Northwest Flow Snow in the Southern Appalachians

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Northwest Flow Snow in the Southern Appalachians 1. Overview Snow associated with northwest low-level winds is a fairly common occurrence in portions of the southern Appalachians during the late fall, winter, and early spring. "Classical" northwest flow snow (NWFS) events develop following the passage of cold fronts when a shallow, moist layer of air covers the Tennessee and Ohio river valleys. Figure 1 is the surface weather map during the northwest flow snow event of December 18-20, 2003. When the air is sufficiently cold, the moist, upslope flow along the Tennessee border produces snow flurries and snow showers. In the WFO Greenville-Spartanburg County Warning Area, the clouds and precipitation during NWFS events are usually confined to the Tennessee border counties, but sometimes brief periods of snow showers or flurries can occur across the southern mountains and foothills of North Carolina and in extreme northeast Georgia. Rather infrequently, flurries also blow across the higher terrain of upstate South Carolina. 2. Climatology Recent research by Dr. Baker Perry at Appalachian State University and Dr. Charles Konrad at UNC-Chapel Hill has provided a great deal of insight into the climatological characteristics of NWFS in the southern Appalachians. Using data from National Weather Service cooperative observers and hourly reporting stations (e.g., Knoxville, Asheville, and Roanoke), important links between terrain characteristics and snowfall distribution have been revealed. For example, the map in Figure 2 highlights areas that experience upslope and downslope flow when the low-level wind is blowing from the northwest. Dark shades identify the windward slopes where lifting enhances precipitation, and the lighter shades depict the leeward slopes where sinking flow suppresses precipitation. Figure 3 provides a general idea of areas that are most likely to experience precipitation during NWFS. Note the general similarities with the windward and leeward patterns in Figure 2. The correlation between snow accumulation and windward and leeward slopes is not exact because strong flow can transport snow downwind from the area where it is generated. No single map can capture the fine details of the spatial distribution of snow occurrence because of the interaction of complex terrain with the variations in wind direction and moisture availability that exist from case to case. Many NWFS events produce only trace amounts of snow or just a dusting. Occasionally, heavy snow will occur. One of the heaviest northwest flow events was the December 2003 case depicted in Figure 1. Click here for a snowfall map for that event. Figure 4 is a satellite image showing the snow cover the day after the snow ended. When NWFS is expected to occur, forecasts will typically highlight the Tennessee border counties as the area most likely to experience snow. In some cases, northern Buncombe County can also receive measurable snow during these events. It is not uncommon for the Asheville Airport to have only flurries while locations from the city of Asheville north through Woodfin and Weaverville can have enough snow accumulation to cause slippery streets and highways. The squally nature of the heavier snow showers can quickly reduce visibility and cause icy patches on the roads. Travelers on primary highways such as Interstate 40 through the Pigeon River Gorge, Interstate 26 through Madison County, and U.S 19E&W in the northern mountains should expect these conditions from time to time during the winter months. During NWFS events, motorists on all mountain roads should be alert for rapidly changing weather. 3. Forecasts The National Weather Service Forecast Office at the Greenville-Spartanburg Airport will issue a Winter Storm Watch for heavy snow within 48 hours of an event if conditions appear favorable for three inches to accumulate in a 12-hour period, or four inches to accumulate in a 24-hour period. A Heavy Snow Warning means the event is imminent or it is occurring. A Snow Advisory is issued when accumulations of an inch or two are forecast but not enough snow to meet the warning criterion is expected. An advisory can also be issued when only a dusting of snow on cold road surfaces might cause hazardous driving conditions leading to numerous accidents.
Fig. 1. Surface weather map at 7:00 am EST on December 19, 2003. Analysis provided by NOAA/NWS Hydrometeorological Prediction Center.
Fig. 2. Locations of windward and leeward slopes during periods of northwest flow. Dark shades highlight windward slopes. Light shades identify leeward slopes. Map created by Dr. Baker Perry of Appalachian State University.
Fig. 3. Typical snow distribution during a northwest flow snow event. Map created by Dr. Baker Perry of Appalachian State University.
Fig. 4. Snow cover in the southern Appalachians on December 21, 2003 following the event of December 18-20, 2003. Aqua MODIS image from Space Science and Engineering Center, University of Wisconsin-Madison.

Northwest Flow Snow in the Southern Appalachians

1. Overview
Snow associated with northwest low-level winds is a fairly common occurrence in portions of the southern Appalachians during the late fall, winter, and early spring. "Classical" northwest flow snow (NWFS) events develop following the passage of cold fronts when a shallow, moist layer of air covers the Tennessee and Ohio river valleys. Figure 1 is the surface weather map during the northwest flow snow event of December 18-20, 2003.

When the air is sufficiently cold, the moist, upslope flow along the Tennessee border produces snow flurries and snow showers. In the WFO Greenville-Spartanburg County Warning Area, the clouds and precipitation during NWFS events are usually confined to the Tennessee border counties, but sometimes brief periods of snow showers or flurries can occur across the southern mountains and foothills of North Carolina and in extreme northeast Georgia. Rather infrequently, flurries also blow across the higher terrain of upstate South Carolina.

2. Climatology
Recent research by Dr. Baker Perry at Appalachian State University and Dr. Charles Konrad at UNC-Chapel Hill has provided a great deal of insight into the climatological characteristics of NWFS in the southern Appalachians. Using data from National Weather Service cooperative observers and hourly reporting stations (e.g., Knoxville, Asheville, and Roanoke), important links between terrain characteristics and snowfall distribution have been revealed. For example, the map in Figure 2 highlights areas that experience upslope and downslope flow when the low-level wind is blowing from the northwest. Dark shades identify the windward slopes where lifting enhances precipitation, and the lighter shades depict the leeward slopes where sinking flow suppresses precipitation.

Figure 3 provides a general idea of areas that are most likely to experience precipitation during NWFS. Note the general similarities with the windward and leeward patterns in Figure 2. The correlation between snow accumulation and windward and leeward slopes is not exact because strong flow can transport snow downwind from the area where it is generated. No single map can capture the fine details of the spatial distribution of snow occurrence because of the interaction of complex terrain with the variations in wind direction and moisture availability that exist from case to case. Many NWFS events produce only trace amounts of snow or just a dusting. Occasionally, heavy snow will occur. One of the heaviest northwest flow events was the December 2003 case depicted in Figure 1. Click here for a snowfall map for that event. Figure 4 is a satellite image showing the snow cover the day after the snow ended.

When NWFS is expected to occur, forecasts will typically highlight the Tennessee border counties as the area most likely to experience snow. In some cases, northern Buncombe County can also receive measurable snow during these events. It is not uncommon for the Asheville Airport to have only flurries while locations from the city of Asheville north through Woodfin and Weaverville can have enough snow accumulation to cause slippery streets and highways. The squally nature of the heavier snow showers can quickly reduce visibility and cause icy patches on the roads. Travelers on primary highways such as Interstate 40 through the Pigeon River Gorge, Interstate 26 through Madison County, and U.S 19E&W in the northern mountains should expect these conditions from time to time during the winter months. During NWFS events, motorists on all mountain roads should be alert for rapidly changing weather.

3. Forecasts
The National Weather Service Forecast Office at the Greenville-Spartanburg Airport will issue a Winter Storm Watch for heavy snow within 48 hours of an event if conditions appear favorable for three inches to accumulate in a 12-hour period, or four inches to accumulate in a 24-hour period. A Heavy Snow Warning means the event is imminent or it is occurring. A Snow Advisory is issued when accumulations of an inch or two are forecast but not enough snow to meet the warning criterion is expected. An advisory can also be issued when only a dusting of snow on cold road surfaces might cause hazardous driving conditions leading to numerous accidents.

Fig. 1.  Surface weather map at 7:00 am EST on December 19, 2003. 
Analysis provided by NOAA/NWS Hydrometeorological Prediction Center.

Fig. 2.  Locations of windward and leeward slopes during
periods of northwest flow.  Dark shades highlight windward
slopes.  Light shades identify leeward slopes.  Map created
by Dr. Baker Perry of Appalachian State University.

Fig. 3.  Typical snow distribution during a northwest flow snow 
event. Map created by Dr. Baker Perry of Appalachian State University.

Fig. 4.  Snow cover in the southern Appalachians on December 21, 2003 
following the event of December 18-20, 2003.  Aqua MODIS image from 
Space Science and Engineering Center, University of Wisconsin-Madison.