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Widespread, intense storm system arrives tomorrow (Wednesday) into Thursday

A widespread, intense storm system arrives tomorrow (Wed) and will last into Thursday. There will be periods of heavy rain and gusty winds (up to slightly over 1″ of rain and 30-40 mph gusts on the Peninsula), particularly between noon to 6pm PDT. After that, periods of rain will continue Friday into early next week.

Weather spotters should self-activate starting Wednesday Mar 12 @ 12 noon PDT.

NWS OPC Pacific forecast for 4pm PDT Wednesday. Note the occluded front passing through the Bay Area at that time.

From MTR AFD:

.SYNOPSIS...
Issued at 357 AM PDT Tue Mar 11 2025

An active period of weather sets in later tonight and persists
through the extended forecast. Periods of heavy rain Wednesday into
Thursday morning can be expected with gusty winds up to 50 MPH at
times along the coast, thunderstorms, potentially high surf and
accumulating snow across the highest elevations of the Santa Lucia
coastal range. Numerous rounds of light to moderate rain will move
across our Friday through the weekend and into the beginning of next
week.

&&

.SHORT TERM...
(Today and tonight)
Issued at 357 AM PDT Tue Mar 11 2025

After one more day of pleasant weather today, the forecast for the
first round of rainfall remains on track. Scattered light rain will
begin to move onshore over the North Bay as early as late this
afternoon and gradually increase in coverage and intensity through
the overnight into Wednesday. By sunrise tomorrow more intense
rainfall rates out ahead of the surface front will be ongoing over
portions of the North Bay. By mid-morning, the more intense rainfall
will begin to impact the Bay Area and East Bay, as the surface cold
front approaches the coast. Convergence along the cold front and
surging southerly winds just out ahead of the cold front will
produce wind gusts up to 50mph along the coast with periods of heavy
rain over the Bay Area likely from late morning through early
afternoon, eventually moving farther inland for the remainder of the
afternoon and early evening. Max wind gusts farther inland will be
in the 30-40mph range as the cold front pushes through with rainfall
rates up to 0.5"/hr. Urban flooding and small landslides in the
coastal ranges are possible where rainfall rates are highest.
Rainfall totals for the North Bay south into the coastal ranges of
Marin, Santa Cruz and Monterey County will approach 2" during the
day tomorrow, with lesser amounts inland, including some rain
shadowing into the South Bay.

&&

.LONG TERM...
(Wednesday through Monday)
Issued at 357 AM PDT Tue Mar 11 2025

Cooler air aloft associated with the center of the trough will
eventually lead to increased lapse rates and more unstable airmass
with a 10-15% chance for strong thunderstorms developing by
Wednesday evening in the post frontal airmass. However, the surface
trough loses some strength as it approaches land tomorrow evening
with weak short wave ridging aloft at the base of the trough. Strong
wind gusts up to 40mph and small hail are the primary concerns with
any updrafts able to remain organized enough to produce isolated
thunderstorms. Any thunderstorms that do develop should be
shortlived into the early hours of Thursday morning. Although rain
chances persist over much of the area through the day Thursday, it
will not be as widespread or intense as Wednesday. Expect mostly
light scattered rain chances gradually shifting south through the
day. The next round of rainfall begins to move onshore over the
North Bay late Thursday night into early Friday morning, a weak and
progressive trough axis embedded in a wide swath of northwest flow up
to 70kts at times shifts east across much of California during the
day Friday. Once again, scattered light rain chances linger into
Saturday, but most of the area will see little to no rain on
Saturday, with lingering cloud cover helping to keep temperatures 5-
10 degrees cooler than normal. Consensus for Sunday is lacking, with
the NBM possibly lagging behind most current NWP guidance, and may
trend drier over the next couple of updates, with potentially little
to no rain on Sunday. Furthermore, the upper level trough expected
for the beginning of next week has taken on a more positive tilt,
which would be drier and more progressive. Keep checking for updates
for the weekend and beyond as the forecast may change significantly
over the coming days.

Weather Update from NWS MTR

From MTR AFD:

.UPDATE...
Issued at 1011 AM PST Thu Feb 13 2025

First and foremost - There are numerous hazards currently in
effect and too many to list here. Therefore, please check our
socials or weather.gov/bayarea for the most up to date
information. Also, please heed all road closures and evacuation
from our local county partners. Finally, if you are driving or
traveling around the Bay Ara and Central Coast this morning allow
for extra time. You may encounter flooded roads and if you do,
turn around. You`re in a car, not a boat.

Now onto the weather...the much anticipated cyclone and associated
AR are definitely impacting the region this morning. Most
notably, the precip associated with the moisture plume streaming
overhead.Satellite PWAT shows nearly 200 percent of normal. In
other words, a very moisture rich environment. The amount of rain
and intensity has led to a variety of hydro issues from minor
flooding to rapid rises on creeks and streams. Rainfall amounts
over the last 12 hours are greatest along orographically favored
locations (coastal mountains): 1.5-3" N Bay coastal mts, 3-5"
Santa Cruz Mts, Santa Lucia 3-5" with a few 6+". In addition to
the rain, winds ramped up as the closed low inches eastward with
gusts 40-60 mph with a few up to 75 mph.

Into the afternoon...did a quick update to the official forecast
this morning. For the most part the update was done to better
reflect reality with timing and coverage. The cold front
associated with the closed low continues to march S and E across
the forecast area. This can be seen on satellite and radar -
distinct line. Behind the front, precip will transition from
widespread stratiform to more cellular and convective. As such,
thunderstorms will be in the forecast for this afternoon. Main
concern will be localized heavy rainfall leading to additional
flooding concerns.

Simply put, stay weather aware today.

Storm updates (Thurs morning)

Weather Spotters should self-activate all day today, Thurs Feb 13, through Friday afternoon.

Hazardous Weather Conditions

All the ingredients are present for a major rain and wind storm. The graphic below from the WPC shows 3-5+ rainfall totals over the entire Bay Area region through 10am PST.

From MTR AFD:

.SYNOPSIS...
Issued at 1205 AM PST Thu Feb 13 2025

The atmospheric river has arrived! Major impacts are expected
through the day as the moderate to heavy rain continues, winds
increase, and thunderstorm chances arrive. The biggest risk is
localized flooding and damaging winds, especially through the
first half of the day. Conditions will improve Friday. Some light
rain is expected in the North Bay on Sunday, but overall the
pattern will dry out over the next week.

&&

.SHORT TERM...
(Today and tonight)
Issued at 1205 AM PST Thu Feb 13 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:

We are in the middle of an atmospheric river, driven by a storm force
low pressure system off the coast of far northern California.
The associated warm front is moving ashore as I type this.
Despite the fact that it`s 1230 AM, it`s currently 61 degrees in
Monterey. That`s the warmest it`s been in a week. As the warm
front moves through and the cold front approaches over the next
couple hours, rain intensity will increase to around 1.0-1.5" per
hour along the coastal mountains, and roughly half that in the
valleys. These high rain rates will cause localized flooding and
likely trigger some shallow landslides. The wind speed is also
increasing, with current gusts of 30-50 mph observed in higher
terrain, and 20-40 mph along the coast. Winds are expected to
increase over the next several hours as the cold front approaches.
High resolution guidance is still developing a SW`rly 50-60 kt
low-level jet at 850 mb from around 2 AM through noon. With the
high rain rates, these winds should have no trouble mixing down to
the surface at times, and widespread gusts of 40-50 mph are
expected across the area, with 60-70 mph possible along the coast
and higher elevations. The cold front is expected to move through
around sunrise, ushering a colder and drier air mass. The PWAT
will drop from around 1.2" to 0.8", while the 500 mb temperature
will drop from around freezing to -10F. While the drier air will
cause the total rainfall to slowly taper off, individual showers
will strengthen as the cold upper air decreases the stability and
allows convection to become the primary precipitation mode. This
threat is highest from late morning through late afternoon. The
Storm Prediction Center continues to show the risk for
thunderstorms throughout the cwa today.

IMPACTS:

Heavy rain is expected to continue for several hours. Minor flooding
impacts are ongoing across the majority of the cwa. We have not
seen any flash flooding yet, but that risk still exist through the
morning for urban areas. Strong wind gusts combined with saturated
soils will likely lead to downed trees and power outages. Numerous
flood advisories are in effect, and the flood watch continues
through 10 PM Saturday as the rivers absorb the run-off. A High
Wind Warning is in effect for coastal areas, with a Wind Advisory
in effect for inland areas, both through 10 AM Friday. We will
likely see notable rises on many major rivers, and the Russian
River at Guerneville and the San Lorenzo River have the highest
chances of going into flood stage. The San Lorenzo is of
particular concern given the rain so far, and how flashy that
basin is. The river gauge is already reporting a sharp rise.
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 this 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
the day.

&&

.LONG TERM...
(Friday through Wednesday)
Issued at 1205 AM PST Thu Feb 13 2025

Showers taper off Friday and the sun will even come out in the
afternoon. Saturday will start cold with mid-30s inland and
mid-40s along the coast. Saturday will stay dry before a much
weaker disturbance moves through on Sunday. We are only expecting
light rain from this, mainly confined to the North Bay. Otherwise
next week looks dry and mild. Long range guidance is somewhat
split, but the majority of solutions point to a much drier second
half of February as our roller-coaster rainy season continues.

&&

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.

&&

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