.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 upstreamjet 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+ ktjet 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 floodwarning 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.
&&
.SHORT TERM...
Issued at 1247 PM PST Mon Feb 3 2025
(This evening through Tuesday)
Key Points:
* Flood Advisory continues for portions of Sonoma County
* FloodWatchgoes into effect 4 PM Monday through 4 AM Wednesday
across the Bay Area and Central Coast
* Mon PM-Tue AR ontrack late Monday with moderate to at times
heavy rain
* Mon PM-Tue will also bring gusty winds and thunderstorms
* Thur-Fri AR is expected to be weaker and aimed farther south
* Cold morning temperatures return mid-week but temperatures
will drop more significantly over the weekend
Being upfront - lengthy AFD as lots of interesting and impactful
weather headed our way.
Near term - KMUX is showing an increase in shower activity off the
North Bay coast and northern Sonoma county. The increase is a
result of better forcing tied to a low pressure off the coast
moving eastward. The approaching low has also allowed for gusty
southerly winds across the region this afternoon. This is only a
preview of things to come over the next 36 hrs.
This evening through Tuesday:
The broader meteorological picture still shows a quasi-stationary
boundary to the north. This front extends west to a surface low
near a triple point 37N 128W and farther west of that is an
occluded low pressure 32N 140W also tracking eastward. Aloft is a
little more interesting as split flow with a high amplified polar
jet (digging SW thru BC and then recurving into NorCal) is showing
some phasing with the sub-tropical jet arriving from the SW. These
upper leveljet features/phasing will aid in overall large scale
ascent. On top of this features, PWAT satellite imagery shows a
Pineapple Express (AR) plume just SE of Hawaii extending north
along the surface boundaries to NorCal.
So what does this mean and how will it impact the Bay Area and
Central Coast? The upstream "triple point" low will move east and
help to lift the stationary boundary slightly, but also allow for
a more defined surface front. This surface enhancement will be
aided by an upper leveljetexit region over NorCal this evening.
Large scale forcing, orographic enhanced, and PWAT transport will
lead to heavier rain spreading over the North Bay this evening.
Could see some decent rainfall rates of 0.5" per hr. Rainfall
coverage and intensity will gradually move S and E over the rest
of the forecast area by Tuesday morning. Not only will rain
increase this evening through early Tuesday, but the winds will
too. The winds will increase rapidly over the northern coastal
waters this evening and then spread to the coastal areas Monterey
Bay northward. A Wind Adv was issued to account for this increase
in winds.
Early Tuesday will feature a brief "lull" in activity as the
"triple point low" fades with an exiting jet region aloft. The
lull will be short lived as even better jetdynamics aloft take
aim at the Bay Area and Central Coast Tuesday mid morning through
Tuesday evening. Expect another rapid increase in winds and precip
on Tuesday as a cold front pushes through the region. This will
bring the most widespread and intense rainfall.
Of particular concern is between 18 and 21 UTC Tuesday, flow
through the column increases. In fact, examining the wind field at
300mb reveals a phased or coupled jet type pattern with a wind
maxima approaching the CA/OR coastline with a broader field of
+120 knot mid level flow. In response, the lowest 1 km wind field
responds and 925mb flow ramping up to around 50 knots. This is
particularly true along and south of the trough/cold frontal
boundary for regions along the Central Coast and Big Sur
Coastline. While instability values will be on the lower end of
the spectrum with under 50 J/kg. Forecast hodographs indicate a
large amount of low level curvature with 0-1 km shear in excess of
30 knots. These types of high shear, low CAPE regimes do support
both the transfer of damaging 55+ mph winds aloft down to the
surface and in some instances brief tornadoes. We`ll continue to
monitor trends in high-resolution output, but as it stands right
now, the favored region for enhanced winds of 55+ mph and a
slightly higher tornado risk will be along the San Francisco
Peninsula Coast (Pacific Ocean side) down to the Big Sur Coastline
late Tuesday morning and through the evening hours. That being
said, have included thunderstorms to the forecast for Tuesday
with specific wording highlighting gusty convective winds.
Heavy rain will transition late Tuesday to more shower activity
from N to S behind the departing cold front. Rainfall amounts
tonight through Tuesday night will 4-6" North Bay and coastal mts
(jackpot up to 9" above Big Sur), 2-4" elsewhere, with lesser
amounts 0.75 to 1.5" Santa Clara Valley/Interior Central Coast.
It goes without saying, but these rounds of intense rainfall will
lead to flooding concerns, especially in areas where rain fell
over the weekend. Do expect Flood ADvisories to be issues for
urban and small stream flooding concerns. If you encounter
a flooded roadway or road closure barricades remember - turn around,
don`t drown!
&&
.LONG TERM...
Issued at 240 PM PST Mon Feb 3 2025
(Tuesday night through next Sunday)
Wednesday can be described as another lull in overall activity as
one storm/moisture plume exits and another approaches from the
west.
Thursday and Friday...a surface low will approach the Central
Coast. Unlike the Tuesday AR this system will have less support
aloft and track a little farther south. Additionally, the moisture
tap is less with IVT/IWV values solidly in the Weak AR realm.
Thankfully, this low also appears to be a fast mover with highest
rainfall totals North Bay and Coastal Mts around 1" with
0.25-0.75" elsewhere. Given the more southern track this system
will be cold with lowering snow levels. Snow level will lower N to
S, but by the time the lowest snow levels arrive the precip may be
tapering off Thursday into Friday. Regardless, some wet snow will
be possible over the higher peaks Thursday into Friday. Not
expecting much in the way of accumulation.
Speaking of colder, the precip threat will quickly transition to a
temperature threat next weekend. Cold advection aloft behind the
departing low combined with clearing skies will lead to some
chilly overnight lows. Both Sat/Sun mornings will feature temps
in the 30s (even toward the coast) with far interior in the 20s.
Will likely need a mix of frost/freeze products.
&&
Moderate to heavy rain now expected to cover Bay Area by about 10am Tuesday.
From MTR AFD:
.SHORT TERM...
(Today and tonight)
Issued at 304 AM PST Mon Feb 3 2025
Key Points:
* Flood Advisory continues for portions of Sonoma County into Monday
* FloodWatchgoes into effect 4 PM Monday through 4 AM Wednesday
across the Bay Area and Central Coast
* Second AR arrives late Monday bringing moderate to at times heavy
rain
* Third AR arrives Thursday-Friday but is expected to be weaker and
aimed farther south
* Cold morning temperatures return mid-week but temperatures
will drop more significantly over the weekend
Light showers continue to the north of our CWA but are expected to
shift southwards again as a second, stronger, AR arrives later
today. Forecast IVT values are in excess of 600 to 700 kg/ms which
is supportive of a more moderate intensity AR arriving mid to late
Monday and continuing into Tuesday. The first round of moderate to
heavy rain will take place later in the day on Monday and a second
round of heavier rain will take place on Tuesday. While the bulk of
the moisture is still located to our north, there will be enough
lingering moisture for light showers across the region Monday
morning. By the late morning/early afternoon, showers will start to
pick up in intensity across the North Bay as troughing deepens over
the Pacific and a shortwave approaches the West Coast. This
shortwave, in combination with the sub-tropical jet lifting
northward, will push the main moisture plume southwards with shower
activity subsequently increasing throughout the day across the Bay
Area and Central Coast. At the same time, a stationary cold front
over northern CA will gradually give way to a more progressive cold
front as a surface low associated with this shortwave moves inland.
The initial rain (on Monday) will primarily be concentrated over the
North Bay through the late afternoon before we start to see more
moderate intensity rain shift south of the Bay Bridge over the
evening hours. Rainfall totals - for Monday - will be highest across
the North Bay Mountains which will see between 1.5"-2". Elsewhere,
initial rain totals of up to an 1" are possible on Monday with the
bulk of the rain from this AR to fall late Monday into Tuesday. As
this occurs, we will see a southwards shift in where the highest
rainfall totals are located with an emphasis on the Santa Cruz
Mountains and Santa Lucia Range (see the long term discussion for
more information). Gusty winds ramp up along the coast and across
elevated terrain Monday into Tuesday. While winds generally stay
below Wind Advisory criteria, coastal areas may flirt with Wind
Advisory criteria Monday afternoon/evening around the time of
frontal passage. For now, continuing to hold off on a Wind Advisory
due to the short duration and more isolated nature of higher gusts
but will continue to monitor and update if things change.
The WPC has issued a day one (Monday) slight risk (15%) of excessive
rainfall for the North Bay and a marginal risk (5%) for portions of
the Bay Area and Central Coast. A FloodWatch will go into effect at
4 PM PST on Monday for the Bay Area and Central Coast as rain starts
to ramp up in intensity and becomes more widespread across the
region. As has been talked about for the past few days, rainfall
across the North Bay and coastal areas has acted to saturate soils
and has essentially "primed" them for flooding to occur. For areas
that have not received as much rain, moderate to heavy rain is
expected to saturate soils fairly quickly Monday/Tuesday with
increased potential for urban nuisance flooding as heavier rain
rates occur. While the most recent CNRFC forecasts continue to limit
mainstream river flooding to the North Bay, smaller creeks and
streams across the rest of the Bay Area and Central Coast may see
sharp rises as the bulk of the rain passes through. If you encounter
a flooded roadway or road closure barricades remember - turn around,
don`t drown!
&&
.LONG TERM...
(Tuesday through Sunday)
Issued at 304 AM PST Mon Feb 3 2025
Heading into Tuesday, heavier showers will spread into the rest of
the Bay Area and Central Coast overnight with the highest
accumulations anticipated in the Santa Cruz Mountains and Santa
Lucia Range. Rain may briefly lighten mid to late morning but
widespread heavy rain will return Tuesday afternoon as a second
pulse of moisture (higher IVT values in exceedance of 700 kg/ms)
arrives. This second pulse will be associated with with the greatest
flood risk as any areas not previously saturated by the Friday to
Sunday rains will be at least partially saturated from the Monday to
early Tuesday morning rains. Moderate to heavy rains will then
persist through the evening hours before rains decrease in intensity
overnight Tuesday into Wednesday. When all is said and done from
Monday to Tuesday, the North Bay will (in total) see an additional
2"-4", an additional 1"-3" south of the Bay Bridge, 3"-5" in the
Santa Cruz Mountains, 1"-2" across the Central Coast, and 5"-8"+ in
the Santa Lucia Range. Given that the moisture plume is oriented
almost perpendicular to the coastline, areas where orographic uplift
is favored may see locally higher totals than previously mentioned.
An additional factor is that over the last few grid updates the
forecasted QPF totals have continued to increase for this event with
particular emphasis on the southern portions of our CWA (Santa Cruz,
Monterey, and San Benito counties). As such, it is important to stay
up to date on the most recent forecast and be aware that there is
elevated potential for flooding to occur. The WPC has issued a
widespread day two (Tuesday) slight risk (15%) of excessive rainfall
exceeding flash flood guidance across the Bay Area and coastal
Central Coast and a marginal risk (5%) of exceeding flash flood
guidance across interior Monterey/San Benito Counties. Gusty winds
will continue through the day on Tuesday before weakening Tuesday
night into Wednesday. Winds continue to look below Wind Advisory
criteria on Tuesday but we will continue to monitor in case frontal
passage slows down and the time period of stronger winds becomes
more prolonged.
Wednesday will be the lull between our Monday-Tuesday AR and our
Thursday-Friday AR. Lingering moisture may be enough to support
scattered, light showers but any totals from this will generally be
below a tenth of an inch. Notably, morning temperatures will begin
to drop Wednesday after cold frontal passage occurs Monday/Tuesday.
This may necessitate the issuance of frost/freeze products for
portions of the Bay Area and Central Coast.
Thursday to Friday will see a third, albeit weaker, AR impacting our
CWA with additional totals between 1"-2" likely across the region.
This system will be spurred by the arrival of another upper levelshortwave and a surface low pressure system with IVT guidance
showing a subtropical moisture tap. The main thing to note about
this system is that, so far, it looks fairly progressive and has
started to trend more southwards (Central Coast into Southern CA) on
recent model runs. This may result in additional fluctuations in
forecasted QPF amounts Thursday and Friday as we get closer to this
AR. Another cold front is expected to pass through the region on
Friday bringing with it locally gusty winds and a more significant
drop in morning temperatures on Saturday/Sunday. Current guidance
suggests temperatures may drop into the 30s to upper 20s. The
combination of prolonged wet conditions, gusty winds, and cooler
temperatures will create hazardous conditions for anyone without
access to adequate heating or shelter. The issuance of additional
Frost and Freeze products are likely Saturday and Sunday.
&&
The Mon/Tues storm forecast for the Greater Bay Area remains on track, with a slight shift to a later onset of heavy rain and gusty winds to later Mon night and lasting through Tuesday.
This graphic shows the 6-hour precip forecast with increased rain intensity moving into the Greater Bay Area by approx 10pm Mon night.
From MTR AFD:
.LONG TERM...
(Monday through Saturday)
Issued at 248 AM PST Sun Feb 2 2025
Light showers continue Sunday into Monday with the next round of
higher intensity rain returning late Monday morning/Monday
afternoon. Taking a look at the synoptic pattern, an upper leveltrough off of the PNW and an upper levelridge over southern CA have
in part kept our moisture plume tighter and pushed farther north
this weekend. By Monday, the upper leveltrough will deepen over the
PNW, pushing farther into northern CA, and ridging over southern CA
will start to shift eastward. This will allow for the moisture plume
to widen and shift southwards again late Monday morning/Monday
afternoon with precipitable water values generally increasing to
greater than 1" across the region. At the same time, the stationary
front over the North Bay will give way to a more progressive cold
front that will move through the Bay Area Monday into Tuesday. Rain
will start to pick up in intensity by mid to late Monday morning
with the initial heaviest rain located in the North Bay. High
resolution models are starting to show precipitation lingering over
the Bay Area (south of the North Bay) and Santa Cruz Mountains
overnight Monday through Tuesday morning. Totals have trended
upwards Monday into Tuesday with an additional 3"-4" possible across
the North Bay, 2"-3" across the rest of the Bay Area, and 1"-3"
across the Central Coast. Locally higher totals (4"+) are possible
in areas where upsloping is favored (North Bay and coastal
mountains). A slight chance (<10%) of thunderstorms continues to be
monitored for Tuesday morning but the probability has been trending
downwards recently.
Wet weather will then continue Wednesday through the end of the week
as troughing persists over the West Coast. Late week rain totals
will be lower, however, with most areas seeing less than 1" of
additional rain. The exception, again, will be where upsloping is
favored within the coastal mountains where totals will be between 1"-
2". This second system will be colder with morning lows expected to
drop into the mid 30s to low 40s across the region. The combination
of damp weather and cooler temperatures may create conditions that
are particularly hazardous for people without access to adequate heat
or shelter.
It is worth noting that the WPC maintains a slight to marginal risk
of excessive rainfall for the Bay Area and portions of the Central
Coast through Tuesday and again on Thursday. Flooding concerns will
remain elevated through the week as our initial rainfall from Friday
to Saturday have caused soils to saturate. This will lead to
increased surface runoff and is likely to lead to additional flood
related products as we move through our next two rounds of wet
weather. In addition, gusty winds are likely to persist for coastal
areas and elevated terrain Monday through Wednesday with the
strongest gusts likely to develop ahead of/around the time of cold
frontal passage Monday into Tuesday. This may result in a few trees
being knocked over and increased potential for power outages to
occur. Make sure to have multiple ways to receive warnings and stay
up to date on the forecast as we move through the upcoming week.
SF Bay Area Skywarn spotters in the North Bay should self-activate immediately. Spotters in the rest of the Bay Area should self-activate starting Monday afternoon / evening.
Rain will continue throughout the Bay Area at least through Tuesday night. The North Bay will be mostly impacted through Sunday. Starting Monday and lasting at least through Tuesday night, the entire Bay Area will get significant rain and winds.
The SJSU WRF 2KM Ensemble Precipitation Forecast 12Z run on Feb 1 shows a rapid increase in the ensemble mean at approx midnight on Monday night / Tuesday morning at KSJC, which indicates a later start to the heavy rain in the South Bay, whereas at KSFO the increase starts in the early to mid afternoon.
This graphic shows the model ensemble 6-hour precip forecast intensifying over the Bay Area during the afternoon (4pm) on Monday Feb 3.
From MTR AFD:
.LONG TERM...
(Sunday through Friday)
Issued at 317 AM PST Sat Feb 1 2025
Showers are expected to continue through the remainder of the
weekend and into next week as an AR continues to impact the West
Coast. Sunday will act as a brief break between rounds of heavier
rainfall on Saturday and Monday with lighter rain expected
everywhere. After the passage of Saturday`s shortwavetrough, the
upper levelflow becomes more zonal (reducing upper level forcing)
and the surface low pressure system shifts northward, becoming
stationary over the North Bay. This will shift the moisture plume
northward as well and continue to train moisture over the North Bay
with locally higher rainfall totals possible there. Comparatively,
the amount of available moisture will be reduced farther south, with
fairly light rain totals anticipated outside of the North Bay. Given
this, an additional 1"-2" of rain is possible over the North Bay, up
to 1" across the Bay Area, and up to 0.25" across the Central Coast
on Sunday.
Heading into Monday, zonal flow will give way to persistent upper
level troughing across the West Coast, kicking off the next round of
moderate rain. The surface low pressure system will become less
stationary as the upper leveltrough develops with the associated
moisture plume widening and spreading southwards on Monday. A cold
front associated with this low will progress through the Bay Area
and Central Coast late Monday night into Tuesday morning. This
system will bring an additional 2"-4" of rain to the North Bay, 1"-
2" to the rest of the Bay Area, up to 3" across the Santa Cruz
Mountains and Santa Lucia Range, and 1"-2" across the Central Coast.
At the same time, cold frontal passage will increase atmospheric
instability and bring a 10-20% chance of isolated thunderstorms over
the coastal waters and along the coastline Tuesday morning.
Thunderstorm chances will peak early Tuesday morning before
decreasing through the remainder of the day.
Additional rounds of light rain (generally totaling to between 0.5"-
1.0") are expected Wednesday through Friday as persistent upper
level troughing continues over the West Coast. Gusty winds persist
throughout the week with cooler morning temperatures returning
Wednesday through the extended forecast. The combination of gusty
winds, cold mornings, and continued wet weather will create
hazardous conditions for those without adequate access to heat or
shelter. Cold weather products are likely to be evaluated for mid
week and beyond as we get closer in time to them. For snow lovers,
it is worth noting that, in the wake of cold frontal passage, snow
levels will drop to between 2500-3500 feet for portions of the
region. This, in combination with the continued wet weather,
means there is a non- zero chance for light snowfall on the
highest peaks of the North Bay Mountains and in the Santa Lucia
Range. While confidence remains low, probabilistic guidance does
show a 30-40% chance of trace snowfall across the peaks of the
North Bay Mountain mid to late week. This is still several days
out and the forecast is likely to change, but, it may be worth
keeping an eye on the Cal Alert Cameras late week to see if you
can spot any mountain snowfall.
Currently the most impactful rain and winds are expected to occur on Monday into Tuesday. The graphic below shows the expected precip (mm) at 4pm PST Tuesday according to GFS. This shows the strongest signal for rain over the Bay Area on the 24-hour GFS runs.
From MTR AFD:
.LONG TERM...
(Saturday through Thursday)
Issued at 302 AM PST Fri Jan 31 2025
If you`ve been sorely missing the rain for the last month, this
upcoming week is for you! Continuing from our Friday system, more
widespread, moderate rain moves in on Saturday with the heaviest
showers expected Saturday morning into the afternoon. Current
guidance suggests 1.0"-2.0" are possible across the North Bay, 0.5"-
1.5" across the rest of the Bay Area, and up to 0.5" for the Central
Coast on Saturday. A slight risk (at least 15% chance) of excessive
rainfall is in effect for the North Bay and portions of the Bay Area
(including the city of San Francisco) for Saturday with a marginal
risk of excessive rainfall in effect for portions of the South Bay
and Central Coast. Heading into Sunday, guidance suggests rainfall
will be lighter and more concentrated over the North Bay as the
moisture plume shifts slightly northward. As such, the WPC has
issued a day 3 marginal risk of excessive rainfall for the North Bay
on Sunday. The next round of widespread moderate showers begins
Monday as another upper level low and associated surface low
pressure system move through the Bay Area. The day four (Monday) WPC
excessive rain outlook shifts the North Bay into a slight risk of
excessive rainfall and portions of the Bay Area into a marginal
risk. By day five (Tuesday), a more widespread marginal risk of
excessive rainfall is expected across the entire Bay Area and
Central Coast. When all is said and done, from Friday to Thursday,
the North Bay will see roughly between 5"-8" of rain, the remainder
of the Bay Area will see between 3"-5", and up to 2.5" is possible
across the Central Coast. Locally higher totals are possible
throughout the elevated terrain of the Santa Cruz Mountains and the
Santa Lucia Range with overall totals between 4"-5" possible by late
next week.
Gusty winds are expected to continue through the week with NBM and
local WRF guidance highlighting Tuesday and Wednesday for
particularly windy conditions. Daytime temperatures will be in the
50s to low 60s through midweek while morning lows hover in the 40s
to low 50s. Cold mornings return again Wednesday through the end of
the forecast period, with lows to drop into the 30s again. This
combination of wet weather, gustier winds, and cold mornings may
create a setup hazardous for those without adequate heat or shelter
mid to late next week.
Friday’s storm will be mostly impacting the North Bay, but the whole Bay Area should expect some rain. Monday’s storm will likely be stronger across the entire Bay Area, and will include potentially heavy rain and strong westerly winds. This graphic shows precip forecast for Saturday 4pm PST according to the GFS model.
From MTR AFD:
.LONG TERM...
(Friday night through next Wednesday)
Issued at 248 PM PST Thu Jan 30 2025
Confidence continues to increase for widespread impactful rainfall to
persist through the remainder of the extended forecast. The first
system arrives on our doorstep late tonight. Much of the North Bay
will have already seen between 0.50-1.00" of rain by late tomorrow
night and early Saturday. The focus of higher rainfall amounts will
then shift south into the Bay Area, East Bay and Santa Cruz coastal
range throughout the day Saturday. A relatively strong upper leveltrough axis will shift onshore late Friday into early Saturday when
increased rainfall rates can be expected from the North Bay into the
Bay area. While the parent low reconsolidates well to our north,
zonal flow with a series of embedded shortwaves will continue to
drive widespread light rain across most of our area, with enhanced
upsloping producing periods of moderate to heavy rain along our
coastal ranges through the weekend.
The main upper low then dives south late Sunday night into Monday.
Deterministic guidance is in decent agreement for the evolution of
the primary low`s shift south, but this part of the forecast will
continue to be watched closely. Any change or retreat north for the
beginning of next week, will result in big changes for rain totals
outside of the North Bay.
The rainfall totals for tonight through early Monday morning should
only lead to minor impacts with respect to flooding across the
North Bay. However, by the time the second round arrives for
Tuesday and Wednesday, we`ll see rivers and streams begin to
respond quicker to the steady fetch of rainfall expected. Even
with the higher end totals anticipated, there is roughly a 10%
chance of flooding for normal trouble spots along the Russian Rive
in Sonoma County and other locations into Napa County and Marin
County. As we monitor the rainfall this weekend, and the evolution
of the larger scale pattern for the beginning of next week, keep
an eye on subsequent forecast updates as these river forecasts
could change considerably. Along with the potential for periods of
moderate to heavy rain leading to flooding, there will also be
moderate to strong gusty winds. The strongest winds will be across
higher elevations, beginning Saturday morning with the passage of
the aforementioned trough axis moving onshore. As some might
expect, the strongest gusts should be expected over higher
elevations like Mt Helena, up to 60 MPH after sunrise Saturday
through much of the day. We`re anticipating the need for a Wind
Advisory beginning at some point early Saturday, with elevated
wind speeds continuing through the middle of next week. While
strong gusty winds at higher elevations may approach 60 MPH, lower
lying areas along the coast and inland valleys can expect max
wind gusts between 35-45 MPH at times.
