The air mass over the New York metropolitan area on Dec. 20, 2011, was a real ice maker. The area forecasts did not note the ice, but AIRMETs and pilot reports did. The NWS Surface Analysis Chart for 1000 depicted a low-pressure center near the Indiana/Ohio border with a stationary front extending east through Ohio into western Pennsylvania. A cold front extended from this point eastward through southern Pennsylvania and southern New Jersey, and continued over coastal waters.

The Area Forecast for northern New Jersey and in effect until 1000 included the following: ceilings overcast at 7,000 ft. with cloud tops to FL 180. The conditions between 1000 and 1600 forecasted ceilings broken at 15,000 ft. The Area Forecast Discussion issued at 0956 did not discuss any icing hazard to aircraft.However, prior to the 0945 AIRMET, ZULU advised of moderate icing between the freezing level (located between 3,000 ft. and 9,000 ft.) and FL 180. More than 80 pilot reports were made over New Jersey, southern New York and eastern Pennsylvania between 0800 and 1300. They included:

An urgent pilot report received at 0749 from a pilot operating a Cessna Citation at 14,000 ft., about 15 nm southwest of Modena, Pa. The pilot reported moderate to severe rime icing between 13,000 and 14,000 ft.

An urgent pilot report received at 1042 from “multiple” types of aircraft at 14,000 ft. near Schooley's Mountain, N.J. The report included severe rime icing between 14,000 and 17,500 ft.

An urgent pilot report received at 0808 from a flight crew operating a McDonnell Douglas MD-83 aircraft at 14,000 ft. over MMU. The pilot reported moderate to severe rime icing between 14,000 and 16,500 ft. One of the flight crewmembers reported that the icing was the worst he had seen in 38 years of flying experience and that he had never seen ice accumulate so quickly. He described “golf ball-sized” accumulation on the windshield wiper.

Later, the captain of a Bombardier CRJ aircraft that was operating in the northern New Jersey area told investigators that the wing anti-ice system could not “keep up” with the accumulation. He estimated 2.5 in. of ice on the protected areas of the wing, and 4.0 in. of accumulation on some unprotected areas in a time span of about 5 min.