Heavy rain and a shortage of air traffic controllers brought travel chaos to San Francisco International Airport on Monday, delaying nearly half of all flights and frustrating thousands of travelers just as Salesforce’s massive Dreamforce conference got underway.

The Federal Aviation Administration issued an advisory for SFO, citing low cloud ceilings and other factors, that forced delays of more than an hour — and in some cases, up to 4 hours. 

The bottleneck was apparently worsened by reduced staffing linked to the ongoing federal government shutdown, which has strained operations across several major U.S. airports.

By day’s end, 434 flights were delayed and seven were canceled, according to FlightAware. 

Conditions improved Tuesday morning, with FlightAware reporting 30 delays and two cancellations. 

United Airlines, SFO’s largest carrier, recorded 142 delays, while Southwest Airlines reported schedule disruptions on two-thirds of its flights. Alaska Airlines delayed 31 flights and diverted several to San Jose Mineta International Airport.

“Unfortunately, Air Traffic Control Restrictions at San Francisco International Airport is causing delays and subsequent flight reroutes,” Alaska Airlines said in a letter to passengers. 

One Chronicle editor flying home from Palm Springs described being diverted mid-flight.

The airline offered ground transportation vouchers from San Jose to San Francisco, but noted it would not cover additional travel expenses. 

“We apologize for any inconvenience and are doing all we can to keep your travel on track,” the statement read.

The airport issues struck while San Francisco began welcoming Dreamforce attendees to the city, one of the largest tech conferences in the world  expected to draw nearly 50,000 people from more than 150 countries.

Meanwhile, torrential rain drenched much of the Bay Area, prompting flood advisories and warnings across the region. Public works crews hustled to clear drains blocked by leaves as localized flooding slowed commutes and inundated intersections.

The National Weather Service warned that low clouds and continued precipitation would likely extend delays into Tuesday. Early morning flight boards showed at least 15 departures already running late, with one cancellation before sunrise.

Despite the rough start, officials said they expect conditions to gradually improve later in the week as the storm system moves east. It was not clear Tuesday morning when the federal government would resume normal operations again.