San Francisco (MTR) — the National Weather Service forecast in plain English · Plaincast

Messages

Rain and wind come through this morning and afternoon

Coastal flooding due to high tides continues through high tide today

Hazardous beach conditions prevail with dangerous conditions Saturday

Periods of rain continue through the next week

Short Term

KMUX radar is showing echoes from pre-frontal showers across the coastal regions from Big Sur to Sonoma County with some rainfall already being reported along the coastal ranges and other favored areas. So far tonight, around 0.14" have been reported at the Big Sur RAWS and 0.05" at the Oak Ridge RAWS in the Sonoma County coastal ranges. The isolated pre-frontal showers should continue through the night.

The weather picture features a deep upper level low centered off the Pacific Northwest that will progress eastwards and bring a frontal system into the region. This frontal system will be the initial round of a series of storms that will extend through the seven-day forecast -- more on that in the next section. For this round, high resolution model output shows the main frontal rain band starting to enter the North Bay around the time of the morning commute, extending into the central Bay Area later in the morning into the early afternoon, and passing through the Central Coast in the afternoon hours. Rainfall totals will top out around half an inch to an inch across much of the Bay Area save the far interior East Bay, Santa Cruz County, and the Santa Lucia range. Rain totals in the Santa Clara Valley and the Monterey-Salinas area lie around a third of an inch to an inch while the interior parts of the Central Coast should expect a tenth to a third of an inch. Strong winds are expected to build as the front approaches with sustained winds generally around 15-30 mph and gusts of 30-40 mph across the region. Locally higher gusts to 50 mph are possible at the immediate coast, especially near Monterey Bay and the Big Sur coast, and through favored gaps and passes including the San Bruno Gap, the Altamont Pass, the Carquinez Strait, Bayshore locations along San Pablo Bay, and the Salinas Valley. Another concern will be the risk of thunderstorms embedded within the main rain band and potentially in the post-frontal environment. The cold low will be centered to our north, but will still bring cool enough air aloft to bring the MUCAPE values up to 100-300 J/kg. That would be within the top 10% of soundings at Oakland for practically any time of year. There is also strong shear aloft enhanced by the region coming within the left exit region of a jet streak visible at the 500 mb level, where the decelerating winds and vertical circulation support an area of upper level divergence, which would support convective activity in the area. A slight (15%) chance of thunder has been added to the SF Bay Area for this morning and afternoon as a result. K-Index values from the European model top out around 30 degrees C, which would support isolated thunderstorms.

Behind the main front, the rain becomes showery and winds will diminish and turn towards the west and northwest. Highs today will be remarkably cooler than yesterday's with the lower elevations seeing highs in the upper 50s to lower 60s while the higher elevations of eastern Santa Clara County and the Central Coast drop into the lower to middle 50s.

Hazardous beach conditions are forecast for the Pacific coast through the evening with dangerous conditions expected to develop into Saturday. A Beach Hazards Statement is currently in effect for the Pacific coast for breaking waves 15 to 20 feet along with an increased risk of sneaker waves and rip currents, while High Surf Warnings will go into effect 6 PM tonight into early Sunday morning due to surf heights of 25 to 35 feet accompanied by a high risk of sneaker waves and rip currents. In addition, coastal flooding concerns due to high tides persist at the bayshore locations along San Pablo, San Francisco, and southern Monterey Bays through the high tide cycle today, along with coastal flooding along the Pacific coast due to wave runup impacts on Saturday. See the BEACHES section for more details.

Long Term

Saturday will feature a lull in the rain with some lingering post- frontal showers in the morning, as highs reach the lower to middle 60s across the lower elevations. Then, a second frontal system comes through Sunday into the early part of Monday. At this time, it looks like the rainfall totals will be similar to Friday's system with generally beneficial effects. Another lull in the rainfall comes later Monday into Tuesday, before a third system comes into the region later on Tuesday through Wednesday and Thursday. It's a little too early at this stage to be confident on how much rain this third system will bring.

As we go through the week, the main point to watch will be soil saturation across the region. The initial rounds of rain will be generally beneficial, reducing our fire weather threat and recharging the snowpack across the Sierra Nevada. Flooding concerns will be minimal with nuisance flooding possible across urban areas and small streams. Outside of these areas, rainwater is absorbed into the ground, which at this time, remains rather dry. As the rain continues to arrive, local soils saturate and are no longer able to hold as much additional rainfall, thus diverting excess water onto the surface and streams and rivers. Wet soils will also increase the chance for shallow landslides across the Bay Area, leading to further damage to homes and properties. Beyond the seven-day outlook, the pattern remains active with a lean towards rainfall totals above the seasonal average through late November. We will need to monitor what this will mean for soil saturation and the possibility for landslides through the next week.