Significant storm today (Thurs) and tonight

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.

&&