After a heat dome brought record temperatures to California and sparked wildfires from Riverside to Tahoe, it looks like the state is being whipped by unusually cooler weather this week.
Cities like Los Angeles and San Francisco are 10 to 20 degrees cooler than they were at the height of the heat wave, and this cooling trend is expected to continue through the weekend as a unusually strong storm settles over the state. So where does this system suddenly come from?
Indeed, a typhoon.
Typhoons are tropical cyclones that develop in the Western Pacific and have the same properties as hurricanes or any other tropical system that forms in the rest of the world.
There are three such typhoons circling off the coast of Japan. Even though it’s over 5,000 miles away from us, we’re likely to feel some residual impacts.
Typhoon Murbok is being pushed north by neighboring typhoons towards the jet stream. Weather models show that Murbok will eventually lose its typhoon status and its remnants will be absorbed by the jet, carrying its moisture to Alaska by Thursday and Friday.
Starting Saturday, the new storm will track south over the West Coast, where European, US and Canadian weather models expect the center of the system to be just off the Bay Area coast. There is a wide range of possible impacts from this storm depending on whether the center remains above water or a few inches above land.
Snowfall forecasts from the U.S. weather model total Tuesday morning around the Sierra, with larger accumulations possible around the peaks south of Lake Tahoe.
Gerry Diaz / WeatherbellEuropean and US weather models have been back and forth on the timing of the first rain showers, but for now it looks like the first showers will arrive in Northern California around Saturday evening. Most of these initial showers will roll over the South Cascades and the Mendocino Coast before eventually rolling into the SF Peninsula, Sonoma Coast and Central Coast by Sunday morning.
Look for rainfall totals by the end of Sunday between a tenth of an inch and up to a third of an inch in the North Bay Highlands, Santa Cruz Mountains, SF Peninsula and Berkeley Hills, while some isolated northwest-facing areas on the coastline can see up to a fifth of an inch of rain. The Central Coast is also looking to get in on the action, with Monterey Bay predicting to see between a quarter inch and three quarter inch of rain. The Big Sur coast could see much higher totals of more than an inch if enough moisture enters the Santa Lucia Mountains on Sunday.
Showers are then expected to develop over the Sierra Nevada by Saturday afternoon, with some models even bringing rain showers to the Mosquito Wildfire perimeter. There’s a lot of spread here, which means some models expect up to four-tenths of an inch of rain, while others lean towards less than a tenth of an inch.
Winds and risk of burns
Strong westerly winds along the Sierra, colloquially called Zephyr winds, are also expected to pick up this weekend, so air quality impacts in the Reno and Tahoe metros cannot be ruled out. A few models even suggest that these strong winds could start as early as Saturday, before the rains arrive, which could fuel fire activity. These Zephyrs can pose problems for firefighting operations if rainfall accumulations remain below a tenth of an inch around Tahoe.
The Canadian weather model’s rendering of the center of the storm system, placing it just west of the Sonoma coast on Sunday morning.
Gerry Diaz / WeatherbellAssuming the drier scenario unfolds, the Tahoe National Forest could be looking at 24 hours of moderate to strong winds blowing at 35-45 mph before the rains move into the Sierra Nevada.
On the other hand, if a lot of rain falls over a short period of time, there is also a risk of landslides and debris flows, which are masses of loose mud, rocks and other material that move quickly over the side of a hill or mountain. Last month we saw debris flows over the Tamarack burn scar in the Sierra, and it was all thanks to the monsoon rains. The region quickly saw totals soar by as much as three-quarters of an inch in just a few hours.
Fortunately, an extreme end to the rain totals in the models seems increasingly unlikely, so for now the hope is that this storm is drawing in enough moisture to bring beneficial rains to the perimeter of the fire.
Some snow potential
The rain will continue through Monday before eventually dying out in the Bay Area. But some showers are looking to hover over southern Tahoe Monday night and Tuesday morning. This is important because this storm system will bring a strong cold front to the Sierra Nevada between Sunday and Monday. And by Monday evening, the cold, drier air over the mountains is expected to drop below freezing.
So if enough residual moisture hangs around Monday night, residents above 3,000 to 4,000 feet can wake up to showers and even a few inches of snow on the ground. The area where the Mosquito fire is burning could also be dusted.
Overall, this appears to be a very unusual storm system with widespread impacts for California.
Gerry Díaz (he/they) is an editorial meteorologist for the San Francisco Chronicle. Email: [email protected] Twitter @geravitywave
