This past winter of 2009-10 has been one to remember. Winter storms plagued the eastern United States almost weekly during the months of December, January and February. Most of the systems had some variation of a southern stream track with low pressure systems forming in the Gulf of Mexico and pushing northeastward south of the area and off the coast.
We were in a Positive PNA (Pacific/North American teleconnection) pattern which is highly influenced by El Niño/Southern Oscillation (ENSO). A positive PNA pattern is simply a ridge in the extreme western United States with a trough in the eastern United States. This resulted in warmer and drier conditions in the west with cooler and wetter conditions in the east. El Niño became very strong in January.
The area was affected by several winter storms with some impact to all or part of Blacksburg’s county warning area. Below are some of the more significant winter storms that occurred across the area.
The December 18-19 storm was a classic Miller A type storm. A Miller A pattern is the most common pattern for bringing our region significant snowfall due to an abundant amount of moisture pushing up from the Gulf of Mexico and off of the Atlantic Ocean. This just means a low pressure system developed along a front in the Gulf of Mexico and tracked northeast along the southeast U.S. coastline to Cape Hatteras spreading precipitation over the cold wedge north and west of the surface front. This was the biggest snowstorm to affect the entire Blacksburg county warning area in over a decade. Figure 1 shows what a Miller-A pattern looks like on a surface map. Figure 1a shows the total snowfall that resulted from this classic storm.
Figure 1: Miller-A Pattern
Figure 1a: Total Snow Accumulations
The ice storm of January 21-22 was a Miller B type storm. Generally, this occurs when an occluded low pressure system is located in the vicinity of the Great Lakes that is nearly stationary or slowly moving northeastward. A weaker secondary low pressure develops near the southeast U.S. coastline along a warm or stationary front that is associated with a cold wedge. The Miller B type storm is the most common pattern for bringing mixed precipitation to the Blacksburg county warning area. In this case, the precipitation was primarily freezing rain and freezing drizzle with a period of sleet mixed in. The highest amounts of ice accumulation occurred along and west of the Blue Ridge Mountains, which was due to the upslope easterly flow producing enhanced cooling along the Blue Ridge. Figure 2 shows a Miller-B pattern while figure 2a shows the total ice accumulations that occurred over the 2 day period.
Figure 2: Miller-B Pattern
Figure 2a: Total Ice Accumulations
The snowstorm of January 29 was classified as an overrunning event. This means the path of the low pressure system developed over the southern Plains and tracked east across the deep South then off the Carolina coast. This pattern keeps a deep wedge of cold air in place over the Blacksburg county warning area and moisture “overruns” the cold air. This pattern often does not pull a lot of moisture into the area. However, in this situation, there was an abundant amount of moisture that had been pulled up from the Gulf of Mexico. Snowfall ranged from 6 to 12 inches across the area. Figure 3 shows an overrunning pattern with figure 3a showing the total snowfall for the 24 hour period.
Figure 3: Overrunning
Figure 3a: Total Snow Accumulations
The winter storm of February 5-6 was a classic Miller B type storm, with a mixture of snow, sleet and freezing rain, similar to the January 21-22 ice storm. However, a much stronger low pressure system along the coast with this February event helped to reinforce colder air that was already in place, which produced more snow across the area. Whereas the January event had a much more shallow layer of cold air near the surface while temperatures aloft were above freezing, which resulted in more icing. Figure 4 below shows most of the higher snow amounts were concentrated in the extreme northern portion of the area, where it remained all snow, while the amounts were less farther south. The mix of sleet and freezing rain with snow caused the snow amounts to be lower in southwestern Virginia and the Piedmont of Virginia and North Carolina.
Figure 4: Total Snow Accumulations
Three snowstorms in February were a product of northwest flow snow events which occur when low pressure systems pulling away from the coast leave behind a strong cold northwest flow. This often results in significant snowfall along the west facing slopes in southeast West Virginia, the mountain empire of Virginia, and the northern North Carolina Mountains. The hardest hit area from the three northwest flow snowstorms was western Greenbrier County in southeast West Virginia. The compilation of these events produced several feet of snow in this area by the end of February. Figure 5, 6, and 7 shows the total snow accumulations for these three events respectively.
Figure 5: Total Snow Accumulations from a Northwest Flow Snow event
Figure 6: Total Snow Accumulations from a Northwest Flow Snow event.
Figure 7: Total Snow Accumulations from a Northwest Flow Snow event.