The 9 January 2007 Northwest Flow Snowfall Event
Blair S. Holloway
Figure 1. Map of event snowfall accumulations as reported by spotters, cooperative observers, and county officials.
Author's Note: The following report has not been subjected to the scientific peer review process.
1. Event Overview
On 9 January 2007, a moderate snowfall event occurred across the mountains of western North Carolina. Snowfall accumulations ranged from trace amounts as far south as northern Rabun County in Georgia, to greater than 5 inches along portions of the North Carolina/Tennessee border counties (Fig. 1). An event maximum snowfall of 7 inches occurred in western Graham County, North Carolina, along the Cherohala Skyway, and in southern Yancey County, North Carolina, at Mount Mitchell State Park (Fig. 1). Overall, snowfall accumulations across the area were characterized by significant horizontal and vertical spatial variability, as is typical of northwest flow snowfall (NWFS) events (Perry and Konrad 2004). Visible satellite imagery from the day following the snow event confirms the variability in accumulations. The bright white shades show the higher accumulations at higher elevations while the darker colors show the lesser accumulations in the mountain valleys and east of the higher terrain (Fig. 2).
Figure 2. TERRA MODIS image from 1627-1640 UTC 10 January 2007. Image from Space Science and Engineering Center, University of Wisconsin-Madison.
On the large scale, this event occurred as a 500 mb shortwave trough moved across the western Carolinas on 9 January. This feature approached the region on the morning of the 9th, and pushed east of the Appalachians later that day (Fig. 3). At the surface, a rapidly moving cold front impinged upon the North Carolina/Tennessee border around sunrise, and was located immediately east of the mountains by mid-morning (Fig. 4). Ahead of the cold front, surface winds that were southwesterly across the North Carolina Mountains, shifted to northwesterly following the passage of the cold front (Fig. 5).
Figure 3. Storm Prediction Center (SPC) objective analysis of 500 mb geopotential height, temperature, and wind at 1200 UTC 9 January 2007 (left) and 0000 UTC 10 January 2007 (right). Click on images to enlarge.
Figure 4. Hydrometeorological Prediction Center (HPC) surface fronts and pressure analysis at 1200 UTC 9 January 2007 (left) and 1500 UTC 9 January 2007 (right). Click on images to enlarge.
Figure 5. Regional surface observations at 1200 UTC 9 January 2007 (left) and 1800 UTC 9 January 2007 (right). Click on images to enlarge.
2. Radar Observations
This snowfall event occurred in two distinct phases of precipitation. The first phase took place in conjunction with the approaching surface cold front. Beginning at 1200 UTC an area of precipitation stretched from central Tennessee through Kentucky (Fig. 6) and lined up well with the analyzed cold front (Fig. 4). This precipitation entered the North Carolina Mountains around 1500 UTC (Fig. 6), and diminished further east of the area by around 1700 UTC (Fig. 7). In the wake of the cold front (Fig. 8), the northwesterly flow phase of the snowfall event began. Snow showers developed across the North Carolina Mountains, and areas immediately upstream at around 2000 UTC (Fig. 9). These snow showers were persistent through 2300 UTC (Fig. 9), but continued to decrease in coverage over the next several hours as seen at 0200 UTC on 10 January (Fig. 10).
Figure 6. Radar reflectivity mosaic at 1200 UTC 9 January 2007 (left) and 1458 UTC 9 January 2007 (right). Image from http://www.rap.ucar.edu/weather/radar/. Click on images to enlarge.
Figure 7. Radar reflectivity mosaic at 1656 UTC 9 January 2007. Image from http://www.rap.ucar.edu/weather/radar/.
Figure 8. Hydrometeorological Prediction Center (HPC) surface fronts and pressure analysis at 1800 UTC 9 January 2007.
Figure 9. Radar reflectivity mosaic at 2000 UTC 9 January 2007 (left) and 2258 UTC 9 January 2007 (right). Image from http://www.rap.ucar.edu/weather/radar/. Click on images to enlarge.
Figure 10. Radar reflectivity mosaic at 0156 UTC 10 January 2007. Image from http://www.rap.ucar.edu/weather/radar/.
During the northwest flow phase, the cellular appearance of the snow showers in the radar reflectivity images, suggested that instability played an important role, similar to the 11-13 February 2006 event. As the upper air observation from Blacksburg, VA at 0000 UTC on the 10th shows, the northwest flow portion of the event occurred in an environment where a nearly dry adiabatic lapse rate was present from the surface up to approximately 700 mb (Fig. 11). Not only is potential/convective instability present in the boundary layer in this sounding up to about 700 mb (where the equivalent potential temperature decreases with height), the sounding analysis also indicated that weak convective available potential energy (CAPE) is present even with an unmodified surface parcel. The presence of such instability in this phase of the event is assumed to be responsible for the periods of heavy snow that were reported across the North Carolina Mountains throughout the northwest flow period. Similarly, convective instability is observed upstream from the southern Appalachians in the upper air observation from Nashville, TN at 0000 UTC on the 10th as well (Fig. 11).
Figure 11. Skew-T log P diagram (upper left) and hodograph (upper right) for upper air sounding at 0000 UTC 10 January from Blacksburg, VA (RNK) (left) and Nashville, TN (right). The tables at the bottom summarize several objective parameters used by the SPC to determine severe weather potential. Image from http://www.spc.noaa.gov/exper/soundings/. Click on images to enlarge.
The 9 January 2007 NWFS event produced snowfall totals ranging from trace amounts up to 7 inches in a few isolated locations. The event occurred with two distinct phases of precipitation. The first was in conjunction with the approach and passage of a rapidly moving cold front. After the frontal boundary cleared the mountains, low-level winds turned northwesterly beginning the second period of precipitation. This phase appears to be at least partially defined by the presence of instability, primarily from the surface to around 700 mb. The cellular appearance of the developing snow showers on radar imagery and the analysis of an upper air observation confirm this assumption. The presence of this instability is similar to a previous NWFS event in the southern Appalachians that took place 11-13 February 2006 and has also been noted in NWFS events in other areas of the country (St. Jean and Sisson 2004).
Perry, L.B., and C. E. Konrad, 2004: Northwest flow snowfall in the southern Appalachians: spatial and synoptic patterns. Proceedings of the 61st Eastern Snow Conference, 179-189. St. Jean, D. and P.A. Sisson, 2004: Characteristics of upslope snowfall events in northern New York state and northern Vermont: diagnostics and model simulations of several northwest-flow cases. Preprints, 20th Conf. on Weather Analysis and Forecasting, Seattle, WA, Amer. Met. Soc., CD-ROM, 18.4.
Many thanks to Larry Lee and Pat Moore for offering opinions on an early draft of this event summary and for help with the html coding. The upper air analysis and sounding graphics were obtained from the Storm Prediction Center. The surface analysis graphic was obtained from the Hydrometeorological Prediction Center. The regional surface plots, satellite imagery, and radar mosaic images were obtained from the University Corporation for Atmospheric Research.