Weather spotters should self-activate starting tonight.

Hazardous Weather Conditions

• Flood Watch in effect from February 12, 10:00 PM PST until February 15, 10:00 PM PST
• High Wind Watch in effect from February 12, 10:00 PM PST until February 14, 10:00 AM PST

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.

&&

Hydrologic Outlook

SF Bay Area Skywarn weather spotters should self-activate for this storm starting Wed night.

From Tropical Tidbits:

From MTR AFD:

.LONG TERM...
(Tuesday night through next Sunday)
Issued at 1250 AM PST Tue Feb 11 2025

Long term forecast thinking remains unchanged. The previous
discussion follows for reference:

First round of rainfall continues early Wednesday morning for areas
south of the Bay, and largely clearing out by mid-to-late morning.
Wednesday afternoon actually has the potential to be mostly dry as
shortwave ridging moves over the region and essentially cuts off our
large scale vertical motion. By Wednesday night, a vigorous
shortwave trough rapidly deepens along with the associated surface
low approaching the CA/OR border. This more intense lift will aid in
advecting the moisture into what would be a more coherent
"atmospheric river" type pattern, whereas the first round of
rainfall Tuesday/Wednesday is a more localized moisture plume,
though not completely detached, and a much weaker degree of upper
level support. So, the Tuesday/Wednesday system currently lacks the
punch and will serve as a precursor event. Then, there is high
confidence that the Wednesday/Thursday system will be the more
impactful given the generous model agreement in robust jet support,
not only supporting heavy rainfall, but also potentially damaging
winds. Let`s juts say that when my SO told me her flight was
Thursday morning out of Monterey, I said, "oh...uhh...gonna be
bumpy!" Given the current location of the moisture axis and
dynamic features, this system will generally favor areas from
about San Francisco southward across the Central Coast. Overall
good news in terms of annual rainfall since southern areas are a
bit behind the curve so far this water year (since Oct 1). An
interesting tidbit to include; the Central Coast watershed which
runs along the coast roughly from Monterey to Arroyo Grande is
about 4.37" below normal for the water year to date. The current
forecast for that watershed by the end of the week is about 3-6".
There is a slight chance (at least 15%) of rainfall rates
resulting in flash flooding conditions area-wide on Thursday. It
is likely that there will be more of a focus as the event nears
and more information about convective rainfall rates becomes clear
from high res guidance. The system does appear to be rather
progressive and should move through the region relatively quickly,
thus hindering any increase in the probability of flash flooding
for now. Guidance keeps this vigorous wave open and embedded in
the flow, so there is relatively low stall potential. As far as
rainfall totals go, the North Bay will still see respectable
totals in the 2-4" range, the greater Bay Area will see a range of
1.5-3" (less in the rainshadowed South Bay, more along the
peninsula), 4-6" and locally up to 8" in the Santa Cruz Mtns, 2-3"
across the Central Coast counties, and 3-6" along the Big Sur
Coast, locally up to 8". A bit about the winds; guidance is
showing reasonable agreement in the placement and strength of the
associated surface low, ultimately bringing it into the CA coast
near Cape Mendocino and a minimum surface pressure of about
985-991 mb. This would likely result in Wind Advisory criteria
being met across a wide swath of the region should guidance begin
to converge on the lower end of that range. Current best forecast
is for gusty winds 40-45 mph along the coast and ridgetops of
coastal ranges. There is also a slight risk for thunderstorms
Thursday afternoon as the cooler air settles in aloft and
instability increases. By Thursday night things should be settling
down with some showers lingering into Friday morning. The weather
into the weekend looks to clear out with another, much weaker,
embedded shortwave trough entering the PNW late Saturday which may
bring more rainfall the the North Bay on Sunday.

&&

SF Bay Area Skywarn weather spotters should self-activate today through tonight. This storm will not be as strong as Tuesday’s, but it will still be impactful to the entire Bay Area today. Given that the ground is already saturated in many areas, and that there will be strong winds peaking at about 2pm today, be on the lookout for downed trees and powerlines, and flooding in low-lying areas.

Alerts:
Wind Advisory in effect from February 6, 10:00 AM PST until February 7, 04:00 AM PST

From Tropical Tidbits:

From MTR AFD:

.SYNOPSIS...
Issued at 1209 AM PST Thu Feb 6 2025

Another round of rain and wind is in store today before a cold
and dry weekend.

&&

.SHORT TERM...
(Today and tonight)
Issued at 1209 AM PST Thu Feb 6 2025

The latest satellite images show that high clouds have reached the
Bay Area. These clouds are the first sign of the approaching low
pressure system in the Eastern Pacific that will drive the
weather today. They will also help limit radiational cooling and
the low temperatures tonight should be a couple degrees warmer
than yesterday. At the same time, a southerly surge of low clouds
are creeping up the Central Coast. The low pressure system and
associated fronts will move directly over the Bay Area today,
bringing rain, wind, and a chance for thunderstorms. Overall it
will be less impactful than the system on Tuesday, but still
significant.

Rain: The rain forecast is interesting with a lot of dynamics in
play. The first drops will begin to fall in the North Bay ahead
of the warm front around 6-8 AM Friday morning. The rain will
start as stratiform, steady rain. As the warm front moves through
in the late morning/early afternoon some stronger showers are
possible. We will then spend the afternoon in the warm sector, and
the rain rates may moderate for a few hours before the cold front
approaches, with the potential for another round of stronger
showers in the evening. After the cold front passes around
midnight, the air mass dries out significantly. Any instability
generated by the colder upper levels is limited by the cool
surface temps overnight. There may be some passing showers into
Friday morning, but nothing impactful. When it`s all said and
done, we`re expecting to pick up about an inch in the North Bay
and roughly 1/2" elsewhere. The exception is the coastal
mountains,especially in Monterey and Santa Cruz Counties, where
the persistent southerly flow will support orographic enhancement
and lead to 2-4 inches through the day. Most of our cwa is under a
marginal risk (5-15% chance) for excessive rainfall which can
cause flash flooding.

Wind: The Ocean Prediction Center is classifying this system as
gale-force, meaning it will generate sustained winds of 39-54 mph
over the water. Thanks to friction, winds won`t be as strong over
land, but we are still expecting sustained southerly winds of
20-30 mph with gusts as high as 40 mph along the coast and higher
elevations. Southerly winds will begin to pick-up around mid-day
and continue through the night. After the cold front passes early
Friday morning, winds will gradually shift to NW and calm back to
a moderate breeze.

Thunderstorms: With multiple frontal passages and a surface low
moving through, we need to take a close look at thunderstorm
potential today. There are certainly enough triggers in place to
realize any instability. The 07Z HRRR shows MUCAPE values over 150
J/kg in the late afternoon/early evening time frame. This has been
trending up with the latest high resolution guidance. Most of the
instability is in the lowest 20,000 ft of the atmosphere, where
the air is warmer than -10 C (bottom of the hail growth zone).
This will limit ice particles in the clouds, which are needed to
generate the electrical charge that causes thunderstorms. On the
other hand, there is a good amount of shear with relatively
strong southerly winds at the surface and roughly 80 kt
westerlies at 500 mb. Even if lightning activity is suppressed,
some of these storms could produce severe wind gusts, and may even
start rotating. This is still an outside chance, as the Storm
Prediction Center only has a general thunderstorm risk over the
North Bay and Sac Valley.

&&

.LONG TERM...
(Friday through Wednesday)
Issued at 1209 AM PST Thu Feb 6 2025

While there is a chance for a lingering shower or two Friday
morning, the air mass dries out very quickly behind the cold
front (PWAT drops to around 0.25"). NW winds will decrease in
strength through the day, and there will be ample sunshine in the
afternoon. The overall pattern flips back to a fairly stable
ridge, which will keep the weather dry and consistent for several
days. After the multiple cold fronts this week, the left-over air
mass will be pretty cold. The 1000-500 mb thickness will hover
around 5,400 m, with 850 mb temps around 0C, or the 10th
percentile for this time of year. Coupled with the enhanced
radiational cooling under clear skies, we turn our attention to
cold morning temperatures. From Saturday-Wednesday we are
expecting coastal lows in the low 40s, with low 30s inland and
upper 20s in the coldest, wind sheltered valleys. Since the air
mass is fairly cold, it won`t be much warmer in the hills, and the
afternoon highs will only get to the mid 50s for nearly everyone.
We are anticipating a lot of frost/freeze prodcuts during this
stretch.

Rain returns to the forecast by Wednesday. Most ensemble guidance
shows we`ll pick up a similar amount of rain as today, but a few
members bring more than 3" to the low-lands. This high-end
scenario is still the underdog, but has gained more support in the
probabilistic guidance over the last 24 hours. If it plays out,
we would likely see more urban, stream, and river flooding. We
will keep a close eye on this system as the forecast evolves.

&&

From Tropical Tidbits:

From MTR AFD:

.UPDATE...
Issued at 915 AM PST Wed Feb 5 2025

Highlights for the morning update: N Bay Valley Frost Adv allowed
to expire, ongoing Flood Warnings in Sonoma continue, drier today.

A chilly start to the day as temperatures plummeted into the lower
30s across some of the interior valleys, especially N and E Bay.
In addition to chilly temps, patchy dense fog also impacted the N
Bay valleys. Makes sense really, ample low level moisture from
yesterday`s rains, weaker winds, and some clearing overnight.
Temperatures fortunately are on the upswing so the Frost Adv was
allowed to expire. Fog is also beginning to dissipate.

Today will be much drier than yesterday. However, KMUX still
remains in precip mode. Colder air with marginal instability
today will lead to some widely scattered showers, especially the
N Bay. Despite the drier conditions and low chance for precip
flooding issues will remain as streams/creeks/rivers water levels
remain elevated.

Morning update will be issues shortly to address potential
Frost/Freeze products for tonight as temperatures will plummet to
critical levels.

&&

.SHORT TERM...
(Today and tonight)
Issued at 1209 AM PST Wed Feb 5 2025

Colder and drier air has settled in following yesterday`s cold
front. The satellite shows some cumulus clouds over the water, but
any showers are expected to be brief. The drier air mass has
precipitable water values of 50-75% of normal. Overall today will
be a reprieve between 2 systems. The colder air mass will be
noticeable, however. The current temperatures are roughly 10
degrees colder than 24 hours ago and a frost advisory is in effect
for the North Bay Valleys this morning. More frost advisories may
be needed tonight, but the increasing cloud cover ahead of
Thursday`s system will suppress radiational cooling and will
likely keep the region a degree or two warmer than this morning.

&&

.LONG TERM...
(Thursday through Tuesday)
Issued at 1209 AM PST Wed Feb 5 2025

By Thursday morning another plume of moisture will reach the
coast as a surface low pressure system develops offshore. There is
an outside chance for thunderstorms off the coast Thursday
afternoon. Compared to the atmospheric river on Tuesday, this
system will be much weaker in terms of both wind and rain.
Rainfall amounts are generally expected to be 0.25-1.25". The wind
will still be noticeable, with SW winds 10-20 mph and gusts up to
35 mph Thursday evening. This system will also bring a
reinforcing shot of colder air as it departs. The 1000-500 mb
thickness will fall to around 5,400 m by Friday. In fact, the snow
levels will drop to around 3,000 ft and there`s a chance some of
the higher mountains will get a few flakes. Otherwise Friday will
be a transition day as the skies gradually clear and the drier
ridging pattern stabilizes.

By the weekend, the only real hazard becomes cold morning
temperatures. Saturday morning will start roughly 10 degrees
colder than Friday morning. Many inland areas will drop into the
low 30s, and widespread frost advisories are expected. Afternoon
highs will only climb into the mid to upper 50s, so it will feel
chilly all day, although there will be ample sunshine. The ridging
pattern stabilizes through mid week, and the weather will be
pretty consistent. The next round of rain looks to arrive around
Wednesday.

&&

Severe Weather Statement
National Weather Service San Francisco CA
433 PM PST Tue Feb 4 2025

CAC081-085-050045-
/O.CON.KMTR.SV.W.0006.000000T0000Z-250205T0045Z/
San Mateo CA-Santa Clara CA-
433 PM PST Tue Feb 4 2025

...SEVERE THUNDERSTORM WARNING REMAINS IN EFFECT UNTIL 445 PM PST 
FOR EAST CENTRAL SAN MATEO AND NORTHWESTERN SANTA CLARA COUNTIES...

At 433 PM PST, a severe thunderstorm was located over Stanford, or 
over Palo Alto, moving northeast at 65 mph.

HAZARD...60 mph wind gusts.

SOURCE...Radar indicated.

IMPACT...Expect damage to roofs, siding, and trees.

Locations impacted include...
  San Jose, Fremont, Sunnyvale, Redwood City, Mountain View, Palo 
  Alto, Newark, Menlo Park, Los Altos, San Carlos, East Palo Alto, 
  Los Altos Hills, Atherton, Woodside, Stanford, North Fair Oaks, 
  Portola Valley, West Menlo Park and Emerald Lake Hills.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Seek shelter inside a well-built structure and stay away from 
windows. This storm is capable of producing damaging winds.

Tornadoes can develop quickly from severe thunderstorms. Seek 
shelter immediately in an interior room on the lowest floor of a 
well-built structure.

Wind damage with this storm will occur before any rain or lightning. 
Do not wait for the sound of thunder before taking cover. Seek 
shelter immediately inside a sturdy structure and stay away from 
windows.

Torrential rainfall is also occurring with this storm and may lead 
to flash flooding. Do not drive your vehicle through flooded 
roadways.

&&

LAT...LON 3755 12220 3753 12217 3752 12216 3752 12214
      3748 12211 3749 12206 3748 12202 3748 12203
      3747 12205 3747 12196 3746 12200 3746 12197
      3734 12218 3741 12231
TIME...MOT...LOC 0024Z 221DEG 60KT 3732 12227

TORNADO...POSSIBLE
HAIL THREAT...RADAR INDICATED
MAX HAIL SIZE...0.00 IN
WIND THREAT...RADAR INDICATED
MAX WIND GUST...60 MPH

$$

BULLETIN - IMMEDIATE BROADCAST REQUESTED
Severe Thunderstorm Warning
National Weather Service San Francisco CA
359 PM PST Tue Feb 4 2025

The National Weather Service in San Francisco has issued a

* Severe Thunderstorm Warning for...
 San Francisco County in northern California...
 Northwestern San Mateo County in northern California...

* Until 415 PM PST.

* At 359 PM PST, severe thunderstorms were located along a line from 
 near San Francisco to near El Granada, moving northeast at 45 mph.

 HAZARD...60 mph wind gusts.

 SOURCE...Radar indicated.

 IMPACT...Expect damage to roofs, siding, and trees.

* Locations impacted include...
 San Francisco, Daly City, South San Francisco, San Bruno, 
 Pacifica, Brisbane, Colma, Broadmoor and Montara.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Seek shelter inside a well-built structure and stay away from 
windows. These storms are capable of producing damaging winds.

Torrential rainfall is also occurring with these storms and may lead 
to flash flooding. Do not drive your vehicle through flooded 
roadways.

&&

LAT...LON 3759 12252 3771 12251 3776 12252 3777 12253
 3779 12253 3783 12241 3782 12241 3780 12238
 3781 12238 3779 12237 3775 12238 3772 12235
 3771 12235 3770 12236 3769 12235 3767 12236
 3754 12252
TIME...MOT...LOC 2358Z 219DEG 37KT 3779 12243 3766 12249 3753 12254

HAIL THREAT...RADAR INDICATED
MAX HAIL SIZE...0.00 IN
WIND THREAT...RADAR INDICATED
MAX WIND GUST...60 MPH

$$

WUUS56 KMTR 042215
SVRMTR
CAC041-042300-
/O.NEW.KMTR.SV.W.0002.250204T2215Z-250204T2300Z/

BULLETIN - IMMEDIATE BROADCAST REQUESTED
Severe Thunderstorm Warning
National Weather Service San Francisco CA
215 PM PST Tue Feb 4 2025

The National Weather Service in San Francisco has issued a

* Severe Thunderstorm Warning for...
  Marin County in northern California...

* Until 300 PM PST.

* At 215 PM PST, severe thunderstorms were located along a line from 
  6 miles southwest of Petaluma to near Point Reyes Station, moving 
  east at 25 mph.

  HAZARD...70 mph wind gusts.

  SOURCE...Radar indicated.

  IMPACT...Expect considerable tree damage. Damage is likely to
           mobile homes, roofs, and outbuildings.

* Locations impacted include...
  San Rafael, Novato, Mill Valley, San Anselmo, Larkspur, Corte 
  Madera, Tiburon, Fairfax, Sausalito, San Geronimo, Lucas 
  Valley-Marinwood, Stinson Beach, Kentfield, Black Point-Green 
  Point, Muir Beach, Tamalpais-Homestead, Point Reyes Station, 
  Tamalpais-Homestead Valley, Strawberry and Ross.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Anyone outdoors should move to shelter inside a well-built structure 
and stay away from windows. These storms are capable of producing 
widespread damaging winds.

Torrential rainfall is also occurring with these storms and may lead 
to flash flooding. Do not drive your vehicle through flooded 
roadways.

Report severe weather to your nearest law enforcement agency or the 
National Weather Service office in San Francisco CA.

&&

LAT...LON 3803 12290 3822 12276 3817 12257 3810 12249
      3806 12247 3802 12249 3798 12244 3796 12248
      3794 12247 3793 12249 3788 12242 3785 12241
      3784 12243 3786 12245 3785 12247 3782 12247
      3781 12256 3790 12268 3789 12274 3798 12282
TIME...MOT...LOC 2214Z 230DEG 21KT 3817 12270 3796 12278 3770 12292

THUNDERSTORM DAMAGE THREAT...CONSIDERABLE
HAIL THREAT...RADAR INDICATED
MAX HAIL SIZE...0.00 IN
WIND THREAT...RADAR INDICATED
MAX WIND GUST...70 MPH

$$

.SYNOPSIS...
Issued at 1202 AM PST Tue Feb 4 2025

An atmospheric river will continue to bring heavy rain, urban and
river flooding, and strong winds to the Bay Area and Central
Coast through Tuesday. There is also an increasing chance for
thunderstorms in the afternoon. While less impactful in the
second half of the week, rain chances continue through Friday.

&&

.UPDATE...
Issued at 918 AM PST Tue Feb 4 2025

Will keep the morning update a little shorter than normal as
operations is rather active this morning a variety of impacts:
flooding, damaging winds, storm force over the coastal waters,
rock slides, and thunderstorm potential this morning.

Latest surface analysis still shows the lingering stationary
boundary to the north, which extends to the SW and connects to a
deepening low pressure. As a result, a moisture rich plume
continues to stream across the Bay Area and Central Coast. KMUX
radar imagery shows widespread rain showers across the region.
Given the rainfall rates, already primed streams/creeks, and
urban runoff issues, flood advisories and flood warnings have been
issued.

The main focus of the near term forecast will be this afternoon.
The aforementioned low pressure will ride along the boundary to
the northeast. As this happens, an upstream jet max will move
eastward an increase to 150kt+. This max will be parked right over
the coastal waters with the surface low on the northern fringe.
Expect rapid increase in surface winds (some CAMS are now showing
80-90mph over the waters) and low level shear. As noted below,
it`s a low CAPE high shear scenario, which may lead to spinning
cells.

If there`s a period over the next 24 hours to be weather aware it
will be this afternoon. A re-newed push of rain, winds, and
possible thunderstorms.

MM

&&

.SHORT TERM...
(Today and tonight)
Issued at 1202 AM PST Tue Feb 4 2025

Synoptic Overview: A stationary boundary is draped across
northern California with low pressure off the coast. The Bay Area
is now in the cross-hairs of the atmospheric river. Over the next
24 hours, a 150+ kt jet max will move over the Bay Area/Central
Coast, causing divergence aloft, and cyclogenesis along the
boundary (Patterson`s Law). This will encourage an active cold
front to move through the region, lifting the warm, moist air mass
on Tuesday.

Rain: Prefrontal rain has been ongoing for several hours in the North
Bay, and has recently spread south across the Bay Area to the
Santa Cruz Mountains. Observed rainfall rates have been as high
as 0.64"/hr in San Francisco. This pre-frontal rain will continue
through the early morning. While mostly stratiform in the warm
airmass, there are some embedded showers bringing stronger down
pours. As the cold front moves through during the day Tuesday, the
rain rates will increase significantly as the cold, dense air
mass lifts the warm, moist air mass, wringing out the atmospheric
sponge. This effect will be enhanced along the windward side of
the coastal mountains.

Flooding: Several area flood advisories are in effect for urban
and small stream flooding across the North Bay, San Francisco and
the northern East Bay. The Green Valley Creek at Martinelli Road
has reached moderate flood stage and continues to rise, verifying
a flood warning in central Sonoma county. In a significant change
to the forecast, The Russian River at Guerneville is now expected
to reach minor flood stage Wednesday morning as more water than
expected has entered the upper Russian. In additions to streams
and rivers, several reports have been coming in of urban and
roadway flooding as heavier bands of rain push through. Expect
this to continue through the day.

Wind: The strong jet max and heavy rain will allow good momentum
transfer down to the surface, especially in gusts. The winds look
strongest in the Monterey Bay region and higher elevations
through the afternoon, although strong winds will be pretty
widespread after the sun comes up and as the front moves through.

Thunderstorms: This is the most meteorologically interesting
aspect of the short forecast. Will we get thunderstorms tomorrow?
Is there a chance these storms will rotate? Overall the
approaching air mass is not that cold. Originating from a maritime
environment, the 1000-500 mb thickness is only around 546 dam
behind the front. That means the instability will be a limiting
factor. Even with good timing of fropa around the maximum day-
time surface heating, the most unstable CAPE values are only
expected to reach around 50 J/kg. On the other side of the
ingredient list, the low level shear looks quite impressive. 0-1
km shear of 35+ kts with nice, curved hodographs and low LCL
heights suggest that if thunderstorms are able to tap into the
moderate instability, they could start rotating. The best window
for this is from 3 PM to 6 PM somewhere south of San Jose where
The HRRR shows the 0-1 km helicity spiking above 300 as the front
moves through. The marine environment will also favor some
rumbles of thunder with a small chance of water spouts.

&&

.LONG TERM...
(Wednesday through Monday)
Issued at 1202 AM PST Tue Feb 4 2025

We will get a break on Wednesday, but it won`t last long. A ridge
will briefly build allowing the new air mass to dry out a bit.
Another moisture plume and surface low pressure will arrive at
the coast and push inland on Thursday. This system looks a good
deal weaker than the current atmospheric river, with less rain and
lighter winds. Outside of the coastal mountains, most areas will
receive a more manageable amount of rain between 1/2" to 1". This
system will dive a little farther south however, bringing a
reinforcing shot of cold air. The 1000-500 mb thickness will drop
below 540 dam. As things dry out Friday and the clouds start to
clear over the weekend, overnight lows will drop back into the
30s. Some places will be more than 10 degrees colder on Saturday
morning, compared to Friday. Once the dry, cold weather sets in
the pattern will stabilize with more of the same through early
next week.

&&

SF Bay Area Skywarn weather spotters should self-activate today through 9pm PST.