Warm weather, high humidity and late afternoon thundershowers are forecast through the Labor Day weekend, the National Weather Service said yesterday.

Senior forecaster Harold Hess said that despite the expected late afternoon thundershowers, Washington area beachgoers will get plenty of sunshine.

The area continued to suffer through 90-degree temperatures, high humidity and the year's sixth air pollution alert yesterday.

For the second day in a row a swath of thundershowers passed through the area in late afternoon, dropping substantial but scattered rain. Unlike Tuesday, when thundershowers caused the thermometer to drop 14 degrees in one hour, the rain did not bring relief from the heat.

Yesterday's high was was 90, measured at National Airport at 2:30 p.m., and marked the fourth straight day of 90-degree temperatures. Until last Sunday cool Canadian air had kept the thermometer under 90 degrees for 15 consecutive days. The Metropolitan Council of Governments extended the year's sixth air-pollution alert - which it had declared Tuesday - at least through today.

A COG spokesman said the air quality index reached a high of 120 in Bethesda at 2 p.m. yesterday. Readings of 110 were recorded by midafternoon in the District of Columbia and Alexandria. Readings above 100 indicate "very unhealthy" air quality.

The current pollution alert was the first for the area since Aug. 4.

The weatherman seems to be aware of the misery recent forecasts have been causing area residents. One late afternoon forecast befan yesterday with this comment:

"Residents of the three-state area no doubt are getting tired of hearing the same forecast day after day, and of experiencing the same warm, humid weather without a break. Unfortunately. . ."