High pressure off the eastern seaboard continues to support south/southeasterly flow towards the Mid-Atlantic and Great Lake states today, ushering in an unseasonably warm and moist air mass (top left, courtesy Accuweather, top right, surface METAR weather observations). This air mass is forecast to remain stationary until mid week, when a cold front pushes through with cooler and drier conditions from west to east. As temperatures climbed this afternoon, so did PM2.5, reaching Moderate (Code Yellow) and Unhealthy for Sensitive Groups (Code Orange) across the aforementioned region (bottom left, courtesy Airnow). Much like the weekend, the Atlantic onshore flow kept clouds around, making it difficult to detect haze and elevated AOD. However, slightly hazy and raised AOD conditions were observed by the MODIS Terra overpass this afternoon (bottom right). The National Weather Service's surface weather observations also reported morning haze at the Philadelphia International Airport.