Thousands of US flights are delayed Sunday due to a major storm system

Winter storm hits the Northeast United States 1:36

(CNN) --

A severe weather system in various parts of the United States prompted the three major airports serving New York City to urge passengers to arrive early for their flights on Sunday, in what was expected It was one of the busiest days of the year.

As of Sunday night, 6,027 flights to, from or within the United States had been delayed, according to FlightAware, and 171 US flights had been cancelled, the website noted.

A fairly large storm system was forecast to move out of the Mississippi River valley Sunday morning toward the northeast, bringing widespread rain showers and gusty winds.

Heavy rain and strong winds were expected to affect millions of travelers from the Midwest to the Northeast and Southeast on Sunday.

New York LaGuardia Airport has urged passengers to reserve parking spaces in advance, which operators said were filling up.

“We cannot stress this enough – plan ahead and arrive early if traveling during the #Thanksgiving holiday travel period,” the airport tweeted.

Newark Liberty International Airport echoed this by suggesting that passengers should plan ahead and try to get to the airport early.

He warned that the roads to the airport were already congested on Sunday morning.

“The holiday season means high passenger volumes.

If you plan to travel through Terminal A or B, take advantage of the FREE EWR VirtuaLine service and secure your spot in the security checkpoint line early," the airport said on Twitter.

"Due to increased traveler numbers during the holidays, please allow additional time for travel, check-in, and security."

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John F. Kennedy Airport also said passengers traveling on Sunday should plan ahead and arrive early.

JFK tweeted from his verified account: "Budget extra travel time to get to #JFK airport, park in your reserved spot, check in and go through security."

– CNN's Ramishah Maruf contributed to this report.