An Early April Northwest Flow Snowfall Event and Cold Air Outbreak
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 Introduction
During 7-8 April 2007, a rather significant late-season northwest flow snowfall (NWFS) event ocurred across portions of the North Carolina Mountains. Overall, total snowfall accumulations for the event ranged from trace amounts across much of the southwestern mountains to a maximum of 10 inches in Minneapolis, North Carolina (Fig. 1). Other notable accumulations include 6 inches in Canton, 6 inches on Mt. Mitchell, and 9 inches on Beech Mountain. This event was also marked by a significant cold-air outbreak that set daily minimum temperature records at the Asheville Regional Airport, the Greenville-Spartanburg Airport, and the Charlotte-Douglas International Airport.
2. Synoptic Features and Radar Imagery
At 500 hPa, the western Carolinas were under the influence of a broad trough rotating around a closed upper level low pressure system located over the eastern Great Lakes region. At 0000 UTC 7 April the primary axis of the upper trough was located to the northeast of the western Carolinas (Fig. 2), and rotated into central Tennessee by 1200 UTC 7 April (Fig. 2). [Note: all times in this report are referenced to Universal Time Coordinated (UTC). To convert to Eastern Daylight Time, subtract four hours from the the UTC time.] At the lower levels of the atmosphere, the event occurred with strong cold air advection at 850 hPa. Beginning at 0000 UTC 7 April temperatures at the 850 hPa level were between -2 and -4 degrees Celsius over western North Carolina (Fig. 3), and cooled to around -12 degrees Celsius by 12 UTC 7 April (Fig. 3). The presence of this low level cold advection is also readily seen in the 0000 UTC 7 April wind profile from the Roanoke, VA (RNK) sounding (Fig. 4), where the low level flow backs from northwesterly at 925 hPa to southwesterly at 700 hPa. As a result, the 1200 UTC 7 April RNK sounding (Fig. 4) reveals a much cooler atmospheric column compared to the 0000 UTC sounding. Snow showers began affecting the counties along the Tennessee border around 0000 UTC 7 April (Fig. 5), and tapered off later in the morning around 1200 UTC (Fig. 5).
Figure 2. Storm Prediction Center (SPC) objective analysis of 500 mb geopotential height, temperature, and wind at 0000 UTC 7 April 2007 (left) and 1200 UTC 7 April 2007 (right). Click on images to enlarge.
Figure 3. Storm Prediction Center (SPC) objective analysis of 850 mb geopotential height, temperature, and wind at 0000 UTC 7 April 2007 (left) and 1200 UTC 7 April 2007 (right). Click on images to enlarge.
Figure 4. Skew-T log P diagram for upper air sounding from Blacksburg, VA (RNK) at 0000 UTC 7 April 2007 (left), and 1200 UTC 7 April 2007 (right). Images from University of Wyoming Department of Atmospheric Science (http://weather.uwyo.edu/upperair/ sounding.html). Click on images to enlarge.
Figure 5. Composite reflectivity imagery from Morristown, TN (KMRX) from 0000 UTC 7 April 2007 (left) and 1159 UTC 7 April 2007 (right). Images from National Climatic Data Center (NCDC). Click on images to enlarge.
3. Observations from Flat Springs, NC
To further examine this NWFS event from radar data, MicroRainRadar (MRR) imagery from Flat Springs, North Carolina, was examined much like in the 17-18 February 2007 NWFS event. As seen in Figure 6, the bulk of the precipitation during this NWFS event occurred between 0000 and 1200 UTC 7 April, with only minimal precipitation noted outside of this period. During the 0000 to 1200 UTC 7 April time period, there are some interesting features that appear with regard to the snow shower activity. The higher reflectivities (denoted by the yellows, oranges, and reds) and periods of heaviest precipitation appear to occur in quite an organized pattern, especially between 0000 UTC and 0700 UTC (Fig. 6). Beginning at 0000 UTC, a short period of high reflectivity moved across this location about once per hour, with the longest break coming between approximately 0200 UTC and 0330 UTC (Fig. 6). This periodicity of approximately 60 minutes between embedded patches of higher reflectivity was perhaps indicative of organized wave motion within the lower levels of the atmosphere in this NWFS event. Overall for the event, Flat Springs received 5.8 inches of snow with 0.32 inches of snow-water equivalent (SWE). This equated to an event snow-to-liquid ratio of approximately 18:1.
Figure 6. MicroRainRadar (MRR) data image for the time period 1800 UTC 6 April 2007 and 1759 UTC 7 April 2007. Image from Dr. Sandra Yuter's Cloud and Precipitation Processes and Patterns Group, North Carolina State University. Made available by Dr. Baker Perry. Click on image to enlarge.
4. Cold Air Outbreak
The other significant aspect of this NWFS event was the record breaking cold air that took place throughout the event. Strong arctic high pressure located over central Canada and a surface ridge that extended southward along the east side of the Rocky Mountains was responsible for the unseasonably cold air (Fig. 7). Overall, records were broken at Asheville, Charlotte, and Greenville-Spartanburg.
Figure 7. Hydrometeorological Prediction Center (HPC) surface fronts and pressure analysis at 1200 UTC 7 April 2007. Click on image to enlarge.
Beginning with the Asheville Regional Airport, a record low of 26 degrees occurred on 6 April, breaking the previous record of 28 which was set in 1982. On 7 April a low temperature of 20 degrees was recorded, breaking the previous record of 25 which occurred in 1982. This also set the all time record for the month of April which was previously 22 degrees on 1 April 1987. Also on 7 April the high temperature of 38 set the record for the lowest maximum temperature, breaking the previous record of 48 which was set in 1973. Finally, a low of 22 degrees was recorded on 8 April breaking the previous record of 26 degrees in 1994.
At the Charlotte-Douglas International Airport, a record low of 25 degrees occurred on 7 April, breaking the previous record of 27 degrees from 1950. Also, on 8 April the morning low of 21 degrees broke both the daily and monthly record lows. The previous daily low temperature was 30 degrees in 1961 and the previous monthly record low was 24 degrees on 1 April 1923.
And finally, at the Greenville-Spartanburg Airport, the low temperature on 7 April was 28 degrees, tying the record low for that date which was previously reached in 1950. On 8 April the low was 24 degrees which set the daily and monthly record low temperature marks. The previous daily record was 32 and was set in 1990. The previous all-time low for the month was 25 and was reached on three previous dates: 4 April 1975, 11 April 1973, and 20 April 1983.
Below are maps reflecting the low temperatures across the western Carolinas and northeast Georgia for the 24 hour period ending the morning of 7 April and the morning of 8 April. The record lows at the locations mentioned above are highlighted in blue numbers. The other values plotted are from data collected by the network of Cooperative Observers across the NWS Greenville-Spartanburg forecast area.
Low temperatures for the 24 hour period ending the morning of 7 April 2007. Numbers highlighted in blue reflect record values at the Asheville Regional Airport and the Greenville-Spartanburg Airport. All other values are taken from Cooperative Observers. Click on image to enlarge.
Low temperatures for the 24 hour period ending the morning of 8 April 2007. Numbers highlighted in blue reflect record values at the Asheville Regional Airport, the Greenville-Spartanburg Airport, and the Charlotte-Douglas International Airport. All other values are taken from Cooperative Observers. Click on image to enlarge.
On 7 April 2007, a late season NWFS event occurred across the western North Carolina Mountains. Overall the event resulted in as much as 10 inches of snow across northern portions of the mountains with smaller amounts extending across the southwestern corner of the state. This event occurred as an axis of cold air advection rotated into western North Carolina in association with an upper level trough. Snowfall around 0000 UTC on 7 April, before tapering off in the morning around 1200 UTC. MRR data from Flat Springs, North Carolina, showed periods of intense snowfall moving over the location at approximately 60 minute intervals, indicating some form of organization in the developing snow showers. All total for the event, Flat Springs received 5.8 inches of snow with 0.32 inches of SWE, resulting in an event snow-to-liquid ratio of approximately 18:1. Another important aspect of the event was the recrod breaking cold air that resulted from a strong high pressure system located over central Canada. Record daily and monthly minimum temperatures were set at Greenville-Spartanburg, Asheville, and Charlotte throughout the event. Overall, a widespread hard freeze occurred across the entire county warning area, with many areas reporting low to mid 20's on the mornings of 7 and 8 April. In fact, freezing conditions extended across nearly the entire state of North Carolina and South Carolina at some time during the event.
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 images were obtained from the National Climatic Data Center (NCDC). A special thanks is extended 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. Finally, thanks to Bryan Mcavoy and Doug Outlaw who compiled the temperature data referred to here.