More than 4,500 flights are delayed or canceled across the US on Sunday evening as California continues to brace for the impact from Tropical Storm Hilary. Many of the flights originated from or were headed to destinations in California or nearby states.

According to flight tracker FlightAware, some of the most affected airports are Harry Reid International in Las Vegas, where more than 300 flights have been canceled, and San Diego International, where some 40% of arriving and departing flights have been canceled.

About a third of flights coming in and out of Ontario International airport outside of Los Angeles have also been canceled, as more than 9 million of the city's residents are under flash flood warnings.

In California, airports in Burbank, Santa Ana, Sacramento have had to cancel more than 20% of their departing flights, with Daugherty Field airport in Long Beach canceling at least 42% of their arrivals.

There are also some delays at international airports in Denver and Phoenix, as the ripple effect from the storm spreads.

The storm has not yet made landfall in the US, but it slammed Mexico's Baja California region earlier Sunday. As conditions worsen, Governors Gavin Newsom of California and Joe Lombardo of Nevada have issued states of emergency.

Hilary is likely to bring severe flooding to parts of California and Nevada, which could experience a year's worth of rain on Sunday.