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(CNN) - More than 100 million Americans will see temperatures drop below freezing this weekend, a stark contrast to last week's sunny weather.
Saturday began with snowfall reported in New York by the National Weather Service.
The NWS tweeted: “Snow in Central Park! In May! The Automated Surface Observation System (ASOS) of Central Park recorded snow. This joins the last record snowfall of this day in 1977 ”.
Snow in Central Park! In May! The Central Park Automated Surface Observing System (ASOS) recorded snow. This ties the record latest snow set on this day in 1977. pic.twitter.com/RBOHenr3ZG
- NWS New York NY (@NWSNewYorkNY) May 9, 2020
Central Park on Saturday morning also hit a daily record low of 1.1 degrees Celsius (34 degrees Fahrenheit).
Central Park on Saturday morning also hit a daily record low of 1.1 degrees Celsius (34 degrees Fahrenheit).
Snow also fell overnight in Pennsylvania and upstate New York, and continued Saturday in Vermont, New Hampshire, Massachusetts and Maine. Shaftsbury, Vermont, was 22.9 centimeters, the National Oceanic and Atmospheric Administration reported.
At least 20 states were under surveillance, warning or frost warning on Saturday morning. Although most of those states are in the Midwest or Northeast, the warnings extend to Georgia and South Carolina.
In some northeastern states it felt more like early March than May. A system was established that began in the Great Lakes region to bring gusty winds and 6 to 30.5 centimeters of wet snow to New England.
"It appears that the interior of New England will see significant amounts of wet snow with increasingly strong and gusty north winds from Friday night to Saturday morning," said the Climate Prediction Center.
For cities like New York, Albany, and Hartford, cold rain will be the main focus, but some snow may mix.
Blame the polar vortex
More than 40 cities across the Midwest and Northeast set daily low-temperature records on Saturday morning, including Pittsburgh at -2.2 degrees (28 degrees Fahrenheit) and Baltimore at 1.1 degrees Celsius (34 degrees Fahrenheit) .
For Fort Wayne, Indiana, it was also the coldest temperature ever recorded until very late in the season and the coldest temperature recorded in May, plus a low daily record (-5 degrees Celsius or 23 Fahrenheit).
The polar vortex was so strong this winter and spring that it led to the largest Arctic ozone hole in history, but it did not produce large outbreaks of cold weather.
The polar vortex, as its fearsome name implies, is a circulation of high-level strong winds that normally surround the North Pole, moving in a west-east direction, that is, a low-pressure polar system.
These winds tend to keep cold air locked up in arctic regions of the northern hemisphere. It is not a single storm. Sometimes this vortex can become distorted and go much further south than it normally is, allowing cold air to tip south.
But this air did not spread to the United States this winter. That is why spring and winter have been mild and the main cities on the east coast recorded low snowfall.
But now, the polar vortex is weakening, allowing Arctic air to move out of its usual place and cause low temperatures in the eastern area over the weekend.
CNN's Haley Brink and Jay Croft contributed to this report.
Low temperatures New York