Air travelers trying to get to and from the East Coast were mostly frustrated Thursday, as a winter storm moved its way up the Atlantic coast, slamming the area with snow and ice and forcing dozens of flight cancellations in the Bay Area.

By 4 p.m. Thursday, spokesman Doug Yakel at San Francisco International Airport said 115 flights had already been canceled, 68 of them arrivals. Airports in Washington D.C., New York City, Boston, and Philadelphia were primarily affected by the storm, which already had dumped 6 to 7 inches of snow in those areas, according to the National Weather Service.

According to Flightstats.com, more than 7,000 commercial flights were canceled nationwide on Thursday and 4,000 on Wednesday as the storm pummeled parts of the South from North Carolina through Georgia to Mississippi.

At SFO, ground delays went into effect at 9 a.m. because of visibility issues created by fog, and flights were delayed by an average of 30 minutes.

Yakel said the airport expects more cancellations and serious delays on Friday due to what he said is "the lap-over" effect.

"It depends on how quickly the weather system moves through the Northeast, and how quickly airlines can recover and get back on track," Yakel said. "But it is reasonable to expect at least some arrival cancellations Friday. If you don't send any flights to New York, you can't expect any to come back."

Yakel said travelers should call their airlines, who will notify them of any flight changes due to the weather and may already have plans in place to allow fliers to change their schedules free of charge.

The weather was having a more minimal impact on flights out of Oakland International Airport, which has far fewer direct flights to the East Coast. One flight departing to Milwaukee had been canceled, as were two other West Coast flights. The latter two were not necessarily due to weather, according to a duty officer in Oakland.

In San Jose, one Thursday night red-eye to and from John F. Kennedy International Airport was canceled, spokeswoman Rosemary Barnes said. No other flights were impacted by the storm.

The weather has been blamed for at least 12 deaths, according to The Associated Press.

Staff writer Erin Ivie contributed to this report. Contact Rick Hurd at 925-945-4789 and follow him at Twitter.com/3rdERH.