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An Early April Northwest Flow Snowfall Event and Cold Air Outbreak

Blair Holloway
NOAA/National Weather Service
Greer, SC

Map of event snowfall accumulations as reported 
by spotters, cooperative observers, and county officials.

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).
Storm Prediction Center (SPC) objective analysis of 500 mb geopotential height, temperature, and wind at 0000 UTC 7 April 2007.Storm Prediction Center (SPC) objective analysis of 500 mb geopotential height, temperature, and wind at 1200 UTC 7 April 2007.

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.

Storm Prediction Center (SPC) objective analysis of 850 mb geopotential height, temperature, and wind at 0000 UTC 7 April 2007.Storm Prediction Center (SPC) objective analysis of 850 mb geopotential height, temperature, and wind at 1200 UTC 7 April 2007.

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.

Skew-T log P diagram for upper air sounding at 0000 7 April from Blacksburg, VA (RNK).Skew-T log P diagram for upper air sounding at 1200 UTC 7 April from Blacksburg, VA (RNK).

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.

Composite reflectivity image from Morristown, TN (KMRX) from 0000 UTC 7 April 2007.Composite reflectivity image from Morristown, TN (KMRX) from 1159 UTC 7 April 2007.

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.
MicroRainRadar data image for the time period from 1800 UTC 6 April 2007 to 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

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.
Hydrometeorological Prediction Center (HPC) surface fronts and pressure analysis at 1200 UTC 7 April 2007. Click on image to enlarge.

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.

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 respectively.  All other values are taken from Cooperative Observers.

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.

5.  Summary
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.
Acknowledgements
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.


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Page last modified: August 26, 2011

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