Widespread storm creates single worst travel day of winter

Yet another storm swept across a large swath of the United States on Thursday, dumping heavy, wet snow that crippled travel options for millions of people. More than 6,500 flights had been canceled by midafternoon, making it the worst single travel day this winter, according to data from masFlight.

This current storm has so far caused more than 11,600 cancellations, compared with 9,700 during last week's weather incident from Feb. 2 through 6, said masFlight, a data company specializing in airline operations.

"I think this has a bigger effect because it's up and down the coast," said Michael Benjamin, CEO of FlightView, a flight tracking site.

By 3 p.m. EST, more than 6,500 flights were canceled and an additional 4,600 were delayed. Airports in Washington, D.C., Philadelphia, Atlanta, North Carolina, New York, Virginia, and Boston were among the locations with the most cancellations, according to FlightAware, a tracking service. (See the latest air travel status here.)

(Read more: Travel trend: 'Get me somewhere warm now')

Delta had canceled more than 700 flights, while Southwest had canceled nearly 600 and regional carrier ExpressJet nixed more than 500, according to FlightAware. US Airways and United also canceled a high percentage of their flights, according to data from FlightView.

"I think there will be a ripple effect the next couple of days," Benjamin said. "It's the reaccommodation of passengers that's going to be the tricky part."

The storm slammed 22 states, caused 468,000 power outages and dumped at least a foot of snow in some areas, including the nation's capital, according to NBC News.

Some airports completely closed their runways at some points of the day, including Washington Dulles International Airport and Teterboro Airport in New Jersey.

The roads and rails were heavily impacted as well.

Amtrak announced that due to the storm it was reducing service on its Acela Express, Northeast Regional, Keystone and Empire lines. Plus there were numerous cancellations for its trains from New York to New Orleans, Miami, Savannah, Ga., and other locations.

(Read more: Half a million without power after Southeast storm)

Both Greyhound and MegaBus reported widespread weather-related cancellations in places such as North Carolina, Virginia, Washington, D.C., Maryland, Pennsylvania, New Jersey, New York, Connecticut, Massachusetts, Vermont and Canada.

So far this winter, including December, 94,400 flights have been canceled, according to masFlight. "The cancellation rate for this winter-to-date has more than doubled since winter 2012/13 because of the sequential storms that have hit the U.S. since December 1," the company said in a statement Wednesday.

That translates into about 4.9 million passenger impacted by flight cancellations and 36.4 million by flight delays

Flight cancellations from Feb. 13, 2014 as of 7:30 a.m.

Airline	Airline Name	# Cancelled	Total	Percent
			Flights	Cancelled
US	US Airways	1591	3367	47.30%
DL	Delta	1328	5277	25.20%
UA	United	1192	5705	20.90%
AA	American	360	3734	9.60%
WN	Southwest	334	3201	10.40%
B6	JetBlue	302	847	35.70%
FL	AirTran	113	349	32.40%
AC	Air Canada	81	437	18.50%
ZK	Great Lakes	36	149	24.20%
VX	Virgin America	36	174	20.70%
NK	Spirit	19	280	6.80%
BA	British Airways	19	130	14.60%
PD	Porter Airlines	15	52	28.80%
WS	WestJet	7	137	5.10%
TA	TACA	4	39	10.30%
F9	Frontier	3	235	1.30%
LA	LAN	3	31	9.70%
VS	Virgin Atlantic	3	40	7.50%
SK	Scandinavian Airlines	2	18	11.10%
QF	Qantas Airways	2	15	13.30%
AV	Avianca	2	27	7.40%
AF	Air France	2	41	4.90%
AS	Alaska	2	884	0.20%
CX	Cathay Pacific	2	34	5.90%
KS	PenAir	2	28	7.10%
XL	LAN Ecuador	1	5	20%
3M	Silver Airways	1	109	0.90%
KE	Korean Air	1	52	1.90%
KL	KLM Royal Dutch Airlines	1	21	4.80%
LH	Lufthansa	1	62	1.60%
EK	Emirates	1	27	3.70%

Source: FlightView

The National Weather Service predicts the heavy snow in the Northeast will continue late Thursday and then taper off to sleet or rain in coastal New England by Friday morning as the storm continues to moves north.

(Read more: Airports ramping up downtown rail connections)

—By CNBC's Amy Langfield. Follow her on Twitter at @AmyLangfield.

Follow Road Warrior on Twitter at @CNBCtravel.