CLIMATE SUMMARY FROM DECEMBER 2007 THROUGH FEBRUARY 2008

by Robert Stonefield

 

Note: Records and normals discussed in this article pertain to NWS Blacksburg's first order climatic sites which includes Roanoke, Lynchburg, Danville and Blacksburg, VA and Bluefield, WV. To view other station climate data that may be closer to your home, visit NWS Blacksburg's Climate web site at http://www.weather.gov/climate/xmacis.php?wfo=rnk or Southeast Regional Climate Center (SERCC) at http://www.sercc.com/climateinfo/historical/historical.html.

Blacksburg County Warning area geographical break down
Figure 1. WFO Blacksburg County Warning area geographical break down.

This past winter (December 2007 through February 2008) was typical for a La Nina event. A La Nina is the cooling of ocean waters in the east-central equatorial Pacific. In January, a moderate to strong La Nina pattern was established with sea surface temperatures around 1.4C below normal. Generally, a winter La Nina pattern (Figure 2) dips the northern Jet Stream south into the Central Plains and Mississippi Valley and up and over the Ohio Valley. Storms systems tend to get organized or develop in the Southern Mississippi Valley and track northeast along cold fronts in the Ohio Valley. This pattern places the Southern Appalachians in a warm sector for much of the time and overall bring above average temperatures for the winter season. This was the case this past winter with the three month average temperature (Figure 3) being 2-4F above normal for the area. Table 1 displays the three month average temperatures for Blacksburg's climatic stations. Danville's, Blacksburg's and Bluefield's average winter 2007-2008 temperature was one of the warmest (top 10) on record. It is also noteworthy that these three stations have the shortest period of record, Danville 1948, Blacksburg 1952, and Bluefield 1959. Roanoke's (1912) and Lynchburg's (1893) period of record is almost twice as long. As one can see in the Winter 2007-2008 temperatures graphs (Graphs 1-5), daily temperatures roller-coastered from above and below throughout the winter season. The cold spells were a result of strong cold fronts (some Arctic) moving through the region. Note that the cold spells were not prolonged, or as much below normal, as the warm spells.

La Nina weather pattern for the United States (December 2007 through February 2008).
Figure 2. La Nina weather pattern for the United States (December 2007 through February 2008).
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Departure of Average Temperature (oF) from Normal (December 2007 through February 2008).
Figure 3. Departure of Average Temperature (oF) from Normal (December 2007 through February 2008).
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Station

Average
Temperature (F)

Normals
(F)
Rankings of Warmest Winter
Roanoke
40.8
38.3
17
Lynchburg
38.8
37.2
47
Danville
43.2
39.1
3
Blacksburg
36.4
33.1
7
Bluefield
38.8
35.5
1
Table 1. Three Month Average Temperatures, Normals, and Rankings (December 2007 through February 2008).
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Daily Highs, Lows, Normal Highs, Normal Lows, and Precipitation amounts
Graph 1: Daily Highs, Lows, Normal Highs, Normal Lows, and Precipitation amounts
from December 1, 2007 to March 1, 2008 for Blacksburg (RNK), VA.

Daily Highs, Lows, Normal Highs, Normal Lows, and Precipitation amounts
Graph 2: Daily Highs, Lows, Normal Highs, Normal Lows, and Precipitation amounts
from December 1, 2007 to March 1, 2008 for Roanoke (ROA), VA.

Daily Highs, Lows, Normal Highs, Normal Lows, and Precipitation amounts
Graph 3: Daily Highs, Lows, Normal Highs, Normal Lows, and Precipitation amounts
from December 1, 2007 to March 1, 2008 for Lynchburg (LYH), VA.

Daily Highs, Lows, Normal Highs, Normal Lows, and Precipitation amounts
Graph 4: Daily Highs, Lows, Normal Highs, Normal Lows, and Precipitation amounts
from December 1, 2007 to March 1, 2008 for Danville (DAN), VA.

Daily Highs, Lows, Normal Highs, Normal Lows, and Precipitation amounts
Graph 5: Daily Highs, Lows, Normal Highs, Normal Lows, and Precipitation amounts
from December 1, 2007 to March 1, 2008 for Bluefield (BLF), WV.
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The precipitation pattern (Figure 4) during a La Nina keeps the the bulk of the precipitation, usually snow, north across the Ohio Valley and into the northeastern United States. For Blacksburg's County Warning Area, precipitation type may vary throughout the winter season but most storms tend to be a wintry mix of rain, freezing rain, snow and/or sleet, as warm air on the eastern side of the storm track gradually overcomes any cold surface air. Precipitation amounts from most of these winter storms are light and less than a quarter of an inch.

La Nina Precipitation Pattern Across the U.S.
Figure 4. La Nina precipitation pattern for the United States (December 2007 through February 2008).
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For the region to receive a significant snow (greater than 4 inches) this past winter, a cold front needed to move through the area and high pressure anchor over the northeast United States to keep the air column cold for a few days. While the air column remains cold, a disturbance needed to bring moisture into the area. This year these disturbances (Figure 5) came in from the northwest (Alberta Clipper) or when the northern jet stream dipped far enough to the south, a surface low tracked from the Gulf coast to the North Carolina-Virginia coast (Gulf Low). This winter season we had 3 significant snow events (05 DEC 2007 Clipper , 17 JAN 2008 Gulf Low , and 13 FEB 2008). The February 13th snow storm was a result of the passing of an Arctic front followed by a trailing upper level disturbance through North Carolina. Significant snow storms of this nature and the location of accumulating snow (south through North Carolina) occur less frequently than clippers and Gulf lows, usually on the order of one per every 2 to 3 years.

Basic track of Alberta Clippers and Gulf Lows.
Figure 5. Basic track of Alberta Clippers and Gulf Lows.
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Snow accumulations from an Alberta Clipper that moved quickly across the Ohio Valley and Mid-Atlantic States on December 5, 2007.
Figure 6. Snow accumulations from an Alberta Clipper that moved quickly
across the Ohio Valley and Mid-Atlantic States on December 5, 2007.
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On January 17, 2008, a weak Gulf Low tracked rapidly northeast from the Gulf of Mexico to along the NC coast.
Figure 7. On January 17, 2008, a weak Gulf Low tracked rapidly northeast from the Gulf of Mexico to along the NC coast.
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Snow accumulations from the Gulf Low on January 17, 2008.
Figure 8. Snow accumulations from the Gulf Low on January 17, 2008.
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Following an Arctic front, an upper level disturbance brought accumulation snow to
Figure 9. Following an Arctic front, an upper level disturbance brought accumulation snow to
Southside Virginia and into the Carolinas on February 13, 2008.
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Visible satellite image on the morning of February 14, 2008.
Figure 10. Visible satellite image on the morning of February 14, 2008.

 

There is another significant snowfall phenomenon that happens a few times per season, but is more geographically based than synoptic (large scale). This event (northwest upslope snow showers) affects western slopes and higher ridges of the Appalachians. Following a strong cold front, northwest winds are able to bring moisture from the Great Lakes and across the Ohio and Tennessee Valleys. As this low level moisture is directed up the terrain of the mountains in a cold environment, snow is produced. During these events, snow may fall for a period of 24, 36, 48 or more hours depositing 6 to 12 inches or more of snow on these western slopes and higher elevations. There were 3 significant upslope events this winter season (16 DEC 2007, 1-3 JAN 2008, 27-28 FEB 2008). Snow accumulations are greatly reduced east of these favored upslope areas. High winds and low wind chill values generally accompany these upslope events.

 

Upslope snow accumulations for December 16, 2007.
Figure 11. Upslope snow accumulations for December 16, 2007.
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Upslope snow accumulations from January 1-3, 2008.
Figure 12. Upslope snow accumulations from January 1-3, 2008.
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Upslope snow accumulations from February 27-28, 2008.
Figure 13. Upslope snow accumulations from February 27-28, 2008.
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Ice storms, on the other hand, are more common during a La Nina winter than significant snow storms. The reason for this is because lows tracking through the Ohio Valley bring warm air in aloft. If this warm air rides over shallow cold (below 32F) surface air (Cold Air Damming, also known as a "wedge", see Figure 14), ice accumulates on objects from freezing rain. Most of the ice events (around 6 or 7) this winter season were minor with only ice forming on elevated surfaces, such as trees, automobiles and bridges/overpasses. Our bigger events that accumulate ice on all surfaces are usually dependent on ground temperatures being below freezing and a very cold wedge of air down the east coast. This year we had two such events (15-16 DEC 2007 and 1 FEB 08). Almost like the northwest upslope snow showers on western slopes, eastern slopes of the Blue Ridge will likely see the highest ice accumulations during a freezing rain event. During these events, easterly winds bring moisture from the Atlantic and up the terrain. As this moisture tracks up the eastern slopes, the air column will cool adiabatically and be colder than other locations. Mountain valleys are also susceptible to ice storms as cold surface air tends to remain trapped in these locations longer. If this air is relatively dry, rain falling into it will cool the air column. This process is know as evaporational cooling.

La Nina Weather Pattern Across the U.S.
Figure 14. La Nina weather pattern for the United States (December 2007 through February 2008).
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Ice accumulations (elevated surfaces and above 2500 feet) from the December 15-16, 2007 ice storm.
Figure 15. Ice accumulations (elevated surfaces and above 2500 feet) from the December 15-16, 2007 ice storm.
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Ice on a bush in Blacksburg, VA (December 16, 2008).
Figure 16. Ice on a bush in Blacksburg, VA (December 16, 2008).

 

The development of the February 1, 2008 ice storm.
Figure 17. The development of the February 1, 2008 ice storm.
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Ice accumulations from the February 1, 2008 ice storm.
Figure 18. Ice accumulations from the February 1, 2008 ice storm.

 

Ice on a pine tree in Christainsburg, VA (February 1, 2008).
Figure 19. Ice on a pine tree in Christiansburg, VA (February 1, 2008).


 

The precipitation pattern for the Southern Appalachians during a La Nina episode is typically below normal along and east of the Blue Ridge. West of the Blue Ridge and into Southeastern West Virginia, precipitation is generally near or above normal and is dependant on the location and position of the cold fronts residing in the Ohio Valley. The three month precipitation (Figure 20) for this past winter season was below normal by 2 to 4 inches across the entire area. Roanoke, Danville and Blacksburg posted one of the top 10 driest winters on record (Table 2). With the drought continuing from last year, December's (3.28 inches) near normal precipitation was a welcome sight. Even though several systems moved through the region in January and February, precipitation amounts for these months were well below normal for the area, 1.48 inches and 2.42 inches respectfully. With the lack of rainfall this winter, drought conditions (Figure 21) continued for the area, especially south into North Carolina.

 

Percent of Normal Precipitation (December 2007 through February 2008).
Figure 20. Percent of Normal Precipitation (December 2007 through February 2008).
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Station

Precipitation Amounts (inches)

Normals
(inches)
Rankings of driest Winter
Roanoke
5.58
9.17
9
Lynchburg
5.87
9.87
11
Danville
6.21
10.60
5
Blacksburg
5.95
9.26
6
Bluefield
7.34
8.79
19

Table 2. Three Month Precipitation Amounts (inches), Normals and Rankings (December 2007 through February 2008).
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Drought Monitor ending on February 28, 2008
Figure 21. Drought Monitor ending on February 28, 2008
(http://www.drought.unl.edu/DM/monitor.html)

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Because of the milder temperatures and the lack of precipitation, snowfall for the winter was also below normal by 5 to 13 inches (Table 3). Strangely enough, one of Blacksburg's warmest climatic sites, Danville, was the only station close to normal snowfall for the winter (3.8 inches fell on February 13, 2008).

Station

Snowfall Amounts (inches)

Normals
(inches)

Roanoke
4.9
17.6
Lynchburg
2.1
13.8
Danville
3.9
5.3
Blacksburg
8.7
17.2
Bluefield
19.3
24.2

Table 3. Three Month Snowfall Amounts and Normals (December 2007 through February 2008)


There is one other weather event that happens every winter but is usually an afterthought, high winds. High winds are more common during the winter months, especially if you live in the higher elevations. These high winds are not directly related to an El Nino or La Nina episodes, but are linked to Nor 'easters off the Mid-Atlantic or New England coast. A high wind events starts with the passing of a cold front. A weak wave of low pressure will track north along this front and intensify as it moves off the coast. In the wake of the cold front, high pressure with strong pressure rises and cold air moves across the Ohio Valley and the Mid Atlantic region. The pressure gradient between the incoming high and the northeast low (Nor 'easter) tighten, creating strong west to northwest winds across the area. Sustained winds of 15-30 mph with gusts up to 60 mph are common during these events, especially across the higher terrain. Accompanying these winds are very cold temperatures. Combining the winds and cold temperatures, wind chill values usually drop into the single digits, sometimes below zero, across the mountains.

On February 10, 2008, an exceptionally strong wind event occurred from this weather pattern. Hurricane force wind gusts of 74 mph or more were reported across some mountain locations. All 40 Counties across Blacksburg's area of responsibility reported numerous power outages, large trees being uprooted, and property damage. Power lines that were downed from falling trees and limbs sparked several wildfires across the area. Three of the largest wildfires were Little Cuba (2700 acres) in Craig County, Black Horse (1500 acres) in Bedford County, and Green Ridge Mountain (about 4000 acres) in Roanoke County. The Black Horse fire in Bedford County (Figure 22-23), was started by an all-terrain vehicle operated on a restricted trail. These fires took state and local agencies and National Guard soldiers 3 days to get under control. Rain falling on the third day was a big help in controlling these fires.

Black Horse wildfire in Bedford CountyBlack Horse wildfire in Bedford County
Figure 22/23. Black Horse wildfire in Bedford County.

Infrared Satellite image display of the wildfires
Figure 24. Infrared satellite displaying wildfires across the region on February 10, 2008.

 

Written by:

Robert Stonefield

Pictures taken by:

Figure 16. Ice on a bush in Blacksburg, VA (December 16, 2008)...Peter Corrigan.

Figure 19. Ice on a pine tree in Christiansburg, VA (February 1, 2008)...Chuck Silverman.

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