The superior resolution of the MODIS instruments can aid in the early detection of fog in the narrow valleys and river channels of the TN Valley region. However, they can also help deter potential false alarm situations as in this unique case.
The image on the left below shows the GOES 11-3.9µm fog product valid at 0815Z on 3rd October (click on the image for a larger version). Notice the narrow yellow colors in portions of northern Alabama and middle Tennessee. The spotty yellow colors from near KHSV (Huntsville Int’l) to KMSL (Muscle Shoals — Northwest Regional Alabama Airport), then turning sharply northward are directly along the TN River channel. The imagery here would indicate the presence of fog formation along the TN River as often happens during cool mornings in the fall season. However, the MODIS imagery taken just about 18 minutes earlier
only indicates the potential presence of fog along the TN River in west/middle TN. Notice there is no indication of fog along the TN River in northern AL in the MODIS image. A visible image later that morning courtesy of GOES shows that fog had
developed along the TN River channel in west/middle TN, and was collocated primarily with the areas of fog indicated by the MODIS image several hours earlier. The runway complex at the Northwest Alabama Regional Airport is prone to fog development, particularly when fog is also occurring just to the north, along and near the TN River. However, no fog occurred on this particular night at the airport. The visibility remained at or above 10 statute miles for the duration of the night. The GOES product alone could have incorrectly suggested development of fog along the TN River was occurring and therefore placement of fog would be necessary in the KMSL TAF. However, the MODIS image suggested fog development was not as likely. So, this was a different situation from the usual early or superior detection of fog by the MODIS instrument, instead, in this case the MODIS instrument would have suggested to the operational forecaster that fog development in the vicinity of KMSL was less likely.