Extremely Dense Fog Events along the I-77 Corridor of the Virginia Blue Ridge

Especially dense fog occurs frequently along the escarpment of the Blue Ridge Mountains, and has proven to be a significant concern for traffic on interstates that pass over this mountain ridge on the eastern edge of The Appalachians. One specific location is near Fancy Gap, Virginia, where Interstate 77 passes through. Due to a history of major accidents along this short stretch of the interstate, the Virginia Department of Transportation (VDOT) has installed a number of sensors along this corridor to monitor weather conditions, including visibility.

This study utilizes archived data from eight of these sensors to document the climatology of particularly dense fog along this stretch of interstate, defined in collaboration with VDOT as visibilities 90m (300 ft) or less detected at more than one sensor for at least an hour. A vast majority of the accidents occur at these extremely low visibilities. The monthly and diurnal frequency of occurrence of these events shows that they can happen at any time of year, but are most common in the early Autumn months and again in the heart of the winter. Furthermore, they can occur at any time of day or night, but tend to be most common late at night through the late morning hours. The distribution of event durations shows that a majority last less than 6 hours, but many last for much longer than that, including over 24 hrs.

In addition, for all extremely low visibility events lasting for more than 6 hours, synoptic scale patterns at several levels are analyzed and categorized to assist forecasters in recognizing any unique larger scale patterns that may be different from more typical fog events. Future work planned regarding correlations with nearby METAR observations and synoptic soundings will be outlined as well.