PM2.5 conditions across the West continued to show improving signs this afternoon (compared to last week), with only a few areas across Oregon, California and Utah experiencing Moderate (Code Yellow) PM2.5 conditions (top left). According to AIRNow, much of the eastern half of the U.S. reached Moderate levels of PM2.5 as well, with the worst conditions, Unhealthy for Sensitive Groups (Code Orange) scattered in Pennsylvania and New Jersey (top right). A stationary frontal boundary stretching from the Great Lakes through New England brought afternoon snow/sleet showers and obscured today's MODIS Terra AOD retrievals with opaque white cloud layers (bottom left). The position of this low pressure system coincides with an area of warm air in the lower atmosphere at approximately 1.5km (850mb). As Ruben and Patricia explained last week, warm air in the lower troposphere acts as a temperature inversion or atmospheric "cap" and can trap pollutants near the surface. The Storm Prediction Center's (SPC) Mesoanalysis Page illustrates low-level warm air (red shades) moving from the west (indicated by orange wind barbs) towards the Northeast today (bottom right). This analysis suggests a "temperature inversion" type atmospheric profile over portions of the Northeast and therefore possibly unfavorable air quality conditions.