The Northeast experienced a dramatic weather whiplash last week, with Central Park hitting 90 degrees on April 15—the hottest temperature for that date since records began in 1869. The unusual heat made flowers bloom early at Longwood Gardens in Pennsylvania, where temperatures reached 87 degrees when 67 is typical for mid-April.

A warm air mass that caused last week's heat has been replaced by cold air moving down from Canada. Freeze watches stretched from Iowa through Pennsylvania as temperatures dropped 25-35 degrees below midweek levels. Longwood Gardens now expects highs barely above 50 degrees with overnight lows around 30. This isn't even the first such swing this year—some Mid-Atlantic spots went from 80-degree highs to actual snowfall in mid-March.

The coldest weather arrived Monday into Tuesday, bringing frost and freeze conditions across the Northeast and Mid-Atlantic. By week's end, temperatures will climb back into the 60s in New York and 70s in Washington. As one Longwood Gardens manager put it: "To be honest, the garden is tough. The plants are adaptable."