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(CNN) -- When she was very young, Kayla Avery was used to playing in the snow. But the former Boston resident hadn't seen much of the white stuff since she moved to Arizona more than a decade ago.
She got a reminder Wednesday.
"We received a blizzard warning last night through the emergency broadcasting system. Most of us didn't believe it," she told CNN iReport. "Most of the news reports said it probably wouldn't snow in the valley, and two hours later it was snowing."
Avery, a 19-year-old student at the University of Arizona, said the snow was still falling Wednesday afternoon.
"My brother and I were staring out the window saying, 'What? It's snowing in Tucson? That's not supposed to happen!' " she said, adding that she's never seen this much snow in the area since her family moved there. Usually the area gets no more than a dusting of snow once a year.
The inch of snow also caused PGA Tour officials to postpone play at the World Golf Championships-Accenture Match Play Championship in nearby Marana, Arizona.
The rare accumulation was all part of a massive winter storm that promises to blanket a huge swath of the nation in crazy weather, from 18 inches of snow or more in Kansas to thunderstorms and flooding along the Gulf Coast.
Trouble for the next three days
In all, about 800,000 square miles in 18 states were under some form of watch or warning Wednesday related to a major storm pouring out of California and into the Central Plains and the Midwest, CNN meteorologist Brandon Miller said.
About 30 million people live in the covered areas, he said.
One of the places that could be hit the hardest is Springfield, Missouri. Among the hazards expected there Wednesday: lightning, wind and ice, with a little snow and sleet thrown on top for good measure.
The biggest threat of heavy snow lies in parts of Kansas, Nebraska and Missouri, with the possibility of whiteout conditions in some places, he said.
In Kansas, where the storm had already started, the National Weather Service was predicting as much as 18 inches of snow. The agency said parts of Nebraska could see more than a foot of snow, with drifts making some roads impassable.
CNN affiliate KSNW in Wichita reported that crews had treated roads Monday but no matter how much salt and sand they had spread, by Wednesday morning, primary roads were snow-packed and slick. Side streets were worse.
There was a break in the weather that allowed crews to get out before the afternoon rush hour and clear some of the major thoroughfares, the station reported.
Kansas State University announced on its website that Thursday classes were canceled.
Forecasters were predicting as much as 2 inches of snow in St. Louis, with the precipitation turning into sleet and freezing rain Thursday. The conditions will have "a significant impact on travel," the weather service warned.
North of where the most snow will fall, Chicago could receive what would be its largest single snowfall of the year -- 4 inches, Miller said. The city usually gets up to 4 feet of snow a year. So far this year, only about a foot has fallen, he said.
South of the snow belt, dangerously icy conditions are possible in parts of northern Arkansas and southern Missouri, he said. That includes Springfield, where windy conditions will precede the possibility of up to 3 inches of snow and sleet Thursday, with the possibility of a quarter of an inch or more of ice to boot.
Heavy rains forecast
Elsewhere, severe thunderstorms moving in from the Gulf of Mexico are expected to dump from 2 to 6 inches of rain over New Orleans, Louisiana, and Montgomery, Alabama, according to CNN's weather center, before rolling up toward Atlanta.
The torrential rains could lead to significant river flooding near the Gulf, as flood watches are still in effect from last week's heavy rains.
Heavy winds, hail and tornadoes are possible, the National Weather Service says. Downpours are expected to continue into Friday.
It was snowing Wednesday in Denver where almost a dozen inbound flights were canceled, according to the flight-tracking website FlightAware.com. More than 200 flights into or out of U.S. airports were canceled, the website said, with almost 175 already called off for Thursday.
Kansas City International Airport posted a statement on its website advising passengers to check their flight status before heading to the airport.
"There is potential for more cancelled or delayed flights as the storm continues," the statement said. Flight Aware reported that more than 100 flights were affected at that airport.
CNN Meteorologists Ivan Cabrera and Dave Hennen contributed to this report.