Thunderstorms, some severe, with very heavy rain have moved into the Mid South and Tennessee Valley today. Flash flooding and severe thunderstorms will be possible from the central Gulf Coast to the Upper Great Lakes. The greatest threat of excessive rains will be in portions of the Lower and Middle Mississippi Valleys. This storm system has a history of producing life threatening flash floods. Read More >
The 17-18 February 2007 Northwest Flow Snowfall Event
Blair S. Holloway
Intersection of Flat Springs Rd. and Stoney Hollow Rd. in Flat Springs, North Carolina around 2300 UTC 18 February 2007. Image courtesy of Dr. Baker Perry, used by permission.
Author's Note: The following report has not been subjected to the scientific peer review process.
1. Event Snowfall Overview
Total snowfall accumulations for the 17-18 February 2007 northwest flow snowfall (NWFS) event ranged from trace amounts to an event maximum of 10 inches on Mt. Sterling in Haywood County (Fig. 1). Other notable storm totals include 7 inches in Graham County along the Cherohala Skyway, 7 inches on Beech Mountain (Avery County), 6 inches in Mars Hill (Buncombe County), and 5.5 inches in Flat Springs (Avery County). The irregular snowfall accumulation and distribution is typical of NWFS events and has been documented in numerous other cases including 11-13 February 2006 and 9 January 2007, just to name a few.
Figure 1. Map of event snowfall accumulations as reported by spotters, cooperative observers, and county officials.
2. Synoptic Features and Radar Imagery
This NWFS event occurred as an upper level trough moved eastward across the eastern third of the country, tracking across the western Carolinas and northeast Georgia between 0000 and 1200 UTC 18 February (Fig. 2). At the surface, a weak low pressure system and frontal boundary was located immediately west of the Mississippi River Valley at 1200 UTC 17 February (Fig. 3). The front then tracked east, moving through the North Carolina Mountains by 0000 UTC 18 February 2007 (Fig. 3). While some precipitation occurred in advance of the approaching frontal boundary, much of the precipitation began following the frontal passage. Snow showers were concentrated along the North Carolina/Tennessee border around 0400 UTC 18 February (Fig. 4), and continued across the area through the early part of 18 February, diminishing greatly by around 1500 UTC (Fig. 4).
Figure 2. Storm Prediction Center (SPC) objective analysis of 500 mb geopotential height, temperature, and wind at 0000 UTC 18 February 2007 (left) and 1200 UTC 18 February 2007 (right). Click on images to enlarge.
Figure 3. Hydrometeorological Prediction Center (HPC) surface fronts and pressure analysis at 1200 UTC 17 Febrary 2007 (left) and 0000 UTC 18 February 2007 (right). Click on images to enlarge.
Figure 4. Radar reflectivity mosaic at 0359 UTC 18 February 2007 (left) and 1456 UTC 18 February 2007 (right). Image from https://www.rap.ucar.edu/weather/radar/. Click on images to enlarge.
3. Observations from Flat Springs, North Carolina
One of the most useful tools available for observing weather, especially precipitation, is radar data. The images available from this type of data allow forecasters to track the movement and intensity of various precipitation features, including anything from thunderstorms to snow. However, the utility of conventional radar data, produced by a Weather Surveillance Radar 88 Doppler (WSR-88D), is quite limited for NWFS events in the southern Appalachians. Over the North Carolina Mountains, the main issues include radar beam blockage and range from the radar. These issues make it difficult to view NWFS snow showers across the North Carolina Mountains, and especially along the Tennessee border. Other types of radars, such as those that are vertically pointing, can provide radar images at specific locations that are blocked from a WSR-88D's view. One such radar is located at 3340 ft. in Flat Springs, North Carolina, in the northern tip of Avery County. The MicroRainRadar (MRR) at this location allows for the viewing of snow showers moving over the radar throughout the NWFS event. A period of snow showers affected Flat Springs between 1900 UTC 17 February and 0000 UTC February (Fig. 5), prior to the passage of the frontal boundary (Fig. 3). Preceeding the passage of the front, 1.7 inches snow accumulated at the location of the radar with 0.10 inches of snow-water equivalent (SWE). The majority of the total snowfall for the event fell after 0000 UTC 18 February, with 3.6 inches of accumulation and only 0.14 inches of SWE. Two main things stand out in the Flat Springs data after 0000 UTC 18 February. First, the snow showers that affected this location were only about 4000 feet deep, with a few periods where the depth was even shallower. In fact, periods of heavy snow have been shown to occur in a relatively shallow cloud layer in other parts of the country (Waldstreicher 2002). Secondly, the snow-to-liquid ratios were as high as 50:1 from 1200 UTC to 1800 UTC, and from 1800 UTC to 0000 UTC 19 February. Overall, for the entire event, 5.5 inches of snow fell with 0.24 inches of SWE. This equates to an event snow-to-liquid ratio of approximately 23:1.
Figure 5. MicroRainRadar (MRR) data image for the time period 1200 UTC 17 February 2007 and 0000 UTC 19 February 2007. Snowfall and snow-water-equivalent (SWE) for each time period indicated. Image from Dr. Sandra Yuter's Cloud and Precipitation Processes and Patterns Group, North Carolina State University. Image made available by Dr. Baker Perry. Click on image to enlarge.
Overall, 17-18 February 2007 was a high impact NWFS event, producing total snowfall accumulations of up to 10 inches across the counties adjacent to the Tennessee border. The event occurred as an upper level trough moved east of the Mississippi River Valley, pushing a weak low pressure system and frontal boundary across the southern Appalachians. The majority of the precipitation during the event affected the counties closest to the Tennessee border and occurred after the front moved through western North Carolina. Detailed observations from Flat Springs, North Carolina, revealed several noteworthy features of this NWFS event. First, much of the snowfall in this area occurred in a shallow cloud layer that was only about 4000 feet deep. Also, snow-to-liquid ratios for the entire event averaged 23:1, with embedded periods of ratios as high as 50:1. These two observations show that only a shallow layer of moisture is necessary for a NWFS event, and that even relatively small amounts of liquid precipitation can result in significant snowfall accumulations.
Waldstreicher, J. S., 2002: A foot of snow from a 3000-foot cloud: The ocean-effect snowstorm of 14 January 1999. Bull. Amer. Meteor. Soc., 83, 19-22.
The upper air analysis and sounding graphics were obtained from the Storm Prediction Center and surface analyses were obtained from the Hydrometeor- ological Prediction Center. Radar mosaic images were obtained from the University Corporation for Atmospheric Research. Also, a special thanks to Dr. Baker Perry, Appalachian State University, and Dr. Sandra Yuter and the Cloud and Precipitation Processes and Patterns Group, North Carolina State University, for the MRR data and observations from Flat Springs.