With the Thanksgiving journey well underway, many have reached their destinations – but for everyone else, a storm stretching from the Rockies to the Midwest and Northeast will bring rain and snow through Thursday, which is likely to impact flights and crowded streets.

Nearly three million people were expected to be screened by the Transportation Security Administration on Wednesday, and a record 71.7 million were expected to drive over Thanksgiving – over 1 million more than last year.

The storm, which dropped snow across the Rocky Mountains Wednesday morning and tapered off in the afternoon, is expected to move east into the Midwest and Northeast throughout Wednesday through Thanksgiving.

It will bring rain with the possibility of ice and snow increasing this evening from St. Louis to Indianapolis and into Pittsburgh.

Overnight, the storm will continue to move east, bringing cold rain along the I-95 corridor from Richmond to Boston during the early hours of Thanksgiving morning.

According to FlightAware, there were nearly 4,000 delays and 45 canceled flights within, to or from the United States as of Wednesday evening.

American Airlines said it operated nearly 6,400 mainline and regional flights on Tuesday and expected more than 650,000 customers to travel on nearly 6,400 flights on Wednesday – that’s nearly 4.5 American flights departing every minute of the day.

Due to a shortage of air traffic controllers, there was a ground delay at Newark Liberty International Airport in New Jersey from 1:00 p.m. ET on Wednesday until 2:00 a.m. Thursday.

In total, there were more than 300 delays from Newark on Wednesday, some lasting at least 95 minutes.

The FAA said Wednesday afternoon that delays were expected in Denver and Salt Lake City due to snow and ice.

In those cities, airport officials were busy de-icing planes to prepare them for departure. Denver led the way with 600 flight delays on Wednesday.

Delays were also common in Boston, Dallas and Las Vegas, where delays were in the triple digits, according to FlightAware.

Fog in Tampa impacted departures Wednesday, with Tampa International Airport reporting 37 delayed departures, according to FlightAware. Delays were also an issue in Seattle and Los Angeles, where low ceilings were expected to have some impact on operations, the FAA said earlier Wednesday.

For road travelers, I-80, I-64 and I-75 were affected by rain Wednesday, and I-25 and I-70 in the Rocky Mountains were affected by snow.

Thanksgiving will bring heavy rain for the I-95 corridor from Florida to Maine and heavy snow for the interior Northeast and New England.

Points north of the Hudson Valley in northern New York State and inland New England can expect 1 to 3 inches of wet snow, with local totals exceeding 6 inches on the highest mountain peaks.

From northeastern Pennsylvania to New Hampshire, 3 to 6 inches of snow – and possibly more – are possible, with windy conditions potentially leading to power outages in the region.

The iconic Macy’s Thanksgiving Day Parade will take place rain or shine, as the Big Apple is forecast to see pouring rain with temperatures in the high 40s. Wind speeds of less than 10 miles per hour are forecast, allowing balloons to fly. The parade’s popular balloons cannot fly when maximum sustained winds reach 23 miles per hour or more or when gusts reach more than 34 miles per hour.

Cities on the East Coast, including Baltimore, Washington, DC, New York, Hartford and Boston, can expect cold rain, with rainfall amounts ranging from 0.50 to 1 inch. Traffic will be smooth as far west as Virginia and Maryland, including along the I-95 corridor into Maine.

Travel conditions will largely return to normal during the overnight hours into Friday morning as the system moves out of the New England region.

However, cold winds will move into New England and a lake effect snow event around the Great Lakes and continue through Sunday.

Airport hubs to keep an eye on Thursday include Atlanta, Charlotte, Washington, DC, Philadelphia, NYC, Boston, Pittsburgh and Buffalo.

On Sunday – typically the busiest travel day of Thanksgiving week as people return home from holiday trips – the cold will continue across much of the country, as will lake-effect snow around the Great Lakes and the Northeast.

A total of 3 million people are estimated to pass through TSA on Sunday, closing in on the record of 3.01 million set the Sunday after the Fourth of July holiday this year.

Chicago and Detroit could experience some problems at their airports on Sunday, but the East and West Coasts appear to be favorable for road and air travel.

Meanwhile, much of the Northern Plains and Upper Midwest will be bitterly cold by the end of the week, with temperatures 10 to 20 degrees below average beginning on Thanksgiving.

Thursday through Sunday will see highs in the 30s and 20s and lows in the teens in Chicago, highs in the 40s and lows falling to around 28°F in New York, and highs in the 50s and 40s in DC -Range and lows in the 30s and high 20s. Minneapolis will see lows in the single digits over the weekend.

As travelers take to the skies to accompany their loved ones, authorities are warning people who park their cars in airport parking lots to be careful about vehicle theft.

This year, more than 300 cars have been stolen from the nation’s busiest airport, Hartsfield-Jackson Atlanta International – nearly three times as many as last year, Atlanta police said.

“Suspects can program key fobs onto vehicles, and that’s what got us to where we are now,” said Maj. Kelley Collier, the Atlanta airport district commander. This year, police are introducing cameras, motorcycle patrols and new fences to curb crime in the 30,000 parking spaces.

More than 50 vehicles were also stolen from Dallas Fort Worth International Airport this year in what police say was a car theft ring that targeted airports in several states. DFW Airport police arrested the suspected ringleader last month.

A woman named Katy told NBC News that she and her husband were on a business trip and returned to Columbus International Airport to find their car was missing.

“Complete disbelief that we had parked at an airport and came out and our car was gone. We just assumed it was safer in that environment,” she said. Police later found it abandoned and completely stripped.

“I hope the airports increase their security so we can travel and come back and have your car there,” she added.

This article was originally published on NBCNews.com