The Nighttime Microphysics RGB helped again to delineate small areas of locally dense fog on the late evening of the 21st and the early morning hours of the 22nd. The Terra MODIS image below (image 1) valid at 0347 UTC shows fog (white-aqua colors) already beginning to develop in the narrow valleys cutting through the far southern end of the Cumberland Plateau in northeastern Alabama, as high clouds in association with a front boundary cleared the region (red-colored clouds). Fog shows up especially well in the Paint Rock River Valley in NE Alabama…a valley which is prone to fog formation during about any month of the year, particularly following rain events. Notice the extent of the fog though in the valleys of eastern Kentucky, which is much larger in extent. This first image was disseminated to collaborative offices before midnight local time, and would have been available for midnight shift forecasters, allowing them to easily see locations of developing fog. Due to the resolution of the imagery and the effiiciency it provides for fog detection, it could be helpful for limiting the extent of areas under advisory, and allowing for more descriptive impacts in advisory products (i.e., fog mainly in valley locations, fog across portions of Hwy 72 north of Scottsboro, etc).
The fog became locally dense in some, if not most, of these locations. Observations from Fort Payne during the morning indicated visibility at or below 1/4 statue miles from 0500 UTC through 1300 UTC. Interestingly, as the loop of available VIIRS and MODIS images shows that morning (image 2), the fog did not become widespread, but was confined mainly to the narrow river valleys of the region. Also, the subdued nature of the colors in the DeKalb Valley (Ft Payne) suggests the fog was probably quite shallow.
Notice the the fog could also be observed in the VIIRS Day/Night Band imagery valid at 0658 UTC in most of the valley locations, but becomes difficult to discern in the DeKalb Valley near Ft. Payne due to the presence of the city lights.