---

From AFD:

.SYNOPSIS...
Issued at 112 AM PST Fri Mar 1 2024

Expect a nasty couple of days ahead as a second cold front moves
through the Bay Area. Strong winds will pick up this afternoon.
Periods of rain will persist through Saturday with cold weather
sticking around into next week.

&&

.SHORT TERM...
(Today and tonight)
Issued at 112 AM PST Fri Mar 1 2024

The rain forecast is holding steady and gauges overnight are more
or less agreeing with the expected accumulation so far. There are
some exceptions where stronger rain bands were able to persist for
several hours. These include over 1.5" at Ben Lomond and similar
totals throughout the higher peaks in the Santa Cruz Mountains.
The Santa Lucias have also over performed, with several gauges
already over 2". Otherwise, the forecast has been very good so
far. With roughly another 1-2" expected over the next 48 hours,
there are still no major flooding concerns expected this weekend.
That being said, the rain combined with a cold wind will not make
for great picnic weather.

Strong SW winds will increase through the day ahead of the
frontal passage tonight. Gusts to gale force are expected,
especially as the frontal boundary moves through in the afternoon
and early evening. After the front passes, winds will shift to
westerly, but remain strong through the night. A wind advisory is
in effect for the majority of the area from noon today through 10
AM Sunday.

&&

.LONG TERM...
(Saturday through Thursday)
Issued at 112 AM PST Fri Mar 1 2024

Saturday will feel like the Pacific Northwest. In addition to
periods of rain, it`s going to start cold and won`t warm up very
much. The official forecast for San Francisco has a low of 46F
with the high only reaching 53F. While that`s nowhere near the
daily record (lowest max temp) of 45F set in 1896, it`s 9 degrees
below normal and more in line with the average March high temp in
Seattle (54F). The majority of the rainfall looks to be in the
morning, with only scattered showers in the afternoon. Winds will
also be lighter than Friday, but there will still be a stiff
westerly breeze through the day, especially along the Central
Coast. The first real snow of the year is expected in the
mountains as the thickness finally drops low enough during the
tail end of the multiday rain event. There`s some instability,
but the air mass will be drying out in the post-front environment.
The 06Z NAM point sounding for Santa Rosa valid at 10AM Saturday
brings the freezing level to 1465` with 142 J/kg of CAPE. The
problem is the precipitable water is only 0.38" at that time, and
continuing to decrease through the day. A few inches are expected
in the higher peaks of the Mayacamas and Santa Lucias, around an
inch in the Diablo Range, and a possible dusting in the Santa Cruz
Mountains. To be clear, the snow levels will remain above 2,000
ft and will not impact the main mountain passes.

The rain will really taper off Sunday, but the cold temperatures
will linger through the week. Monday morning looks to bring the
coldest temperatures with most areas dropping into the mid to
upper 30s before the sun comes up. The remainder of the week will
have lows in the upper 30s to mid 40s and highs in the upper 50s.
Some light rain is possible mid week. The uncertainty is much
higher than normal beyond Thursday with the global models all over
the place by next weekend.

There is some uncertainty about the timing and amount of precip expected with this storm. There is concern about wet and windy conditions, and the possibility of significant snow accumulation in the mountains. Stay tuned for updates, and stay safe!!

---

From AFD:

.SYNOPSIS...
Issued at 157 AM PST Thu Feb 29 2024

A large, cold low pressure system will result in rain, colder than
normal temperatures and gusty winds today and into the weekend. High
elevation snow in the Bay Area is likely this weekend. The pattern
will remain mostly active and unsettled through the middle of next
week.

&&

.SHORT TERM...
(Today and tonight)
Issued at 318 AM PST Thu Feb 29 2024

The extent of arctic air coupled to anomalously deep 500 mb
heights in a planetary scale long wave trough along western North
America is evident on infrared satellite imagery. There`s a very
large field of cellular clouds, a signature of instability, latent
and sensible heat transfers are occuring from sea surface up
through the atmosphere upstream over the Pacific all the way to
the Aleutian Islands. Condensation (clouds) releases sensible heat
furthering instability, the colder surrounding environmental air
does additional work to force relatively milder air parcels
upwards and precipitation often ensues, including when air parcels
are forced upwards in the eventual ascent over mountains hence the
additional cooling (decreasing pressure, increasing volume) and
increasing precipitation rates.

The northern hemispheric pattern is active, a large dynamic
rotation rate, the western North America long wave is one of
several. Winter time synoptic pattern recognition for times of
west coast cold air intrusions with little over water trajectory
quite often require a sharp 500 mb unstable ridge over Alaska, the
ridge is more to the west this time; it`s the short wave troughs
within the long wave trough doing all the work in this case. The
overall slow progression of the long wave trough will bring an
extended period of colder than normal temperatures, rain, rain
mixed with snow over our mountains, including measurable snow on
the highest peaks in the Bay Area. Higher mountain peaks of the
west will get multiple feet of snow; this is a dangerous scenario,
with major to extreme conditions, if you haven`t already please
check with your local NWS office(s) for the latest.

For our forecast area the amount of rain we get from this strong
mid-high latitude influence will be because of less water vapor
availability mixed with a relative lack of fully developing smaller
scale low pressure systems (short wave troughs) within the slowly
eastward moving long wave trough. If the meso-scale models don`t
have a good handle on the smaller scale lows then rainfall rates
may end up higher in our forecast area, but as noted previously
there`s been good model agreement so far. Gradually increasingly
colder air aloft will move across our forecast area mainly from
the west and west-northwest, taking a strong over sea trajectory
there will be sea surface based air mass modification, but also
upward heat and moisture fluxes, westward facing slopes will
benefit the most in precipitation.

By later today the initial cold front will arrive, southwesterly
prefrontal winds today breezy to gusty and rain developing across
much of the forecast area. Light measurable rain of a few one
hundredths has already started over Sonoma and Marin counties
early this morning. Later today, tonight post frontal winds will
become westerly to west-northwesterly in direction.

&&

.LONG TERM...
(Friday through Wednesday)
Issued at 318 AM PST Thu Feb 29 2024

A high surf advisory is in effect for the entire coastline 10 am
Friday to 4 pm Saturday. Large break waves along west-facing
beaches will result in dangerous surf conditions and localized
beach erosion. Never a good idea to turn your back on the ocean,
large waves can sweep across the beach without warning, pulling
people into the sea. Coastal flooding is not expected since
observed and forecast high tides are running lower.

Otherwise plan on chilly, wet weather this weekend along with
periodic breezy to gusty winds. Of course will have to keep a
close eye on radar, satellite (plus model forecasts) over the
weekend if any potential last minute changes in rainfall and/or
wind forecasts. A break in the rain arrives either late in the
weekend or early next week. At and beyond 120 hours (5 days) model
solutions diverge, disagreement appears centered on the location
of the long wave trough, thus if the long wave trough stalls then
additional eastern Pacific low pressure system(s) will have an
opportunity to develop next week. March can be a powerhouse with
regard to precipitation, for many locations in our forecast area
long term climatology shows it`s normally the fourth wettest month
of the year.

---

Excerpt from recent AFD:

.SHORT TERM...
(Today and tonight)
Issued at 1233 AM PST Tue Feb 27 2024

High pressure will prevail at the surface and aloft, with dry
conditions, northerly breezes, and cooler temperatures. A few higher
clouds may filter in at times, but overall look for plenty of
sunshine and an absence of coastal stratus after this morning.
Highs each afternoon will reach the upper 50s to lower 60s, with a
few of the warmer inland places (Santa Clara and southern Salinas
Valleys) perhaps nearing the 70 degree mark. Lows will be chilly
and in the 30s (inland) to lower 40s for tonight, with mostly 40s
for tomorrow night.

&&

.LONG TERM...
(Wednesday through Monday)
Issued at 1233 AM PST Tue Feb 27 2024

An unsettled weather pattern will develop as a low pressure system
slowly migrates from the Gulf of Alaska towards the West Coast. The
pattern looks rather dreary with cloudier conditions, gusty winds,
and periods of moderate rainfall. Light rain should first reach the
North Bay by Thursday/Thursday night, with showers overspreading the
remainder of the area on Friday into the weekend. Most of the
rainfall looks to occur on Friday into Saturday, with showers
lingering into early Sunday. Deterministic models, their ensembles,
and the consensus blended guidance all remain in relatively good
agreement regarding rain totals, and no major changes have occurred
since the previous forecast. Storm total amounts still look to
average 1-2" for most locations, with perhaps another inch or so in
the coastal ranges. This suggests that rain-related impacts from
this system should be much more modest as compared to previous storm
systems this winter. Breezy to gusty westerly winds are also
expected with peak speeds generally between 25-35 mph. This will be
a colder system and snow levels this weekend will lower to between
2,000 feet in the North Bay to 3,000 feet for the Central Coast.
This suggests some higher elevation snowfall is possible above these
elevations. The greatest potential for this appears to be across the
Santa Lucias where cold temperatures and precipitation should best
overlap. But can`t rule out light accumulations for some of the
highest peaks in the Mayacamas and Diablo range as well, depending
upon how things play out. At this point it appears snow levels will
remain high enough such that no impacts are anticipated for any of
the passes or major travel corridors.

Colder temperatures return for early next week with frost and freeze
potential for inland areas on Sunday and Monday nights.

---

Excerpt from today’s AFD:

.SHORT TERM...
(Today and tonight)
Issued at 120 AM PST Tue Feb 20 2024

Upper troughing encompassed much of the northeast Pacific earth this
morning with a closed upper circulation just offshore from the N
CA/OR coast. A 160 kt basal jet was located along the equatorward
side of the upper trough and was pointed into SoCal as of this
writing. An occluding surface low was noted off the NW CA coast,
with low-level troughing extending SSW from it. Showers and
thunderstorms were ongoing ahead of the trough around 250 nm
offshore from the Central Coast, with additional showers ongoing
across the Bay Area. For today expect the upper trough to slowly
progress through the region. As temperatures aloft drop this will
steepen lapse rates and increase instability with forcing for ascent
enhanced via the approaching surface trough. Expect an uptick in
shower coverage later this morning with shower and storm potential
peaking this afternoon and evening. Several southwest to northeast
bands of (mainly) light showers were noted early this morning, and
some of the convection allowing models suggest similar
banding/training of showers is possible later today. HREF ensemble
mean probabilities for seeing 1"+ in a 3 hour period peak at 10% as
early as 18Z/10AM for the North Bay this morning and spread through
the remainder of the Bay Area between 21Z/1PM and 00Z/4 PM before
reaching the Central Coast between 03Z/7PM and 06Z/10 PM. Given this
setup we`ll need to keep an eye on flooding potential yet again
today. For most areas this means primarily minor urban and small
stream flooding. However flash flooding is a possibility if any
heavier rain bands are able to train over an urban or
hydrologically sensitive area over a prolonged period.
Thunderstorms are a hazard today as well and given lowered
freezing levels lightning, small hail, and a few stronger wind
gusts are the primary concerns. Thunderstorm potential should be
rather limited early this morning but should increase into the
15-25% chance range over the Bay Area by this afternoon and
perhaps the Central Coast by this evening. Additional rain amounts
for most areas will be in the 0.25"-0.5" range, with around 1" in
the orographically favorable coastal ranges. Localized heavier
amounts remain possible for localized areas that receive
thunderstorms however. Temperatures today will top out in the 50s
(North Bay) to lower 60s elsewhere. Lows tonight will dip into the
mid to upper 40s.

CAC081-200030-
/O.CON.KMTR.SV.W.0001.000000T0000Z-240220T0030Z/
San Mateo CA-
402 PM PST Mon Feb 19 2024

...A SEVERE THUNDERSTORM WARNING REMAINS IN EFFECT UNTIL 430 PM PST
FOR CENTRAL SAN MATEO COUNTY...

At 401 PM PST, a severe thunderstorm was located near Woodside, or
near San Carlos, moving northeast at 35 mph.

HAZARD...60 mph wind gusts.

SOURCE...Radar indicated.

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

Locations impacted include...
San Mateo, Redwood City, Menlo Park, Foster City, Burlingame, San
Carlos, Belmont, Hillsborough, Atherton, Woodside, North Fair Oaks,
Highlands-Baywood Park, Highlands, and Emerald Lake Hills.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Remain alert for a possible tornado! Tornadoes can develop quickly
from severe thunderstorms. If you spot a tornado go at once into the
basement or small central room in a sturdy structure.

For your protection move to an interior room on the lowest floor of a
building.

&&

LAT...LON 3759 12234 3759 12232 3758 12231 3756 12225
3755 12224 3754 12220 3751 12221 3752 12220
3751 12217 3751 12214 3739 12232 3742 12238
TIME...MOT...LOC 0001Z 210DEG 32KT 3745 12232

TORNADO...POSSIBLE
HAIL THREAT...RADAR INDICATED
MAX HAIL SIZE...<.75 IN
WIND THREAT...RADAR INDICATED
MAX WIND GUST...60 MPH

$$

McCorkle

---

Excerpt from today’s AFD:

.UPDATE...
Issued at 815 AM PST Mon Feb 19 2024

Another gorgeous morning for most as much of the large scale
forcing has moved out of the area with main front now situated
along the Central Valley and down through the Central Coast. Moist
upslope flow continues throughout the day as the low remains
spinning offshore, so locations like the Santa Cruz Mtns will
continue to see periods of light rain throughout the day. With the
clearing this morning, this will aid in further destabilizing the
atmosphere as the surface warms and the air aloft continues to
cool. Forecast surface based CAPE on the order of ~500 J/Kg and
effective bulk shear 30-40 kts for areas mainly north of the
Monterey Bay will be more than enough for a few thunderstorms to
initiate and sustain themselves as they move inland off the
waters. As of writing, isolated cells moving through the waters
off the North Bay coast have been showing consistent signs of
rotation. Would not be surprised to see a waterspout or two today
along the coast with a marginal chance for a very brief tornado
threat to land, particularly from Monterey Bay northward and
inland areas from about Livermore through Napa County. This low
threat of a brief tornado decreases as we lose surface heating
heading into the evening and the vertical wind field becomes more
uniform. A brief wind threat is also possible from these storms
as well. Given steep low level lapse rates allowing for efficient
vertical mixing, a brief gust or two of up to 60 mph appears
plausible from any of the stronger cells.

Behringer

&&

.SHORT TERM...
(Today and Tonight)
Issued at 1200 AM PST Mon Feb 19 2024

SW-NE oriented rain bands have been persistent across the North
Bay and the Santa Cruz Mtns/Santa Clara Valley/East Bay regions
as the surface triple point (intersection of occluded, warm, and
cold fronts) continues to approach the Bay Area. Flood Advisories
continue for much of the area with minor impacts ongoing. More
rises on creeks/streams are being observed as well as increased
reports of roadway flooding early this morning.

Looking ahead...High resolution guidance has been consistently
hinting at the potential for a heavy/convective rain band setting
up across the Bay Area later this morning. There is still
uncertainty on if/where this narrow feature will set up and if it
will stall. Upper level support will be favorable for large scale
ascent through the afternoon, drawing in all that subtropical
moisture (PWATs of around 1"). If convective bands were to train
and stall for many hours today, this could result in more serious
flooding impacts. We`ll be watching this closely through the
morning. Chance for thunderstorms still look decent through early
this vening, especially for the North Bay and the interior East
Bay bordering the Sac Valley. A few stronger storms are likely,
with gusty winds, hail, and perhaps a brief tornado. Continue to
be weather aware today and have many ways to receive weather
alerts.

Starting at 3pm PST Sunday afternoon, 02/18/2024, the SF Bay Area Skywarn group will be activated. I will be monitoring the K6MPN repeater (444.500 MHz, +5.0 MHz offset, PL=100Hz CTCSS tone) and the N6NFI repeater (145.230 MHz, -0.600 MHz offset, PL=100Hz CTCSS tone) for severe weather and storm damage reports through Tuesday morning at least. Please keep in mind that we are SECONDARY users on the K6MPN repeater, which is a primary resource for the South County Amateur Radio Emergency Service (SCARES) group.

There are multiple alerts in effect for the SF Bay Area issued by the NWS WFO in Monterey, CA. See sfoskywarn.org/alerts for details. See https://poweroutage.us/area/state/california for current power outages in CA, listed by county.

Please email photos showing storm related damage or flooding to ckolovson – at – gmail.com.

Stay safe!
73, Curt Kolovson W6RQ
SF Bay Area Skywarn Coordinator

---

Quoting from today’s AFD:

.SHORT TERM...
(This evening through Monday night)
Issued at 122 PM PST Sun Feb 18 2024

Current radar showing rain showers increasing in coverage as the
warm front approaches the coast this afternoon. Evident on radar
and visible satellite, the cold front follows close behind as a
cold occlusion sets up offshore north of the Bay Area going into
this evening. The eastward movement of these features and further
destabilization due to cold air advection aloft should result in
increasing rain rates over the next few hours. Heading into the
evening hours once the warm front moves over the Central Coast and
the cold front nears the coast, it is expected that rainfall rates
will further increase, especially for orographically enhanced
areas from the Santa Cruz Mtns southward. Areas of 0.5-0.6 inches
per hour appear likely along the Big Sur Coast, Santa Lucia, and
Santa Cruz Mtns late this evening, persisting for a longer period
over the Santa Lucia, where rainfall rates could approach up to 1
inch per hour in isolated areas. This has prompted the issuance of
a Flash Flood Watch for the River Fire burn scar for the
possibility of debris flows, mudslides, and flash flooding in the
burn scar area and low-lying areas along the Carmel River near the
burn scar. As we move into the overnight period, further
destabilization occurs with additional cooling aloft creating a
more favorable environment for thunderstorms. At the moment, it
appears that the initial threat will be for areas more near the
coast and the North Bay during the overnight period. A widespread
break in the rainfall appears likely Monday morning as the
initial wave pushes through further east into the state. This
opens the door for the second wave of rainfall later Monday
morning where a greater likelihood for thunderstorms exists over
a larger area. Current thunderstorm-related threats include gusty
winds and lightning, and a slight threat for waterspouts along the
coast. High res guidance has been relatively consistent at
depicting a line of heavier showers and thunderstorms training
into the Bay Area Monday afternoon. Should this be the case, we
could end up with isolated locations along the peninsula and East
Bay that end up with a bit more rainfall than expected. However,
these such features are notoriously difficult to nail down, so
will keep that in the back pocket as a low possibility for now.

Current best forecast for rainfall amounts is on the order of
2-3.5" across the North Bay, 1.75-2.5" for the SF Peninsula, 1-2"
for lower elevations of rainshadowed valleys (Santa Clara,
Salinas, East Bay, Southern Monterey Bay), 3-5" Santa Cruz Mtns,
and 3-6" for the Big Sur Coast with isolated areas of 8-9" in the
Santa Lucia.

In terms of wind, current observations show the ballpark to be
about 25-35 mph across the area as of this early afternoon. Expect
these winds to continue into tonight and through Monday afternoon.
Once the cold front nears the area this evening, expect gusts to
increase generally into the 35-45 mph range, with isolated gusts
to 50 mph along the coast and 60 mph along ridgetops. These gusts
will lessen a bit overnight before increasing again with the next
wave of rain and thunderstorms Monday morning.

Starting at 6pm PST Sunday night, 02/18/2024, the SF Bay Area Skywarn group will be activated. I will be monitoring the K6MPN repeater (444.500 MHz, +5.0 MHz offset, PL=100Hz CTCSS tone) and the N6NFI repeater (145.230 MHz, -0.600 MHz offset, PL=100Hz CTCSS tone) for severe weather and storm damage reports through Tuesday morning at least. Please keep in mind that we are SECONDARY users on the K6MPN repeater, which is a primary resource for the South County Amateur Radio Emergency Service (SCARES) group.

There are multiple alerts in effect for the SF Bay Area issued by the NWS WFO in Monterey, CA. See sfoskywarn.org/alerts for details. See https://poweroutage.us/area/state/california for current power outages in CA, listed by county.

Please email photos showing storm related damage or flooding to ckolovson – at – gmail.com.

Stay safe!
73, Curt Kolovson W6RQ
SF Bay Area Skywarn Coordinator

---

Quoting from today’s AFD:

.SHORT TERM...
(This evening through Sunday)
Issued at 230 PM PST Sat Feb 17 2024

A weakening cold front that is drawing in subtropical moisture
continues to slowly push east towards the CA coastline, resulting in
generally light rain across the region. MRMS surface radar precip
rate indicates rain rates are around 0.15" per hour or less. This
checks out given that 12 hour rainfall totals as of 1PM have been
on the light side with 0.25-0.30" across the coastal mountains
and <0.20" across the interior with less than 0.10" across rain
shadowed valleys. As a result, there have not been any flooding
reports thus far from CHP and gauges on creeks/streams are not
responding and holding steady, which is good news. However, the
rain isn`t over yet and an additional 0.15-0.75" is expected
through the evening today with locally higher rain rates possible
as the front gets closer, offering better forcing. But still
expecting minor impacts from today`s system (system #1).

As system #1 weakens and exits the Bay Area and Central Coast this
evening, a break in the rain and wind is still expected overnight
through early Sunday morning. This break will not last long with
system #2, a deep Pacific low, approaches the CA coastline and will
start impacting our region by Sunday mid-morning. The Flood Watch
that encompasses the entire forecast area is set to start at 10 AM
Sunday.

By tomorrow mid-morning, the warm front will approach the region,
bringing warm, stratiform rainfall. Winds will start to increase
as well as a southerly low level jet approaches the coastline,
bringing gusts of 30 to 45 mph (stronger near the coast and at
higher elevations). After the warm front passes sometime tomorrow
afternoon- evening, calmer conditions may briefly prevail with
diminishing winds and rain activity. However, heavier, more
convective rain showers and persistently strong southerly winds
will be quickly develop as the cold front inches closer to the
coastline late Sunday into Monday.

Thunderstorms continue to look increasingly probable Sunday
overnight through much of Monday as colder air aloft moves in,
destabilizing the atmosphere. Heavier rain, locally gusty winds,
small hail, and lightning are all hazards associated with
potential thunderstorms that may develop. Locally heavier rain
rates have the potential to create more flash flooding concerns.

While flooding impacts will be minor at most today, the soils have
become saturated which will further exacerbate flooding concerns
as system #2 rolls through tomorrow-Mon. Tomorrow we will likely
see more reports of roadway flooding, downed trees, and power
outages. Use caution and if able, probably best to stay at home to
avoid problems on the roads.