Earlier in the day the pilot had spoken with the owner of the airplane, who had cautioned him to monitor the weather. It tends in late spring to be highly predictable, but this time the area forecast was a bit optimistic. Issued at 7:45, it called for marginal VFR conditions -- ceilings between 1,000 and 3,000 feet, visibilities between three and five miles -- in the coastal plain, with the inland valleys remaining clear of clouds until 11 p.m. After that, the outlook called for a 1,000-foot ceiling with tops between 3,000 and 4,000, and three miles visibility in mist. The terminal area forecast for Van Nuys called, a little more pessimistically, for a broken ceiling at 1,500 feet by 10:30. Actually, by 10 o'clock, when the pilot arrived in the vicinity of Whiteman, the overcast had already formed.