SNOW SHOWERS THAT GO OUT WITH A BAND

by James Hudgins

 

Many post frontal cold air surges during the late fall and winter months are often accompanied by upslope snowfall across the western slopes of the Appalachians.  Although most of the significant snowfall accumulation with these events is confined to the higher west facing slopes mainly across western North Carolina, southwest Virginia, and eastern West Virginia, a few of these events produce isolated but intense snow bands just east of the Appalachians.  This phenomenon has most often been observed near the end of the upslope snowfall events and mainly during the late night and early morning hours.

These banded snow events appear to be linked to low level northwest flow under which moisture and instability are sufficient to produce heavy snowfall.  The depth of the moisture and unstable air has been shown to be the key ingredients to produce such events with trajectories originating from the Great Lakes (Fig 1).  The local development of these snow bands appears to occur across the higher peaks or ridges of the Appalachians, with the bands translating downstream depending on the exact direction of the steering wind.

Snowfall during these events is usually short lived, but often results in very heavy snowfall rates (similar to Lake Effect) with localized accumulations of two inches or more in less than an hour. In specific the heavier bands have been seen to affect locations across the New River Valley especially from Giles County across Montgomery and Floyd counties, as well as over Wythe, Pulaski and Carroll counties (Fig 2).  These events usually occur when snowfall chances are waning but often impact the morning commute with hazardous road conditions and near zero visibility at times in very narrow swaths across parts of the region.  An example of an early morning snow band event is depicted by the National Weather Service Doppler radar in Fig 3.

*Note this is ongoing research with future emphasis on the origination (ridges vs. mountain gaps) of these bands and the trajectories that they follow, plus the role that the Great Lakes and wind shear play. Additional efforts will also include an attempt at local modeling of the bands and the specific terrain features associated with them.

 

 

Fig 1. Plot showing favored trajectory of air (red line)
Fig 1. Plot showing favored trajectory of air (red line)
needed for the snow band events.

 

Fig 2. Some typical locations (white arrows) of more significant snowbands seen across the region.
Fig 2. Some typical locations (white arrows) of more significant snow bands seen across the region.

 

Fig 3. Example of early morning snowbands across parts of the region as
Fig 3. Example of early morning snow bands across parts of the region as
depicted by the Blacksburg NWS Doppler radar.