A severe weather system across several parts of the United States has prompted the three major airports servicing New York City to urge passengers to arrive early for flights on what is expected to be one of the busiest travel days of the year. As of Sunday night, 6,027 flights into, out of or within the United States have been delayed, according to FlightAware, and 171 US flights have been canceled, the website noted. A rather large storm system will shift from the Mississippi River Valley Sunday morning into the Northeast bringing with it widespread rain and gusty winds. The heavy showers and high winds are expected to impact millions of travelers from the Midwest to the Northeast and down to the Southeast Sunday. New York’s LaGuardia Airport has urged passengers to pre-book parking spaces, which operators said are filling up. “We can’t stress this enough — plan ahead and arrive early if you’re traveling during the #Thanksgiving holiday travel period,” the airport tweeted. Newark Liberty International Airport echoed LGA in suggesting passengers should plan ahead and aim to arrive at the airport early. It warned roadways into the airport were already congested Sunday morning. “The holiday season means high passenger volumes. If you’re planning to travel through Terminal A or B, take advantage of the FREE EWR VirtuaLine service and secure your spot in the security checkpoint line in advance,” the airport said on Twitter. “Due to the increased number of travelers during the holiday, allow extra time for travel, check-in and security.” John F. Kennedy Airport also said passengers traveling Sunday should plan ahead and arrive early. JFK tweeted from its verified account, “Budget extra travel time to arrive at #JFK Airport, park in your reserved spot, check in, and get through security.” – CNN’s Ramishah Maruf contributed to this report.