.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.
&&
