Snow fell in Washington yesterday for the first time since April 6, as the metropolitian area was blanketed with cold Canadian air that is expected to plunge temperatures below the freezing point tonight.

Although there were no accumulations of snow reported in the region, forecaster George Schielein of the National Weather Service estimated the snowfall here at a "couple of million flakes."

He said the cold weather - yesterday's high of 47 degrees was 11 degrees below normal - and flurries were caused by the southward flow of Canadian air that generally is confined to western New York and Pennsylvania at this time of year.

Schielein predicted that today would bring the coldest weather of the season, as the center of the cold air mass arrives in the Washington area.

Temperatures in Washington should fall to 30 or 31 degrees tonight, with even lower temperatures expected in outlying suburbs, he said. The normal low temperature for this time of year is 38 degrees.

By Tuesday the weather should begin warming up and on Thursday, temperatures will probably rise above normal again, as they did in the early part of the month, according to Schielein. "We could push 62 or 63 degrees," he said.

The high temperature of 47 degrees was measured at National Airport yesterday at 11:30 a.m. The day's low reading of 38 was recorded at 6:40 a.m. The peak wind gust was 28 miles an hour, from the north-northwest at 3:52 p.m.