Bay Area to see heat wave after major forecast shift

In a major forecast twist, a three-day heat wave is now expected to hit the Bay Area from Tuesday through Thursday, triggering a heat advisory for the region beginning at 10 a.m. Wednesday and lasting through 10 p.m. Thursday. San Francisco, Oakland and San Jose could see their hottest temperatures so far this year, with the National Weather Service forecasting temperatures 10 to 20 degrees higher than normal in some places. Exceptionally warm nights will make the heat wave much more extreme across the entire region.

Earlier forecasts indicated remnants of Hurricane Marie would keep temperatures cool and bring Northern California a chance of rain, but that changed. Marie’s lingering circulation is now expected to be well off the coast, far enough away to disrupt the typical cooling sea breeze in San Francisco but close enough to send tropical clouds streaming over the Bay Area.

That combination of light winds and high-altitude clouds will combine for hot days and warm nights across the entire region. The National Weather Service said a heat advisory will probably be needed Wednesday and Thursday.

Here’s the forecast for this week:

Temperatures are expected to soar Tuesday due to a jumbled atmospheric pattern.

High pressure will build over the surface of northern Nevada as a cool, dense air mass lingers from the weekend chill. At the same time, Hurricane Marie’s remnant low pressure will pull air toward it and temporarily halt the northerly winds that dominate along the California coast during summer and early fall.

This combination of high pressure inland and low pressure off the coast will slow or completely shut down the cooling ocean breeze across San Francisco Bay. Without this natural air conditioning, downtown San Francisco should warm well into the 80s, San Jose should reach 90, and mid-90s will prevail in the interior East Bay and North Bay.

Tropical clouds, associated with the hurricane’s lingering high-altitude winds, will move over the Bay Area on Tuesday night. These clouds will provide a blanket overnight and cap lows in the 60s to low 70s across the region.

While the clouds will provide insulation Tuesday night, they are expected to clear out by midday Wednesday and strip the Bay Area of shade during peak afternoon heat.

Under abundant sunshine and little to no sea breeze, Wednesday has a chance to be the hottest day so far this year in San Francisco, Oakland and San Jose. Those numbers to beat are 90, 93 and 98, respectively. Highs Wednesday are expected to be right around those marks in each city.

Triple-digit heat is possible anywhere away from the immediate coast. Even the Highway 101 and Interstate 280 corridor along the Peninsula could see upper 90s to low 100s.

Wednesday night will be brutally warm for people without air conditioning. Moisture lingering high in the atmosphere will provide a greenhouse effect and trap heat at night, even without clouds in the area.

Low temperatures may not drop below 70 degrees in the interior valleys of the East Bay and South Bay. A very weak sea breeze may briefly drop temperatures into the 60s in downtown San Francisco and Oakland in the late evening, but heat radiating off of buildings and roadways will warm the cities back up once the winds die down after sunset.

San Francisco’s Twin Peaks, Diamond Heights, Golden Gate Heights, the Oakland and Berkeley hills and other high-elevation neighborhoods away from the water will probably remain in the mid- to upper 70s overnight.

Whatever is left of former Hurricane Marie will probably be too far away from California to significantly affect Thursday’s weather.

Northwest winds are expected to resume in the afternoon, but not before another hot morning in San Francisco and Oakland, where temperatures may reach 80 before noon. The breeze will be delayed in the inland valleys. San Jose will likely still get well into the 90s, and triple-digits are again possible in parts of the North Bay and East Bay.

Natural air conditioning will resume for the entire region on Thursday evening. Low temperatures are forecast to drop to the upper 50s to mid-60s for most of the Bay Area. Elevations above 1,000 feet may remain in the 70s one more night, but a deepening marine layer through the weekend should cool the mountains down, too.

High temperatures are expected to return close to September averages by the weekend, in the 60s at the coast, 70s along the bay shoreline and 80s inland.