Winter 2009-2010 Climate Summary

by Marc Chenard

The 2009-2010 winter months were characterized by below normal temperatures, and above normal precipitation and snowfall.

Figure 1. Winter temperature anomalies

Figure 2. Winter precipitation anomalies

Figure 3. Total Winter Precipitation

Figure 4. Average winter temperature

Climate Station Graphs

Blacksburg Climate Graph from Nov 2009-Feb 2010

Roanoke Climate Graph from Nov 2009-Feb 2010

Danville Climate Graph from Nov 2009-Feb 2010

Lynchburg Climate Graph from Nov 2009-Feb 2010

Bluefield Climate Graph from Nov 2009-Feb 2010

Cooperative Observer and Airport Snow Totals

These tables show unofficial snow amounts for various sites across the Blacksburg County Warning Area for the winter season.  Due to the nature of the way the data is collected, these snow amounts are subject to error. The only locations considered official amounts are shaded in blue.

Interesting Facts/Winter Records

• Winter 2009-2010 was the snowiest on record for Bluefield, WV. They received 81.1 inches, breaking the previous record of 77.8 inches during the 1995-1996 winter.
• The beginning of January was the coldest start on record for some locations across the Blacksburg County Warning Area. Blacksburg’s average temperature over the first 12 days of the month was 18.2 degrees, breaking the 18.9 degrees set in 1981. The first 12 days of January also ranked as the 6th coldest at Roanoke, 1st at Bluefield, 1st at Danville, and 8th coldest at Lynchburg.
• Some locations across the region also set a record for most days with at least 1” of snow on the ground.  Blacksburg had 71 days with at least 1 inch of snow on the ground. The previous high was 56 days in the 1977-1978 winter.  Lynchburg had 42 days, with the previous high being 36 days in the 1977-1978 winter. Roanoke had 40 days, tying the record set in 1977-1978. Many locations also set records for number of days with higher snow depths. Overall, the 2009-2010 winter was unprecedented across many locations when it came to the number of snow covered days.
• Winter 2009-2010 was also unprecedented when it came to the lack of warm days. Most winters feature at least a few periods when temperatures jump well above average. However this winter featured very few major warm-ups. In fact, Blacksburg set a record for most consecutive days without hitting 60 degrees. The 99 days of consecutive temperatures below 60 degrees between November 29th and February 7th broke the old record of 90 days set in 1977.
• Winter 2009-2010 featured above normal precipitation and broke some precipitation records. Roanoke and Blacksburg both had their record wettest December with 8.02 and 6.66 inches. December and January were also the wettest ever combined, with Blacksburg seeing a combined 10.91 inches, and Roanoke 12.83 inches.

Here are the more details to the records broken at our climate stations:

Bluefield, WV
Record daily rainfall:
January 24th 2010: 0.87 inches. Previous record 0.81 set in 1999.
February 5th 2010: 1.20 inches. Previous record 0.78 set in 1960.
March 22nd 2010: 0.77 inches. Previous record 0.55 set in 1968.

Record daily cold high temperatures:
January 2nd 2010: 16 degrees.  Previous record 18 set in 2008.
January 3rd 2010: 17 degrees. Previous record 20 set in 1999.
January 4th 2010: 17 degrees. Previous record 20 set in 1999.
February 16th 2010: 22 degrees. Ties previous record set in 1991.

Blacksburg, VA
Record daily rainfall:
December 13th 2009: 1.34 inches. Previous record 0.75 set in 1998.
January 17th 2010: 0.91 inches. Previous record 0.74 set in 2008.
February 5th 2010: 1.12 inches. Previous record 0.75 set in 1975.

Record daily cold high temperatures:
January 2nd 2010: 18 degrees. Previous record 20 set in 1977.
January 3rd 2010: 20 degrees. Previous record 22 set in 1957.

Record daily warm low temperatures:
March 12th 2010: 50 degrees. Previous record 50 set in 1990.

Roanoke, VA
Record daily rainfall:
December 9th 2009: 0.98 inches. Previous record 0.93 set in 1931.
January 24th 2010: 1.34 inches. Previous record 1.33 set in 1958.

Lynchburg, VA
Record daily rainfall:
February 5th 2010: 1.23 inches. Previous record 0.96 set in 1960.
March 12th 2010: 1.92 inches. Previous record 1.38 set in 1935.

Record daily cold low temperatures:
January 31st 2010: -2 degrees. Previous record 5 set in 1966.

Danville, VA
Record daily rainfall:
December 2nd 2009: 1.02 inches. Previous record 0.66 set in 1996.
January 17th 2010: 1.23 inches. Previous record 0.49 set in 2008.
February 5th 2010: 1.41 inches. Previous record 1.19 set in 1975.
March 28th 2010: 3.53 inches. Previous record 1.37 set in 1994.

Record daily cold high temperatures:
January 2nd 2010: 32 degrees. Previous record 32 set in 1977.

Record daily warm low temperatures:
March 12th 2010: 59 degrees. Previous record 59 set in 1967.

Winter Explanation

There are many factors that go into determining whether a winter will be cold or warm, snowy or dry. One major factor is the El-Nino Southern Oscillation Cycle (ENSO).  This cycle is defined as the warming and cooling of sea surface temperatures across the central and east-central equatorial Pacific Ocean. The warming phase is called El-Nino and the cooling phase LA-Nina. Even though ENSO is a cycle of ocean temperatures quite a distance away from us, it still has a significant impact on our winter weather. This winter featured an EL-Nino, and as can be seen from the graphs below, an El-Nino tends to bring below normal temperatures and near to above normal precipitation to the southern United States. This is due to an active southern branch of the jet stream during EL-Nino years, which brings more storms and cooler temperatures to the area. Given these tendencies, it’s no surprise that this winter was colder and wetter than normal.

Most El-Nino years, while cold, are not always cold enough for widespread snow in the south. However, other global and regional factors came together to bring the extreme cold to the area. This cold, combined with the active storm pattern produced by EL-Nino, brought a cold and snowy winter to the region. Some of these other factors were the North Atlantic Oscillation (NAO) and the Eastern Pacific Oscillation (EPO). The NAO is defined by the strengths of areas of high and low pressure over the Atlantic Ocean. A negative phase implies a weaker high and low, which tends to allow colder air to push from Canada into the Eastern United States.  The EPO is similar except over the Pacific Ocean. A negative EPO usually implies colder air over central and eastern Canada, while a positive EPO implies a lack of arctic air over Canada. This winter featured both a negative NAO and EPO for a majority of the time. The EPO phase provided the source of cold air and the NAO phase a means of transporting it south. Thus these two oscillations combined to play a major factor in our much below normal winter.

Figure 5. Typical pattern during El Niño Winters

Figure 6: Left: Average temperature anomalies during El Niño winters. Blue is colder than normal and tan is warmer than normal.
Right: Average precipitation anomalies during El Niño winters. Green is above average, brown below average.

Individual Months

December 2009

December temperatures averaged a few degrees below normal across the Blacksburg County Warning area.  The month was dominated by an upper level trough over the central part of the country. The coldest anomalies were thus seen in the central United States, with much of the east coast seeing near to slightly below normal temperatures.  The southern part of the jet stream was active, bringing several significant storms to the area. Total precipitation amounts and snowfall amounts were both above average, with some areas seeing record precipitation.

Figure 7. December Precipitation and Temperature Ranks

January 2010

January was a month that featured both cold and warm spells across the Blacksburg County Warning Area, with it ending up slightly below normal at most locations.  The month started with an anomalously strong trough over the eastern United States, bringing very cold temperatures to the area. After this cold start, a large ridge developed in the east, allowing temperatures to warm to above average levels for much of the middle of the month.  Precipitation and snowfall were slightly above average across most of the County Warning Area. The month started out dry with most storms passing well to our south, however as the trough weakened towards the middle of the month, the precipitation increased. The month came to an end with a significant winter storm.

Figure 8. January precipitation and temperature ranks

February 2010

February featured a mean trough over much of the country. This kept temperatures well below average for much of the month across the region. It was a relatively dry month, with the southern branch of the jet stream pushed south, keeping most of the moisture and associated precipitation out of the area. However there were a few moderate events, and with temperatures on the cold side, these storms produced mainly wintry precipitation, leading to another month of above average snowfall.  The mean trough also allowed for a continuous northwest wind for much of the month. This northwest wind helped produce numerous upslope snow showers across the western slopes of the higher elevations, mainly in southeast West Virginia.  The following process is a simplified version of how upslope snows occur:  cold air is forced up the mountain…the air cools as it rises…when it cools enough a cloud forms…and when enough moisture is present, and temperatures are cold enough, continuous snow showers fall, with significant accumulations on western slopes. The northwest winds then sink down the east side of the mountain…this sinking warms the air…this warming and sinking dissipates the cloud and associated snow showers…thus only a few snow showers make it to the east of the mountains, with significantly less accumulations.

Figure 9. February precipitation and temperature ranks