UPDATED FORECAST : Here's where the highest chances of thunderstorms are across the Bay Area
Tuesday's unusual late-season storm is making its rounds across Northern California and will introduce a slew of weather impacts , including strong winds, downpours and thunderstorms, along with the risk of small hail. Most of this severe weather will peak Tuesday morning and steadily drop off by the afternoon across most of the Bay Area, but lingering showers and thunderstorms are forecast to hang around through Wednesday afternoon — well after the worst of the storm has passed.
- Track live updates on the storm from the San Francisco Chronicle today.
Heavy rain and wind threat: Tuesday
The heaviest downpours are forecast to roll into Northern California on Tuesday morning, lasting until 1 p.m. in the Bay Area and 5 p.m. in the Sacramento Valley. Some downpours are forecast by the high-res rapid refresh weather model to introduce gusts up to 40 mph across most of the Bay Area and up to 55 mph on some of the hillsides in the North Bay highlands, East Bay hills and the west side of San Francisco. On Mount Tam, Mount Diablo, Mount Hamilton and Mount Umunhum, 70 mph gusts aren't out of the question.
These strong northwest winds will help crank up rainfall rates to half an inch per hour around San Francisco, Oakland and Santa Rosa between 9 a.m. and noon.
The rest of the Bay Area can expect rainfall rates around a third of an inch per hour. That doesn't account for the risk for thunderstorms, which would add heavy rain to the mix and boost rates closer to half an inch per hour in the Santa Cruz coastline — including Aptos, Santa Cruz and Capitola — if a lone storm or two come ashore over the course of the day.
By Tuesday evening, most of the wind and shower activity across Northern California will begin to fizzle, leaving behind quieter conditions as winds and rain peter out. But the storm won't be done just yet.
Thunderstorm threat continues: Wednesday
The storm's center will continue to travel south toward the open waters off the coast of Morro Bay on Wednesday, leaving traces of atmospheric energy in its wake. This energy, known scientifically as convective available potential energy, or CAPE, is a measure of how many joules per kilogram are in a column of the atmosphere — in other words, how much energy the atmosphere has available to convert into thunderstorms.
For thunderstorms to form over California's coastline and the Central Valley, the atmosphere typically must hold around 50 to 100 joules per kilogram of energy. The North American weather model, along with a couple other models like the European and Canadian, are trending toward there being more than enough energy in the atmosphere for thunderstorms to surface off the coast and in the Central Valley.
But just how many of these thunderstorms could evolve into supercells, or rotating thunderstorms, capable of producing powerful gusts over 35 mph, small hail and an additional inch of rain? Currently, the high-res rapid refresh and North American weather model are suggesting that a fair amount of these types of thunderstorms will be possible.It looks like the greatest chances on Wednesday are slated for parts of the coastline along Highway 1 between Mendocino and Santa Cruz counties, along with most of the Central Valley from Chico to Fresno.
There is also the possibility of weak, short-lived tornadoes that could run up to the coastline or form in the Sacramento Valley. For now, these chances for severe thunderstorms and their subsequent weather features remain low to moderate.
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San Francisco: A stormy morning is on tap for much of the city as gusts climb above 35 mph through noon. Some of the windiest spots will be along the outer Sunset and Richmond districts, where gusts will briefly surpass 50 mph. These winds could lead to power outages across the city that recently dealt with outages in previous storms. Winds will also draw downpours, with rainfall rates around a third of an inch per hour throughout the morning. A few areas around Twin Peaks, Mount Davidson and Golden Gate Park could reach rainfall rates up to half an inch per hour before showers taper off this afternoon.
Light rain and weaker winds are on tap after 2 p.m. as the bulk of the storm's rainfall exits the city. The cold air left behind will keep daytime temperatures in the lower 50s while nighttime temperatures quickly fall to the upper 40s in the Mission, SoMa and downtown areas overnight. The rest of the city can expect nighttime temperatures closer to the mid-40s as skies clear overnight.
Pacific Coast and Peninsula: A slew of unsettled weather conditions will make for dangerous driving conditions on Highway 1 between Pacifica and Half Moon Bay, with downpours, brief thunderstorms and gusts most of the morning. Rainfall rates will range between a quarter to a half an inch of rain per hour, while winds will gust above 45 mph — and up to 60 mph on Highway 92 between Half Moon Bay and San Mateo.
These gusts will spread east over the morning and into the San Bruno Gap, raising 45 to 60 mph gusts in Daly City, San Francisco International Airport and Millbrae. Residents in the rest of the Peninsula along I-280 and 101 will experience fewer impacts thanks to the rain-shadow effect off the hillsides of San Mateo County, but can expect occasional downpours and a couple of thunderstorms.
Conditions will gradually improve by the afternoon as showers and winds taper off, but brief thunderstorms capable of downpours, strong winds and even isolated tornadoes are expected to last though 6 p.m. Cloud cover overhead will help keep daytime highs in the lower 50s, while nighttime temperatures are forecast to fall to the mid-40s as showers clear out and skies become clearer.
North Bay: Some of the heaviest rainfall is forecast for residents downwind of the Mayacamas Mountains, including Calistoga, Santa Rosa, St. Helena, Windsor and Guerneville. Rainfall rates around a half an inch per hour are expected in the morning, with some of the heaviest rain between 9 a.m. and 11 a.m. The heavy rainfall will be carried in by powerful winds gusting to 60 mph in the highlands of Napa and Sonoma counties. On Mount Tam winds could gust up to 70 mph at the summit. Look for widespread 35 to 55 mph gusts and rainfall rates around a third of an inch per hour across most of Solano, Napa, Sonoma and Marin counties through 1 p.m.
Weather conditions will improve this afternoon as winds and rain ramp down, but isolated thunderstorms will be possible along the Sonoma and Marin county coastline, including Point Reyes and Bodega Bay. Brief heavy rain, small hail and even a couple of funnel clouds can't be ruled out for the immediate coastline.
Gloomy skies overhead will help keep daytime highs in the Santa Rosa, Napa and Petaluma valleys closer to 50 degrees, while residents closer to San Pablo Bay, the delta and the Pacific coast can expect slightly warmer temperatures in the lower 50s. Cold air from the storm's cold front will settle in the region and drop nighttime temperatures to the upper 30s inland and lower 40s by the water. Watch for overnight frost and any icing on remote roadways off 101.
East Bay: A windy and rainy morning is on tap for Alameda and Contra Costa counties, with 35 to 55 mph gusts for residents in Richmond, Albany, Berkeley, Oakland, Fremont and Hayward. Brief swells could raise coastal flooding concerns in Alameda and along the bayshore as morning winds raise rough waters on San Francisco Bay. These winds will be accompanied by heavy rainfall, with rates up to a third of an inch per hour possible along the I-80 and I-880 corridors, along with the delta region that includes Concord and Pittsburg. The bulk of these downpours will also spread east of the Caldecott Tunnel over the morning, raising heavy rainfall and gusts in the San Ramon and Livermore Valleys. Walnut Creek, Pleasanton and Dublin will bear some of the most serious impacts this morning as showers spill into the valleys and briefly reach rainfall rates of half an inch per hour.
Conditions will gradually improve after 1 p.m. as winds ramp down and showers fizzle, but there will be a slight risk for thunderstorms throughout the afternoon. Chances remain generally low at around 20% to 25% across the East Bay. Thunderstorm risks include small hail, damaging gusts over 35 mph and even a couple brief spin-ups.
Temperatures will generally peak in the lower 50s across the wider East Bay before dropping to the lower 40s overnight.
South Bay and Santa Cruz Mountains: The stage is set for an active weather day on the Santa Cruz coastline. Gusts to 55 mph will be possible along the waters of Capitola, Aptos, Soquel and the city of Santa Cruz this morning. These winds will carry heavy rainfall toward the shoreline, raising rainfall rates to a half an inch by midmorning. The rainfall and winds will peak around 11 a.m., spreading to the rest of Santa Cruz and Santa Clara counties before the storm's bands fizzle away this afternoon.
That said, swollen rivers and infrastructure damage from this morning's winds and rain are likely to delay flood and damage relief operations from last week's storm along Highway 17. There may be significant travel delays if gusts cause more widespread damage in the Santa Cruz Mountains.
By Tuesday afternoon, the storm's impacts will be more isolated. The forecast is calling for thunderstorms with gusts possibly exceeding 35 mph, small hail, brief downpours up to half an inch per hour and brief spin-ups. The areas with the highest chances for afternoon thunderstorms are the Santa Cruz coastline and the 101 corridor between Gilroy and Morgan Hill. Clouds and cold air will keep daytime temperatures in the lower 50s. By nightfall, thunderstorm chances will fizzle and temperatures will drop to the lower 40s by the water and upper 30s in the Santa Clara Valley and the Santa Cruz Mountains.
Reach Gerry Díaz: [email protected]; Twitter: @geravitywave
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