Millions of Americans who had to navigate a bomb cyclone and a Midwest windstorm on the way to Thanksgiving celebrations will encounter more inclement weather when returning home Sunday and Monday as a major winter storm takes shape in the Northeast and a new storm hits the San Francisco Bay area.

In the Northeast, heavy snow, mixed precipitation and strong winds are expected to develop in many areas beginning as early as Sunday. Freezing rain was already falling in parts of Pennsylvania on Sunday, making roads hazardous, and the stage is set for a burdensome Monday morning commute for many from New York to Portland, Maine.

Flights were disrupted Sunday afternoon as a cluster of airports in the Northeast experienced average delays of at least an hour because of wind, snow or mixed precipitation, according to the Federal Aviation Administration. The biggest delays were reported at Newark International Airport in New Jersey, where the typical arrival was slowed by about 2 1/2 hours.

The delays were even longer across the country at San Francisco International Airport, where high winds caused arrivals and departures to stall by more than four hours on average. The Bay Area is under a high wind warning through the early hours of Monday.

Several major airlines, including Delta and American Airlines, were waiving flight-change fees for passengers likely to be stranded by the storms.

Flight disruptions are likely to continue in New England on Monday into early Tuesday, especially from Philadelphia northward to Boston, where a mix of rain and snow, along with strong winds, is anticipated.

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Aside from the East Coast storm, more than five inches of snow had fallen Sunday by afternoon in Minneapolis. Minneapolis-St. Paul International Airport is a major hub for Delta Air Lines.

Road travel in northeastern Minnesota was not advised, because of a combination of light snowfall and high winds, which was creating blizzard conditions near Duluth.

While motorists on the bulk of roadways in the rest of the Midwest were traveling at or near normal speeds as of Sunday morning, there were several slowdowns in northern Michigan, including along Interstate 75. Heavy snow was just exiting the region after dropping more than half a foot.

Delays mounted along the West Coast, as the area was hit by a significant storm system known as an “atmospheric river” because of the amount of water vapor it contains.

That storm caused high winds of up to 50 mph throughout the Bay Area, slowing air traffic into airports in San Francisco, Oakland and San Jose. Rain has fallen intermittently in San Francisco, but the heaviest showers are concentrated south, near Monterey. The storm is also dumping heavy snow in the northern Sierras.

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As of Sunday evening, the system that likely will evolve into a major storm in the Northeast was centered over Cleveland, with a secondary low pressure system taking shape along the Mid-Atlantic coast.

The storm’s expansive precipitation shield extended like an outstretched arm well to the east, with heavy rains and downpours reaching as far south as Augusta, Georgia, and northward into New Hampshire. That slug of moisture – which slowed traffic on Interstate 95 along much of the Eastern Seaboard – wrapped all the way up through Washington; Baltimore; Philadelphia; Buffalo, New York; and into Detroit, before curling northwest as snow in northern Wisconsin and Michigan.

The precipitation initially fell as sleet and freezing rain into metro Philadelphia, New York City and Hartford, Connecticut, on Sunday afternoon.

From Sunday into Monday, the storm will transfer its energy to another system Monday, spreading heavy precipitation up the New England coast and wrapping colder air around the storm.

One effect of the storm is that air travelers who endured extremely bumpy flights Wednesday are also having to deal with moderate to at times severe turbulence Sunday, and are likely to do so on Monday as well, as the low-pressure area transfers its energy to a coastal storm.

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Wintry weather was settling over the Appalachians on Sunday morning behind the storm’s cold front, where winter storm warnings peppered the high terrain of eastern Tennessee and western North Carolina.

For parts of Pennsylvania and West Virginia, though, the story was one of freezing rain – enough to create a treacherous glaze that could coat outdoor surfaces. With up to a quarter-inch of icing expected, the National Weather Service warned of “nearly impossible” travel conditions and sporadic power outages. The bulk of it was forecast to wrap up for central and western Pennsylvania by midafternoon, though road travel could remain difficult to impossible for hours afterward.

As the storm evolves, it will bring snow to a small part of Pennsylvania, specifically the northeast, beginning late Sunday and extending through Monday and possibly into early Tuesday. There, amounts could build to as much as nine or 10 inches as one nears the New York border.

Snowfall totals of a foot-plus are possible in eastern New York state east of Interstate 81 by the time the storm ends. Pockets of up to 18 inches may punctuate some spots in the Catskills and Hudson Valley.

Travelers getting a later start on their post-Thanksgiving journey are likely to encounter more severe weather in the Mid-Atlantic and Northeast as the storm slowly congeals, particularly on Monday.

Anyone traveling there Monday morning to beat the Sunday rush is likely to encounter more problematic conditions as precipitation gets heavier and winds increase along the coast. As cold air works into the storm’s western flank, snow will creep closer to parts of metro Philadelphia, New York City and Boston by Monday night.

While forecast uncertainty is high for the major cities themselves, areas just west of the I-95 corridor could see from several inches to over a foot of snow, particularly in metro Boston, where precipitation will be heaviest and last the longest.

All precipitation should wind down west to east Tuesday morning, clearing all but Maine by early afternoon Tuesday. Regions east of I-95 in Maine could see four to six inches, but more is possible if the rain-snow line edges further eastward.

On Sunday evening, the National Weather Service increased the snowfall forecast for the Philadelphia; New York City; Providence, Rhode Island; and Boston areas, noting that final tallies may be higher depending on the storm’s exact track and intensity. A snowfall gradient of 10 to 15 inches could be found in just a few miles from east to west across the big cities, based on forecasts issued Sunday.

In interior Massachusetts, more than a foot is likely in at least two areas – one just west of the Blackstone Valley from Worcester to Fitchburg, with another along the east slopes of the Berkshires. The Weather Service is calling for as much as 14 to 16 inches there. A foot of snow may even fall closer to Boston, near Route 128, making the morning commute difficult there to some extent on Monday and into Tuesday.

Heavy snow is also forecast to extend into central and southern Vermont and New Hampshire. The Hudson Valley of New York and portions of northeastern New Jersey could pick up at least several inches of snow before the storm exits on Monday, as colder air filters into the region.

Winds at the height of the storm will also gust near 40 mph near the coast, with less forceful breezes anticipated inland.