After a weak cold front passage earlier this week through the Tennessee and Ohio valleys and the precipitation associated with a shortwave during the mid-week, fog and low clouds developed in the wake of these events due to moist ground and calm winds overnight into Thursday (4/19) morning. Can you see the fog and low cloud areas in the Night-time Microphysics RGB (NTmicro) imagery above from MODIS? Take a look below at the MODIS 11-3.9 difference product used for fog and low clouds. Wide spread areas of yellow indicate low clouds and fog, but with such a large area it can be hard to differentiatethe two. The METAR ceiling and visibility observations help to interpret the MODIS 11-3.9 product.
Take another look below at the NTmicro RGB from MODIS with the METAR observations now overlaid. Note some of the color variations and associated observations. First, note that the NTmicro areas in the lower right portion of the swath (VA, NC) have some tan coloration which indicates a thicker cloud, likely stratus. Observations in these areas typically show multi-layer clouds, often with ceiling and visibility criteria within the VFR range (but not always; could have fog or low ceiling under the stratus). Next look for the bright areas of aqua and light blue in the region. These areas correspond to low clouds where visibility conditions are often VFR, but ceilings of thick stratus are low enough to cause IFR conditions. Lastly, look in the western portions of the RLX WFO (Charleston, WV) county warning area and parts of the JKL and MRX WFOs futher to the southwest. These have very dull aqua colored regions that appear nearly gray to dull purple. Several observations show 1/4 mile visibility with ceilings of 100 feet. It’s as if some of the violet color representing the land surface is showing through the fog and mixing with the aqua. In fact, the surface emissions are likely influencing the 3.9 channel to be warmer than surrounding low clouds. Hence the 11-3.9 difference used in the green component of the RGB will be smaller than areas of low, thick stratus and therefore contribute less green to the RGB.