More than 23 million travelers are expected fly for Thanksgiving.  Millions more are hitting the roads.

Story highlights

Thanksgiving travelers face two major storms -- in the Northeast and the Pacific Northwest

AAA: 42.5 million Americans will travel at least 50 miles during the Thanksgiving weekend Los Angeles, Chicago's O'Hare and Orlando airports will be the busiest

All eyes are on the weather, which can turn a simple trip into a nightmare

CNN  — 

Millions of travelers headed home on one of the busiest days of the year Sunday, many of them fighting crowds but avoiding what had been feared could be even worse delays caused by treacherous weather.

Two major storms in the Northeast and the Pacific Northwest are complicating matters on one of the busiest travel days of the year.

At the same time, the Department of Homeland Security and the FBI have issued a joint intelligence bulletin saying terrorists could choose to strike during the holidays and everyone should stay vigilant.

CNN has obtained a copy of the bulletin which says law enforcement is “not aware of any credible threats to the Homeland specifically timed to coincide with the 2011 holiday season.”

But the bulletin notes last year’s Oregon plot to set off a bomb at a Christmas tree lighting. That one allegedly involved a homegrown suspect. And the bulletin says that as of February 2010, al Qaeda was considering attacks on the United States on symbolic dates such as Christmas, but there’s no information to indicate that led to actual al Qaeda plots.

These sorts of bulletins have come out at holiday time in past years. But this is the first once since the death of Osama bin Laden.

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Meanwhile, rain has prompted flood watches in parts of Pennsylvania; up to 8 inches of snow have fallen in central parts of Maine; showers are soaking northern Florida; and wind gusts of up to 98 mph have been reported along the Oregon coast, CNN meteorologist Jacqui Jeras said.

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“It’s going to be a rough go for a whole lot of people,” she said.

Flight delays were spreading Wednesday afternoon, with Newark International, LaGuardia, San Francisco International and Boston’s Logan International airports reporting problems.

Check Federal Aviation Administration flight delay information

That’s bad news for those who are on the move for the annual November trek to see family and friends, feast on turkey and pumpkin pie, and rediscover the joys of their hometowns on Thanksgiving.

AAA projects that 42.5 million Americans will travel at least 50 miles during the Thanksgiving weekend, an increase of 4% from last year.

Slightly fewer people are choosing to get to their destinations by plane.

About 23.2 million travelers will fly over a 12-day period surrounding Turkey Day, a 2% drop from last year, according to a forecast by the Air Transport Association of America.

Unlike last year, when the National Opt-Out Day movement against airport body scanners threatened to snarl security lines across the country, air travelers are not facing the prospect of protest-induced delays. (The Opt-Out Day turned out to be a nonevent.)

The Transportation Security Administration says it has prepared its work force for a “smooth holiday travel experience for travelers.”

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The busiest air travel days for the Thanksgiving holiday period are expected to be Sunday and Monday, the Air Transport Association of America said.

If you’re flying into or out of Los Angeles International, Chicago’s O’Hare International or Orlando International, brace yourself for lots of company. Those will be the nation’s busiest airports this Thanksgiving, based on flight bookings, according to

(And in case you’re wondering, Mineta San Jose International in California and Kahului Airport in Maui, Hawaii, will be the least busy.)

No matter where you start your journey, flight attendants say it’s a week when they see lots of extra-grumpy passengers worried about flight delays and cancellations.

“There’s a little bit more at stake. Oftentimes, people are trying to get someplace to be with their family, so naturally they would be upset if they’re not going to make it,” said Rene Foss, a longtime flight attendant for a major U.S. airline.

“However, in general, it’s also kind of a festive time. Sometimes, contrary to what you might think, people are in a good mood because they are going for something kind of fun as opposed to just a business meeting or something related to work.”

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Flying with food or gifts? Check out the TSA’s guide to what you can and cannot bring through an airport security checkpoint. The agency reminds you not to wrap gifts you are taking on the plane because security officers may have to unwrap them if they need to take a closer look.

The TSA also offers tips on how to get through the security line faster, including packing coats and jackets in checked bags whenever possible and putting your shoes directly on the conveyor belt instead of a bin when they go through the X-ray machine.

Then, there are things you can’t control: Snow, fog or rain may mean you won’t fly on time or at all.

To avoid being stuck at the airport, sign up for airline alerts and check your flights frequently online before you leave home. If your flight is canceled, get in line for assistance and try your airline by phone or online at the same time to get an edge over other fliers who are trying to rebook.

Winter weather travel tips

If you’re driving through an area that’s expecting wintry weather, AAA recommends that you keep at least half a tank of gas in your car at all times and pack a cell phone, plus blankets, gloves, hats, food, water and any needed medication in case you’re stranded.

Stay safe out there, and happy travels.

CNN’s Michael Martinez, Zohreen Adamjee and Carol Cratty contributed to this report.