textproduct: San Francisco Bay Area

SYNOPSIS

Issued at 1238 AM PST Thu Dec 26 2024

The first in a series of rainmakers comes through the North Bay through the morning, with more systems impacting the region through the weekend. New northwest swell arrives today, driving dangerous surf and beach conditions into the weekend.

SHORT TERM

(Today and tonight) Issued at 1238 AM PST Thu Dec 26 2024

Radar products show generally light rain setting up across the San Mateo Peninsula, the fringe of a larger system centered in the eastern Gulf of Alaska that is primarily impacting the Pacific Northwest and northern California. Impacts are generally limited to mainly beneficial rain over the North Bay, with rainfall totals ranging from 0.1-0.3" in the valleys up to 0.5-0.75" in the higher elevations. Elsewhere in the forecast region, the highest rainfall totals are generally around 0.1", and this is for favored locations in the coastal ranges, many inland regions should remain dry into the afternoon. Otherwise, a second push of northwest swell should arrive in the coastal waters today, leading to a second round of dangerous surf and beach conditions (see the BEACHES section for more information).

A more significant round of rain comes through later tonight into Friday with a low pressure system expected to develop and travel a little further south, heading into the Pacific Northwest. This will result in a more southerly distribution of rainfall impacts for us, and will bring higher rainfall totals to the Bay Area and Central Coast. The North Bay remains the wettest region with 0.5-1" in the valleys and up to 3" in the higher elevations, while 1-2" of rain is expected along the Santa Cruz Mountains, 0.25-0.75" in the Bay Area valleys, Monterey-Salinas region and the Big Sur Coast (locally up to 1" in the Santa Lucias), and up to 0.1" in the interior Central Coast valleys. The risk of flooding does increase with each passing system, with soils getting ever more saturated (especially in the North Bay), but confidence in flooding impacts is too low to issue any products at this time. In addition, gusty winds are expected to develop with this system. The coastline, the ridgetops, and the gaps and passes could see gusts of 30-35 mph with this system, and 20-25 mph gusts are possible in the valleys. These gusts are not expected to reach Wind Advisory criteria, but they could still create hazards, such as objects being blown away and difficult driving for high profile vehicles.

High temperatures reach the upper 50s to middle 60s in the lower elevations and into the lower to middle 50s in the higher elevations. Low temperatures remain quite chilly this morning with the inland valleys reaching the low to mid 40s, near 50 at the coast, before warming into Friday morning with readings in the upper 40s to middle 50s across the lower elevations.

LONG TERM

(Friday through Wednesday) Issued at 1238 AM PST Thu Dec 26 2024

A couple more rounds of rain are in store through Sunday, and with the storm track migrating northward, the greatest impacts will once again be focused on the North Bay. Cumulative rain totals from Friday afternoon through Sunday evening are 0.5-1.5" across the North bay, 0.25-0.5" near the coastal ranges down to Santa Cruz, up to 0.25" in the inland East Bay and coastal Monterey County, and up to 0.1" in the inland Central Coast.

Ensemble model guidance continues to show signals of potential rainfall in the first days of the New Year. Cluster analysis reveals a stronger signal for rain from the American GEFS and Canadian GEPS models, while the European ENS model leans more towards a drier solution. As of now, cluster analysis suggests that the chances for any rainfall in the first days of the New Year lie around 40-50%. We've kept the NBM forecast guidance for the later part of the 7-day grids for now, as there's not enough confidence as of yet to nudge the precipitation totals higher.

AVIATION

(12Z TAFS) Issued at 342 AM PST Thu Dec 26 2024

The southern end of a PNW atmospheric river is bringing light rain and IFR-MVFR ceilings to the regional terminals this morning. These conditions will improve through the morning before a more robust system moves through this evening through Friday morning. Widespread MVFR conditions with light to moderate rainfall and strong southerly winds will move from N to S through the night.

Vicinity of SFO...MVFR ceilings will lift to low VFR for the majority of the daylight hours before returning overnight. There is high confidence in MVFR ceilings, light to moderate rain and strong southerly winds overnight. SFO is close to gusting to 35 knots, which would trigger an airport weather warning. We will need to assess the nocturnal stability to determine if the winds will mix down to the surface, but with moderate rain rates and a dry slot around 900 mb, the momentum transfer will likely be efficient.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...These terminals are far enough south to reamin out of the overcast for the moment, but that will likely only last for another few hours. The wind are rain overnight will be less impactful here, although moderate southerly winds and IFR ceilings are still possible early Friday morning.

MARINE

(Today through Tuesday) Issued at 342 AM PST Thu Dec 26 2024

Disturbed weather has returned to the coastal waters and buoys are showing sea heights have starting to rebuild. Strong SW winds and rough seas will persist through Saturday, before a cold front flips the wind direction to NW on Sunday. Conditions will improve next week as high pressure rebuilds over the area and the swell abates.

BEACHES

Issued at 342 AM PST Thu Dec 26 2024

A High Surf Advisory is in effect from 10 AM this morning through 4 PM Sunday. Large NW swell is arriving to the coastal waters and will persist through the weekend. Satellite observations and wave models continue to suggest the swell will be roughly 15 feet high with a 15 second period. That translates to 20-30 foot breaking waves, depending on the beach orientation and bathymetry (beach slope). The waves now look to peak Friday and abate through the day Saturday. If that trend holds, we may have to shorten the advisory duration. While these waves are smaller than the monsters we got on Monday, the coast is still dangerous. Inexperienced swimmers should stay out of the water, wave watchers should maintain a a safe distance, and no one should ever turn their back on the ocean. Let's get through this weekend without another ocean fatality.

MTR WATCHES/WARNINGS/ADVISORIES

CA...High Surf Advisory from 10 AM this morning to 4 PM PST Sunday for CAZ006-505-509-529-530.

PZ...Small Craft Advisory from 9 PM this evening to 9 AM PST Friday for SF Bay N of Bay Bridge.

Small Craft Advisory from 3 PM this afternoon to 9 PM PST Friday for Mry Bay.

Small Craft Advisory until 9 PM PST Friday for Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm.

Small Craft Advisory from 9 AM this morning to 9 PM PST Friday for Pigeon Pt to Pt Pinos 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.

Small Craft Advisory until 9 PM PST this evening for Pt Arena to Pt Reyes 10-60 NM.

Gale Warning from 9 PM this evening to 3 AM PST Friday for Pt Arena to Pt Reyes 10-60 NM.