Friday, July 18 – Saturday, July 19: In the eastern US, a persistent surface high-pressure system continues to produce light winds over the region, limiting pollutant dispersion. These conditions, combined with pollutant carryover from previous days, will result in widespread Moderate and Unhealthy for Sensitive Groups AQI levels across the East, with several sites in New Jersey, Maryland, Virginia, and Connecticut expected to reach Unhealthy AQI levels for ozone.

In the west, fires in northern and central California continue to burn, releasing pollutants into the atmosphere and aiding ozone and particle formation. In addition, warm temperatures and sunny skies will further enhance ozone production. These conditions will result in Moderate AQI levels throughout the West, with sites in California reaching Unhealthy for Sensitive Groups AQI levels. Surface high pressure over Colorado will result in light winds and limited pollutant dispersion, resulting in Unhealthy for Sensitive Groups AQI levels in eastern Colorado.