The last time it was this dry in January, there were barely more than 500 people in San Diego. The year was 1850, and only a quarter-inch of rain had fallen in six months. It made the region ripe for wildfire — but that wasn’t much of a concern. The city’s population was clustered along the coast, far from the backcountry, where most fires burned. Today, San Diego is the seat of a county that’s home to 3.3 million people spread over an area about the size of Connecticut, and many of them are nervous. Unusually persistent Santa Ana winds are stoking wildfires in a region that’s gotten about 0.16 inches of rain since July 1 — significantly less than has fallen in parts of the Mojave Desert. People want to know why this is happening, and when it will end. And their questions have intensified over the past three days. On Monday, firefighters had to knock down a blaze in Poway. On Tuesday, they did the same in the Fallbrook and Bonsall areas. On Wednesday, a blaze erupted in Rancho Bernardo, not far from the National Weather Service’s offices. More fires could erupt Thursday, when the Santa Anas again roar to life, forecasters said. Wednesday’s fire came at an ironic moment: Exactly one year ago, a super-powerful rain cell stalled over southeastern San Diego, unleashing heavy precipitation that helped lead to flooding that damaged more than 1,000 homes and businesses. The wild weather happening now is on some levels well-understood but on others confusing and mysterious. San Diego County is experiencing historic dryness because the northern jet stream hasn’t been guiding winter storms into Southern California as it did the previous two years. The path of the jet stream is known to vary. But scientists don’t know how to accurately predict what it will do during the winter months. Even near-term forecasts can be dicey. Forecasters say a weak extension of the jet stream could produce one-half to 1 inch of rain in the mountains and valleys this weekend and about 0.25 inch at and near the coast. But it’s not a guarantee. They’ve learned not to guess about such things. In 2015, NOAA said the jet stream would produce a wet winter in Southern California. The opposite happened. Some scientists say the dryness might also be tied to some degree to La Niña, a periodic and natural climate change that sometimes leads to mild winters on the West Coast. But researchers have yet to prove their hunch about current conditions. In the meantime, Santa Ana winds have caused the region’s vegetation to further wither. Their presence is no surprise; the Santa Anas usually blow, on and off, from October through January, and sometimes into February. And while the current series of windstorms is unusual, it’s not unprecedented. About 30 wildfires hit Southern California during a roughly month-long period beginning on Oct. 20, 2007 — among them the Witch Creek fire near Julian, which destroyed more than 1,100 homes and buildings and forced more than 500,000 people to evacuate. Scientists cannot accurately forecast whether a Santa Ana wind season will be mild or severe. But they’ve made some interesting observations about what might happen in the future. In 2019, for instance, researchers at UC San Diego’s Scripps Institution of Oceanography said it appeared the winds would become less common by the end of this century due to the effects of climate change. Further insights are expected in the coming months — partly due to the efforts of people like Julie Dinasquet, a marine biologist at Scripps. On Wednesday, she was continuing to help guide the collection of water samples off Los Angeles County, where ash from the recent wildfires fell into the ocean. In a bit of serendipity, NOAA had a research ship offshore when the Palisades and Eaton fires erupted. “Our first job is to characterize the ash,” Dinasquet said Wednesday. “What are the potential toxicants in there? What are the potential nutrients? We’ll build up the story. It will take time to understand if there is a cascading effect on the food web in the ocean.”