headlands, Big
Sur/Carmel coast, and Monterey Peninsula. The commute Thursday
morning will undoubtedly be nasty and downright dangerous in some
cases (thinking Hwy 17 in particular). When all is said and done,
the North Bay will likely end up with 2 to 4 inches, and the Bay
Area will see a range of 1.5 to 3 inches depending on if you`re in
the South Bay rain shadow or not. The Santa Cruz Mtns and Big Sur
coast will be the winners with 4 to 7 inches, locally up to 8-10.
Despite some locations seeing some of the lower end of rainfall
amounts, wind is still expected to pose a threat to life and
property through Thursday.

&&

.LONG TERM...
(Friday through next Tuesday)
Issued at 330 PM PST Wed Feb 12 2025

Shower activity will be coming to an end Friday morning with dry
conditions into Saturday. Medium range ensemble guidance is showing
the potential for another much weaker storm system on Sunday that
would brush the North Bay, but the upcoming storm system this
Thursday morning is the only majorly impactful period that we see in
the 7 day forecast period. There will likely be a lull in storm
activity along the West Coast through next week.

&&