Northwest flight 2501, was scheduled to operate between New York and Seattle via Minneapolis and Spokane. At approximately 19:31 the flight departed from LaGuardia Airport. At 21:49, when over Cleveland a cruising altitude of 4,000 feet was requested by the flight and approved by ARTC. Forty minutes later the flight was requested by ARTC to descend to 3,500 feet because there was an eastbound flight at 5,000 feet over Lake Michigan which was experiencing severe turbulence and difficulty in maintaining its assigned altitude. ARTC estimated that the standard separation of 1,000 feet would not be sufficient because of the turbulence. At 22:51, Flight 2501 reported that it was over Battle Creek at 3,500 feet, and that it would be over Milwaukee at 23:37. When in the vicinity of Benton Harbor, at 23:13, the flight requested a cruising altitude of 2,500 feet, however, no reason was given for the request. Acknowledgement that ARTC could not approve descent to 2,500 feet was received at 23:15, and this was the last communication received from the flight. An intensive search of the Lake Michigan area was commenced at daylight June 24. On the following day, at 18:30, a United States Coast Guard cutter found an oil slick, and aircraft debris in Lake Michigan approximately 18 miles north-northwest of Benton Harbor. At the time the aircraft crashed, it was flying through an area of considerable thunderstorm activity. The crew knew about the thunderstorm activity and the possible development of a squall line forecast of a squall line, but had not been given a forecast describing the development and location of a squall line that had been issued 100 minutes before the accident. PROBABLE CAUSE: “Insufficient evidence upon which to make a determination of probable cause.”