U.S. Air Quality

Dangerous Wildfires in Los Angeles Area Continue; Smoke from Fires Spreads into Midwest; Good Air Quality across Most of U.S. Despite Influence of Fires

The dangerous fires in the Los Angeles, California area continued today, generating dense smoke that has spread into the Midwest. Today's Aqua MODIS true color image, overlaid in Google Earth with AQI values and the locations of the large fires in the region (below on left), shows the extent of smoke from the fires in southern California. Significant plumes extend inland to the northeast and also to the southwest over the Pacific Ocean. The Aqua MODIS AOD image for Region 9 (below on right) indicates very high AOD values corresponding to these smoke plumes.

The National Interagency Fire Center reports that of the major fires in the Los Angeles area, only the Morris and Cottonwood fires are contained. The Station fire, which killed two firefighters on Sunday, is the largest fire in the region, encompassing 105,296 acres with only 5% contained. The unusual aspect of these fires is that they are growing with relatively light winds. Typically, wildfires in California are driven by Santa Ana winds, which are katabatic (downslope) winds driven by high pressure over the Great Basin desert. Because Santa Ana winds are katabatic, they are very dry and hot (as air descends, it warms; as it warms, it dries), which leads to prime fire weather. The current Los Angeles fires are not being fed by strong winds, but rather by consuming very dry forests that haven't burned in several decades, and because the fires are burning on mountainous terrain that makes firefighting difficult. Temperatures have been high and humidity low, too, but not because of Santa Ana winds.

A complicating factor for the spread of the fires may be Hurricane Jimena, a Category 3 hurricane on the Saffir-Simpson scale that is approaching Baja California from the south (GOES visible satellite image below on left). Jimena is forecasted to emerge into the Gulf of California this weekend and proceed into the Southwestern U.S. (5-day forecast cone below on right). What is left of Jimena could intensify winds over the Los Angeles area, which would help spread the fires, but without providing any rain that might aid in firefighting efforts. The most recent advisory from the National Hurricane Center has downgraded Jimena's intensity significantly, so it is possible that it will not affect Los Angeles late in the week, but the situation bears watching.

The good news is that despite the large, dense smoke being generated by the fires, air quality at the surface has remained in the Code Green (Good) to Code Yellow (Moderate) range today, as shown by the loop of AQI values for the Los Angeles area (below). There is no doubt that air quality in the vicinity of the smoke plumes must be very dangerous, but the monitors are not recording high pollution levels today, which I hope means that the air quality in urban areas of Los Angeles, where most people live and work, is still at safe levels.

It is noteworthy that smoke from the fires in Los Angeles has spread into the Midwest today, as seen in today's Aqua MODIS true color and AOD images (below). NOAA's Hazard Mapping System reports that remnant smoke from the fires in California and southern British Columbia and southwest Alberta is covering most of the central U.S. and all of southwest and south-central Canada. Again, the good news is that the smoke does not appear to be impacting surface air quality, with conditions remaining at Code Yellow and Green levels across the Midwestern U.S.

Posted by Amy Huff at September 1, 2009 10:54 PM