More than 100 million Americans will see temperatures drop below freezing this weekend, a harsh contrast to last week's sunny weather.
Saturday started with a tied record for the latest snowfall in New York City's Central Park, with light snow reported by the National Weather Service.
The NWS tweeted: "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."
Central Park on Saturday morning also hit a daily record low of 34 degrees Fahrenheit.
The white stuff also fell overnight in Pennsylvania and upstate New York, and continued Saturday in Vermont, New Hampshire, Massachusetts and Maine. Shaftsbury, Vermont, got 9 inches, the National Oceanic and Atmospheric Administration reported.
At least 20 states were under a frost or freeze watch, warning or advisory on Saturday morning. Although most of those states are in the Midwest or Northeast, the advisories stretch all the way down to Georgia and South Carolina.
In some Northeast states, it felt more like early March than May. A system that began in the Great Lakes region was set to bring gusty winds and 6 to 12 inches of wet snow to New England.
"It appears that interior New England will see significant amounts of wet snow with northerly winds becoming increasingly strong and gusty late Friday night into Saturday morning," the Weather Prediction Center said.
For cities like New York, Albany and Hartford, cold rain will be the main focus, but some snow could mix in.
Blame the polar vortex
More than 40 cities across the Midwest and Northeast set daily low temperature records Saturday morning, including Pittsburgh at 28 degrees and Baltimore at 34.
For Fort Wayne, Indiana, it was also the coldest temperature ever recorded so late in the season and the coldest temperature recorded in May, in addition to a daily record low (23 degrees).
The polar vortex was so strong this winter and spring that it led to the largest-ever Arctic ozone hole, but not a lot of cold weather.
The polar vortex, as its scary name suggests, is a circulation of strong, upper-level winds that normally surround the northern pole, moving in a west to east direction -- a polar low-pressure system.
These winds tend to keep the bitter cold air locked in the Arctic regions of the Northern Hemisphere. On occasion, the vortex can become distorted and dip further south, allowing cold air to spill southward.
But it didn't spill out into the US this winter. That's why the spring and winter have been mild and major East Coast cities saw low snowfall amounts.
Now the polar vortex is weakening, allowing Arctic air to spill out -- leading to plunging temperatures in the East this weekend.