Hurricane Kay will bring very unusual weather to the Bay Area

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So long, heat dome. The long-awaited cooling trend is coming to the Bay Area and across the state Friday with temperatures falling through the weekend. Temperatures are also set to steadily drop in the Sierra Nevada, which is great news for firefighters battling the Mosquito Fire.

This fire, burning in Tahoe National Forest and Placer and El Dorado Counties, has grown dramatically since its ignition. That’s thanks to the extremely hot and dry weather from the statewide heat wave earlier this week. And weather models are hinting that a rare system will fan some of its flames this weekend.

Some good news: Kay’s remnants might bring some rain to Northern California.

Winds are set to steer from the east this weekend thanks to Kay’s remnants.

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Our neighbors in Southern California are amid a dramatic shift between this week’s statewide heat wave as tropical-storm-force winds whip into the San Diego-Tijuana metro, churning rough waters off the coast of Southern California. Kay is charging up the coast and bringing historic rains to much of Baja California, while Southern California is set to see upward of 1 to 4 inches of rain this weekend, with even higher totals possible along the San Jacinto and San Bernardino ranges.

So can we expect the Bay Area and Sacramento to tap into some of this remnant tropical moisture? The European, Canadian, and American weather models are all leaning toward yes!

Precipitable water values are forecast to run well over 100% and 200% above normal over large parts of Southern California, with some of that moisture making it up into the Sacramento Valley and parts of the Bay Area by Sunday and Monday.

Precipitable water values are forecast to run well over 100% and 200% above normal over large parts of Southern California, with some of that moisture making it up into the Sacramento Valley and parts of the Bay Area by Sunday and Monday.

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Kay’s remnant moisture will be hoisted north by southerly winds to help fill the void left behind by this week’s heat dome. This will raise precipitable water values — which measure how much water vapor there is in the atmosphere — up to 100% of normal in Northern California by Sunday. That may not be as high as the PWAT values that will be running 200% to 300% of normal in Southern California, but we will take what we can get.

There’s just one catch: Thunderstorms will also be possible.

The latest European weather model’s outlook on rain totals is looking low for the Bay Area, with only a few sprinkles expected along the coast and higher mountains. Half an inch to an inch of rain looks more favorable along parts of the Sierra.

The latest European weather model’s outlook on rain totals is looking low for the Bay Area, with only a few sprinkles expected along the coast and higher mountains. Half an inch to an inch of rain looks more favorable along parts of the Sierra.

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Unfortunately, the air in the Central Valley and the Bay Area is still running very dry, largely because the heat dome was centered right over Central California for more than a week. This means that despite all the moisture moving in this weekend, a lot of it might end up evaporating in this dry layer before it hits the ground in a process called virga.

This is not good news for firefighters in the Mosquito fire, who may go up against strong isolated gusts, on top of any prevailing winds from Kay, near Tahoe this weekend.

There’s already been extreme fire activity on this fire, with at least two pyrocumulonimbus clouds having erupted Thursday. Not only will the Mosquito Fire be fanned by winds from the remnants of this hurricane, but these leftovers will waft smoke to the Sacramento Valley. This means it will be muggy and hazy in much of the state this weekend.

The Bay Area Air District has issued an advisory for Friday to account for haze through the rest of the weekend. In the Tahoe area, air quality will be extremely hazardous as AQI levels surpass 300 Friday through Sunday, making it dangerous for even the healthiest people to breathe in.

The moisture moving into the Bay Area and Central Valley will interact with an unstable atmosphere, helping to fan a slight chance for thunderstorms. If any storms develop, they may produce lightning in parts of the Sierra and Bay Area.

The moisture moving into the Bay Area and Central Valley will interact with an unstable atmosphere, helping to fan a slight chance for thunderstorms. If any storms develop, they may produce lightning in parts of the Sierra and Bay Area.

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The remnants of this system will also be bringing unstable air into the region, but any virga in the region has a low chance of becoming dry thunderstorms. As of Friday, model runs show the odds of dry thunderstorms happening in the Northern California, Bay Area and Central Coast regions are less than 5%.

Should any dry thunderstorms develop, they may produce lightning that could ignite new wildfires. But if the dry air in Central California absorbs more moisture than what the models are suggesting, pockets of the Bay Area and the Sacramento Valley may end up with more beneficial rains from these isolated storms. If this event does produce any precipitation, the chances are greatest on the Central Coast and in the Diablo Range, Santa Cruz Mountains and the Sierra Nevada.

For now, we will monitor to see if any changes come up in this unique weather setup.

Weekend breakdown

• San Francisco: The sea breeze will continue to cool off the west side Friday and into the weekend, with the chances for patchy fog steadily increasing Saturday and Sunday. The sea breeze will raise wind gusts up to 20 mph each afternoon on the west side.

Cloud cover will increase Saturday and Sunday as remnants from Kay roll into the Bay Area. Look for partly to mostly cloudy skies, breezy winds, and temperatures in the mid 60s on the west side while east of Sutro Tower they’ll hover in the 70s.

• The Pacific Coast: Patchy fog will be back by Friday along Highway 1 between Half Moon Bay and Pescadero, with temperatures hovering in the mid-60s. Look for a slight chance of sprinkles on Sunday on the immediate coast.

Cloudy skies will cover much of the Peninsula, helping to shave off temperatures this weekend. Look for widespread upper 60s and low 70s in Daly City and neighboring areas, while 80s will be confined to the Highway 101 corridor south of Belmont.

• North Bay: For the first time in nearly 10 days, Napa, Sonoma and Petaluma valleys are forecast to see temperatures only in the upper 70s and low 80s Friday and through the weekend. Enjoy the heat relief, along with plenty of cloud cover by Saturday and Sunday to keep the sun away.

South of Novato, Tiburon and San Rafael will enjoy similar temperatures to the rest of the North Bay thanks to the return of the sea breeze.

• East Bay: The big cooldown is here with temperatures Friday through Sunday forecast to be in the upper 70s to low 80s every afternoon in Oakland and along the I-880 corridor. Clouds will steadily build this weekend, with partly sunny skies on tap for Saturday and Sunday.

Residents east of I-680, including those who live in Concord and Fairfield, are looking at the biggest heat relief out of everyone in the Bay Area. Mid-90s on Friday will make way for partly cloudy skies and daytime temperatures below 90 degrees for the first time in almost two weeks.

• South Bay and Santa Cruz: Santa Clara Valley cities such as Mountain View and San Jose will breathe a sigh of relief as temperatures on Friday get up only to the mid-90s. The cooling trend will continue for Saturday and Sunday as cloudier skies promote widespread 80s for much of the valley.

Over the hill, residents along Highway 17 will see 80s on Friday and upper 70s by Saturday and Sunday thanks to the added cloud cover. Low to mid-70s will dominate the Santa Cruz coast this weekend, with skies steadily becoming cloudier by Saturday and Sunday.

Gerry Diaz (he/they) is a San Francisco Chronicle newsroom meteorologist. Email:gerry.diaz@sfchronicle.com Twitter @geravitywave

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