Winter Outlook, Another Mild Winter?
by Jan Jackson
Winter of 2006 - 2007...looking back
Most of you probably remember last winter (2006 - 2007) as milder than normal. Snowfall totals for last winter, (Figure 1), for the Blacksburg County Warning Area were well below the average winter snowfall, (Figure 2), except for western slopes areas from Greenbrier County WV, southward to the Bluefield, Tazewell and Burkes Garden areas. Note that the largest deficits extend from the New River Valley and North Carolina mountains, eastward into the foothills and piedmont areas of Virginia and North Carolina.
Figure 1. Snowfall Totals From October 2006 - May 2007
Figure 2. Average Annual Snowfall
In addition to the lower snowfall totals, average temperatures during the 3 month climatological winter period of December 2006 through February 2007 (Figure 3) were warmer than normal at all 5 climate sites in the the Blacksburg County Warning Area.
Figure 3. Three Month December through January Average Temperatures
However, averages can tend to mask the extremes. Almost all of our wintry weather occurred in a 3 week period last season. The period from the last week in January 2007, through the middle of February 2007, had daily temperatures 10 to 20 degrees below normal. Even though the end of February warmed up, the average temperature for the month ranged from 3 to 9 degrees below normal, (Figure 4).
Figure 4. Departure of Average Temperature from Normal February 2007
We had a significant ice storm for parts of the area during our 3 week "winter", on February 13th and 14th. Areas right along the Blue Ridge in southwest Virginia saw ice coatings of up to an inch thick, (Figure 5). This caused widespread downed tree limbs and power lines.
Figure 5. Total Ice Accumulation February 13 - 14, 2007
In addition, our most widespread snowfall event occurred during this period, on February 6th, when an Alberta Clipper storm system brought 4 to 6 inches of snow from southeast West Virginia, eastward to Bedford and Amherst counties in Virginia, (Figure 6).
Figure 6. Storm Total Snowfall February 6, 2007
So, while overall it was a milder than normal winter in 2006 - 2007 based on average temperatures and snowfall, we did have a period of harsher winter weather. Keep that in mind when we discuss the upcoming winter outlook for our area.
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Winter of 2007 - 2008...looking ahead
The winter of 2007 - 2008 is likely to be milder than normal, and with less snowfall. Here is an overview of the reasons why:
We have entered a cold episode of the El Nino/Southern Oscillation (ENSO) cycle, called La Niña, and these conditions are expected to further develop and possibly strengthen into December. What this means for our area is that much of the east coast, and particularly the southeast US coast, is likely to be drier than normal. On the other hand, the Tennessee and lower Ohio valleys are likely to be wetter than normal, (Figure 7). Much of the southern, and eastern US is warmer than normal. Note the area of near normal temperatures over a large portion of central and southwest Virginia, and small portions of northwest North Carolina and southeast West Virginia, (Figure 8). This closely matches the area that experiences Cold Air Damming (CAD), and implies a lot of mixed wintry precipitation events into a shallow cold airmass. In addition, the precipitation anomalies with wetter than normal conditions over the Tennessee and lower Ohio valleys implies storm tracks west of the Appalachians, which would lead to more CAD and mixed precipitation events.
|Figure 7. La Nina Precipitation Anomalies||Figure 8. La Nina Temperature Anomalies|
Below are the official Climate Prediction Center's winter forecasts of precipitation, (Figure 9), and temperature, (Figure 10), which take into account several other factors besides La Nina.
|Figure 9. Climate Prediction Center 3 Month Winter Outlook for Precipitation||Figure 10. Climate Prediction Center 3 Month Winter Outlook for Temperature|
Local 3 month Temperature Outlook (L3MTO)
There is a way to get a site-specific winter temperature outlook, using the Local Three-Month Temperature Outlook available from our web page here. There are 14 sites with this 3 month temperature outlook in the Blacksburg CWA (Figure 11). For detailed instructions on how to use L3MTO, click here.
Figure 11. Local 3 Month temperature Outlook Sites
The forecasts are for average daily mean temperatures over a 3 month period. They are provided in graphical, tabular, and text formats for thirteen separate three month periods, (to cover a year's worth of time). These forecast periods begin at 0.5 months after the current month, to 12.5 months after the current month. The forecasts are updated once a month, after the 15th of each month. So, one can easily select the 3 month December-January-February "winter" period for any of the above sites and view its winter temperature outlook.
Below, (Figure 12), are the 3 month "winter" temperature outlooks for all of the sites in the Blacksburg CWA. Note that all except for Bluefield, Jefferson, Lexington and Wytheville show higher chances for above normal temperatures.
Figure 12. Three Month Winter, (Dec 07 - Feb 08), Temperature Outlooks for all Sites in the Blacksburg County Warning Area
To summarize...with a La Nina that is expected to strengthen, this coming winter will likely be mild overall, with warmer than normal temperatures, and less than normal snowfall. Much of the Blacksburg forecast area has equal chances of above or below normal precipitation, but eastern piedmont areas have a slightly better chance of being drier than normal, while western slope areas in the mountains have a slightly better chance of being above normal. There are some indications that the storm track in La Nina winters in the eastern U.S. is mainly west of the Appalachians, which would tend to produce more mixed wintry precipitation events for the Blacksburg forecast area with shallow Cold Air Damming events. Thus, while snowfall will likely be below normal, we may still see a lot of sleet and/or freezing rain.
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