UPDATED 2/3/19 – 4:45pm – Monday is an Air Quality Action Day for the State of Delaware for Code Orange (Unhealthy for Sensitive Groups) fine particles! The state is in the midst of an uncommon, but not unprecedented, wintertime poor air quality event. High pressure over the Mid-Atlantic region and warm air arriving aloft has promoted high particle concentrations all along the I-95 Corridor over the weekend, including across Delaware. Very stagnant weather conditions will persist on Monday. Surface winds will be calm, and an extremely strong surface inversion will inhibit vertical mixing. Given these factors in conjunction with the increase in local pollutant emissions on Monday due to weekday traffic, a second consecutive day of Code Orange air quality is expected.
**Extended Forecast: A weak cold front will move through the forecast area on Tuesday afternoon. Particle concentrations will drop into the Moderate range in response to breezy northwesterly winds and enhanced vertical mixing in the late afternoon and evening. Tuesday’s cold front will move back northward on Wednesday as a warm front, bringing clouds and a chance for rain in the late afternoon to evening. Moderate particle concentrations will continue.

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Sunday, February 3, is forecast to be Code Orange for particulate concentrations in Delaware!

Particle concentrations rose much faster than expected on Saturday morning, due to an extremely strong surface inversion enhanced by Friday’s snowfall. As of 2 PM Saturday, hourly particle concentrations are dropping in response to increasing surface winds and a break in the strong temperature inversion at the surface. Daily average particle concentrations will end up in the upper Moderate for Saturday, however, which is much higher than normal for this time of year. **Sunday’s Forecast: Sunday’s weather conditions are expected to be extremely stagnant. Surface winds will be calm all day. Another very strong morning surface inversion will form, but the difference from Saturday is that this inversion is not forecasted to break in the afternoon. As a result, atmospheric mixing will be minimal for the entire day. Given how quickly particles rose on Saturday, in conjunction with the very stagnant forecast, particle concentrations will reach the Code Orange (Unhealthy for Sensitive Groups) range on Sunday.


Code Orange air quality forecast for ozone is a level of pollution that can be unhealthy for sensitive groups, such as young children, the elderly, and those with heart and/or respiratory conditions. Such persons should limit outdoor activities, especially those that require a high level of exertion. Any health related questions should be directed to the Division of Public Health at 302-739-6619. All other questions should be directed to the Division of Air Quality at 302-739-9402.