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.

Figure 2. La Nina weather pattern for the United States (December 2007 through February 2008).(back to paragraph)

Figure 3. Departure of Average Temperature (oF) from Normal (December 2007 through February 2008).(back to paragraph)

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).(back to paragraph)

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.

Figure 4. La Nina precipitation pattern for the United States (December 2007 through February 2008).(back to paragraph)

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.

Figure 9. Following an Arctic front, an upper level disturbance brought accumulation snow to
Southside Virginia and into the Carolinas on February 13, 2008.(back to paragraph)

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.

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.

Figure 14. La Nina weather pattern for the United States (December 2007 through February 2008).(back to paragraph)

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.

Figure 20. Percent of Normal Precipitation (December 2007 through February 2008).(back to paragraph)

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.

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

Figure 24. Infrared satellite displaying wildfires across the region on February 10, 2008.