Washington was witness yesterday to a full-dress rehearsal for summer, as a blazing sun boiled damp, sticky air to the 90-degree level, producing the kind of meteorological discomfort that is likely to be all too frequent in weeks to come.

Later, the heat and humidity that anticipated the calendar and produced the steamy, summery afternoon were transformed into the vivid flash and angry rumble of electrical storms. Sweeping south and east over much of the area, they brought high winds and heavy rain in spots.

"It was pretty scary," said a utility company spokeswoman, describing her own observations. "It just kicked up and let loose."

A total of 5,700 Potomac Electric Power Co. customers, scattered across Montgomery County, lost power during two brief separate thunderstorms yesterday evening and last night, officials said.

Areas particularly affected included Bethesda, Rockville, Gaithersburg, Poolesville and Darnestown. Only a handful of outages were reported in Northern Virginia.

Yesterday's 90-degree high, reached at 3:30 p.m. at the official National Weather Service measuring station at National Airport, was two degrees short of the record for the date, set in 1974.

It was only the second time this year that the mercury had reached 90. The first was on May 7, when the temperature was 91. The official start of summer is June 21.

The heat and humidity that spawned and sustained last night's thunderstorms are expected to return today. The National Weather Service said the mercury could reach as high as 89 degrees.

Although some parts of the area were pelted by rain, it was only for a few minutes, and some sections received only a few drops.

Elsewhere, the storms were all distant sound and fury, with no rain at all to refresh lawns and fields parched by a severe spring dry spell.