Arctic air hits New York State, along with some flooding.

As the first flakes of a predicted bomb cyclone snowstorm began to fall in New York State on Friday, officials sent out warnings for the region to brace for flooding, flash-freezing and Arctic air, cautioning that conditions would only get tougher as the day went on.

A state of emergency declared by Gov. Kathy Hochul took effect at 6 a.m. on Friday; it included bans on commercial vehicles on chunks of the New York State Thruway, as well as closures on interstate highways in Western New York.

By 9:30 a.m. with whiteout conditions widespread across the state’s east, officials announced a travel ban covering all of Erie County. Nearly 20,000 people were without power, according to Mark Poloncarz, the county executive. “Conditions have deteriorated rapidly,” Mr. Poloncarz wrote in a Twitter post. “The Blizzard is here.”

In Buffalo, a city predicted to be among the hardest hit by what the National Weather Service has called “a once in a generation” storm, morning wind gusts had pushed Lake Erie past flood levels. The waters were still expected to rise by another four feet, according to the Weather Service.

Still, as rain turned to snow on Friday morning, people in Buffalo still ventured out to purchase last-minute supplies and even to do some holiday shopping. Nick Ceglia, 49, who works at a store selling wine and liquor, pulled up at Dash’s Market in a minivan along Hertel Avenue for necessities. Mr. Ceglia said school classes were canceled for all three of his children on Friday, and his workplace was shuttered. He was frustrated.

“I think it’s overhyped,” he said of forecasters’ predictions for the storm. “I lost a shopping day.”

The storm is predicted to fall unevenly across the state, but the effects will be felt across the region: In New York City, where on Friday morning the high temperature was still 55 degrees, officials warned residents to brace for temperatures to drop nearly 40 degrees by Saturday morning. Floodwaters, raised by the storm as well as a new moon, were already rising in vulnerable shoreline communities, the city said, urging residents to “make an emergency plan now.”

In East Rockaway on Long Island and other parts of the South Shore, floodwaters had already submerged some streets, according to the Nassau County executive, Bruce Blakeman. By midmorning, the high tide had begun to dissipate he said. “However, the bad news is, in a few hours we are going to start experiencing the winter storm,” Mr. Blakeman said. “It’s going to be a long 24 hours.”