By Ryan Ellis NWS WFO Raleigh, NC
On January 12th, a dense fog event over central NC prompted a Dense Fog Advisory across the CWA. This was the second night in a row where an extremely warm and moist airmass by January standards was in place over the region ahead of a strong cold front that brought significant rain to western NC and southern VA. This is the first time since having access to the VIIRS RGB Nighttime Microphysics Imagery that fog has shown up this widespread. The fog is clearly indicated with a blue color. This makes sense because dense amount of blue in the image is directly related to the 10.8 micron thermal channel which is related to the temperature of the surface. This is the first opportunity we have had to see fog with the micro physics imagery with a warm surface. Usually the fog appears as a light green color and it is tougher to tell between fog and low stratus. Since this was a slam dunk fog event there was no doubt about what we were seeing.
As a forecaster it was helpful to know which interstate corridors had the worst conditions. For our CWA this turned out to be the I-85 corridor north of Raleigh and the northern half of the I-95 corridor. In fact, later that morning a multi-vehicle accident was reported on I-95 southbound near Wilson that shut down the southbound lanes for several hours. While not exactly in the dense fog at the time of these images, this location is in the general area. According to these images, this location might have had some higher clouds over the area that were masking the fog. It is also conceivable that the fog had moved southward in the several hours between the time the image was captured and the time that the accident was reported. Either way, this was a great situational awareness tool to let us know that these two interstates in particular were in more danger than some of our other high traffic corridors.
Also in these images it is very easy to tell where fog ends and higher level debris clouds associated with the approaching cold front begin. This is clearly evident in southern VA where the higher red clouds quite definitively end and the blue color representing the fog begins. This is a big advantage over the general visible satellite imagery and the 11-3.9 micron channel that we usually use to forecast fog. While this imagery is only available a couple of times per morning, it has still proven to be a useful tool in operations.