A heat wave will hit California early next week, with temperatures likely to reach 90 degrees or higher in San Francisco and along the Bay Area coast.

After a week of dramatic weather fluctuations, temperatures will slowly rise over the weekend. This temperature trend will intensify on Monday, marking the start of a multi-day heat wave that may extend to the coast.

A ridge of high pressure over the Four Corners has brought record-breaking temperatures in Phoenix this week. Over the weekend, this ridge will extend west into California, pushing temperatures into the 80s and 90s in the interior of the Bay Area. Closer to the water, a sea breeze will bring highs in the 70s and 80s along the bay and highs in the 60s along the immediate shoreline.

Then the actual warm-up begins.

It’s clear that a ridge of high pressure will dominate the weather in California through Monday, but its exact location and strength will determine how hot it gets in San Francisco and along the coast.

The uncertainty about the intensity and duration of the heat is related to a chaotic atmospheric pattern over North America. The same pattern is steering Hurricane Helene toward Florida and stalled Hurricane John, resulting in heavy, catastrophic rainfall near Acapulco. Tropical systems are strong enough to alter large-scale atmospheric flow, and weather models are having a hard time determining what the flow will look like next week, which has a direct impact on how the high pressure ridge over California will develop.

Some weather models suggest that another, stronger ridge of high pressure will develop over the Pacific Ocean as this ridge forms from the east. What happens between these two high-pressure systems makes forecasting even more difficult.

It’s unclear how an upper-level storm system swirling offshore between the two ridges could contribute to the effects of the heat wave. As of Thursday afternoon, it appears that this bit of atmospheric energy will help limit inland temperatures, maintain strong sea breezes and marine layer, and keep coastal areas cooler through the weekend.

Some weather models suggest the two ridges of high pressure will merge into one large system by Monday. This would effectively suffocate the upper level storm system separating them and developing more west to east.

When a high-pressure system aligns this way in the past, it promotes strong offshore winds that can block sea breezes — especially in early fall, when San Francisco typically experiences the warmest weather of the year. This type of configuration can cause temperatures to climb into the 90s or even higher in San Francisco. This scenario is definitely on the table for the city, especially on Tuesday and Wednesday.

When the high pressure ridge develops in this manner, the extended period of offshore winds also increases the risk of fire weather. Several days of prevailing offshore winds could keep relative humidity in the teens, while temperatures are expected to rise 15 to 30 degrees above normal. This coincides with an extended period of unusually dry conditions currently occurring across much of the state.

If offshore winds are strong enough to suppress sea breezes, fire danger will extend to the Coastal Range and Santa Cruz Mountains, which the Northern California Geographic Coordination Center (NIFC) highlights in its enhanced fire weather forecast.

The timing is the most uncertain aspect of the forecast, with weather models showing even greater inconsistency beyond Wednesday.

As of Thursday afternoon, two scenarios are possible. The more likely scenario is that a storm system will move down from the upper reaches of Alaska, breaking up the high pressure ridge and ending the heat wave by Wednesday. The other scenario, however, keeps the ridge of high pressure in place, extending both the heat and increased fire danger through the end of next week and into the weekend.

Anthony Edwards contributed reporting. Reach Greg Porter: [email protected]