By Greg Porter, San Francisco Chronicle The Tribune Content Agency
A heat wave is set to hit California early next week, with the potential for temperatures to soar into the 90s or higher in San Francisco and along the Bay Area coastline.
A gradual weekend warmup
After a week of dramatic weather swings, temperatures will begin to slowly trend upward through the weekend. This trend in temperatures will intensify on Monday, marking the start of a multi-day heat wave that may stretch all the way to the coast.
A ridge of high pressure centered over the Four Corners has been creating record breaking temperatures in Phoenix this week. Over the weekend, this ridge will begin to extend westward into California, pushing temperatures into the 80s and 90s across the interior regions of the Bay Area. Closer to the water, a sea breeze will keep highs in the 70s and 80s along the Bay and highs in the 60s along the immediate coast.
It's clear that a high-pressure ridge will dominate California's weather by Monday but its exact position and strength will determine how hot it gets in San Francisco and along the coast.
The uncertainty around the intensity and duration of the heat is linked to a chaotic atmospheric pattern over North America. This same pattern is steering Hurricane Helene toward Florida and has stalled Hurricane John, causing heavy, catastrophic rainfall near Acapulco. Tropical systems are strong enough to alter the large-scale atmospheric flow and weather models are having a hard time resolving what the flow will look like next week, which has a direct impact on how the ridge of high pressure will develop over California.
Some weather models are suggesting that while this ridge builds from the east, another, stronger ridge of high pressure will develop over the Pacific Ocean. What's happening between these two high-pressure systems is further complicating the forecast.
It's unclear how an upper-level storm system swirling offshore, sandwiched between the two ridges, could play in to heat wave impacts. As of Thursday afternoon, it appears that this piece of atmospheric energy will help cap temperatures inland, maintaining a strong sea breeze and marine layer, keeping coastal locations cooler through the weekend.
The heat's full potential
By Monday, some weather models are suggesting that the two ridges of high pressure will merge into one large system. This would effectively smother the upper level storm system that's been separating them and developing in a more west to east direction.
Historically, when a high pressure system aligns itself this way, it promotes strong offshore winds that can shut down the sea breeze - especially in early autumn, when San Francisco typically experiences its warmest weather of the year.This kind of setup can cause temperatures to spike into the 90s, or even higher, in San Francisco. That scenario is definitely on the table for the city, especially on Tuesday and Wednesday.
If the ridge of high pressure develops in this way, the extended period of offshore winds will also raise the fire weather risk. Several days of prevailing offshore winds could keep relative humidity levels in the teens while temperatures are expected to rise by 15 to 30 degrees above normal. This coincides with an extended period of abnormally dry conditions currently in place across much of the state.
If the offshore winds are strong enough to shut down the sea breeze, the fire risk will spread all the way to the coastal range and Santa Cruz mountains, something the Northern California Geographic Coordination Center (NIFC) highlights in their extended fire weather forecast.
How long will the heat last?
The timeline is the most uncertain aspect of the forecast, as weather models show even more disagreement beyond Wednesday.
As of Thursday afternoon, two scenarios are possible. The more likely one involves an upper-level storm system moving down from Alaska, breaking up the high-pressure ridge and ending the heat wave by Wednesday. However, the other scenario keeps the high-pressure ridge in place, extending both the heat and the elevated fire risk through the end of next week and into the weekend.
Anthony Edwards contributed reporting. Reach Greg Porter: [email protected]