A 30-mile-wide swath of thunderstorms brought heavy showers to scattered sections of the metropolitan area yesterday afternoon, dropping the temperature at National Airport from 90 to 76 degrees between 5 and 6 p.m.

At the same time the third consecutive day of 90-degree temperatures, coupled with humid and stagnant air, moved the area's Council of Governments to declare the sixth air-pollution alert of the year.

A spokesman for COG said the air quality index reached 120 in Alexandria and Bethesda and 100 in the District. Readings of over 100 indicate a "very unhealty" condition.

The National Weather Service predicted more warm and humid weather with high temperatures and little rainfall for the rest of the week. Temperatures today are expected to be in the 88-to-93-degree range.

Bob Werner, a National Weather Service forecaster, said the area once again had come under the influence of a Bermuda high, a high-pressure ridge holding in a stationary position off the mid-Atlantic Coast. It triggers southerly winds and moisture from the Atlantic Ocean and the Gulf of Mexico.

D.C. police reported that Oxon Run at Wheeler Road and Valley Avenue SE overflowed briefly in heavy showers, areas adjacent to the creek were under four feet of water at one time. No injuries were reported.