The entire eastern half of the U.S. is bracing for a monster storm, the likes of which one forecaster says occur "once every 20 years."

A storm system heading east from the Rockies is expected to strengthen over the Atlantic and form a huge nor'easter that will barrel up the East Coast, bringing gale-force winds, driving rain and heavy snow to much of the eastern half of the country.

Forecaster Brian Korty says the entire eastern part of the U.S. will feel the effects in the coming days. He calls it the kind of storm that happens "once every 20 years."

Accuweather.com predicts the storm will flood low-lying areas and cause coastal erosion. Inland areas could see a record-breaking snowstorm.