A Tale of Two Weather Patterns---One Cold and Snowy, the Other Warm and Rainy
Updated: March 19, 2011
Through much of December and January, the Mid State became accustomed to some unusually long periods of Very Cold temperatures, along with frequent snowfalls. The upper level weather systems formed a pipeline extending all the way from the Arctic Circle, to Middle Tennessee and the Gulf Coast, and kept frigid air masses sliding southward out of Canada. One such air mass in December ran into a moist air mass from the Gulf of Mexico and produced a rare White Christmas across the Tennessee Valley. Below, you can see the blanket of white covering Newsom Station, near Bellevue (just southwest of Nashville) on December 25th:
(Photo Credit: Dave Rosenberg, 2010)
Some enjoyed the cold weather. Others waited patiently for a trend to warmer conditions. The high temperature of 68 degrees at Nashville on December 31st gave those folks a momentary respite. Then, we slipped back into the deep freeze...and had one of our biggest snow storms in quite a while. Just like at Christmas, the cold air ran into another moist air mass from the Gulf of Mexico, and produced an accumulating snow in the Mid State...with a heavy 8 to 13 inches reported across our most extreme southern counties on January 9-10th. Amounts of 1 to 3 inches were common across the north...with the Nashville Airport measuring 2.2 inches. Ricky Dodson, pictured below, found himself digging out of 10 to 12 inches of snow at Pulaski.
(Photo Credit: Ricky Dodson, Pulaski, TN)
The southern Magnolias, like this one in Donelson (just east of Nashville) were covered in white.
(Photo Credit: Debbie Pounders, 2011)
For those looking for an explanation for all the cold weather during the bulk of this winter, or a culprit on which to blame it...you can find a large part of your answer in the so-called Arctic Oscillation (AO).
The AO is a weather pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases. The negative phase allows cold air to routinely plunge out of Canada into the Tennessee Valley. The positive phase, on the other hand, produces generally mild or warm weather in Middle Tennessee. [note: you can read more about the AO at http://nsidc.org/arcticmet/patterns/arctic_oscillation.html]
The National Aeronautics and Space Administration (NASA), recorded the temperature anomalies for January 9 to 16, 2011 for the United States, Canada, eastern Siberia, and Greenland, compared to the same dates from 2003 through 2010 (See map below). Looking at the map, you will notice a wide swath of colder-than-normal temperatures (colored in blue) extending from far northwestern Canada, all the way southeastward to New Orleans and Mobile!
(Photo credit: NASA, http://earthobservatory.nasa.gov/IOTD/view.php?id=48882)
The negative AO that dominated the weather pattern over Canada and the United States through December weakened during the waning few days of the month, and the first week or so of January. Then the AO slumped back to another minimum at about the time of our January 9-10 snow storm (although this minimum failed to reach the magnitude of the AO oscillation back around December 18th). Around the same time as the next AO minimum was achieved, Middle Tennesseans received yet another significant snowfall on January 26th. This minimum, however, was not as low as the one achieved on the 10th, and nowhere close the minimum achieved back in December. Toward the end of January, the AO finally recovered into positive territory--for the first time since November 2010. As a result, the last few days of January ended up much like the last day of December--unseasonably mild. The high temperature at Nashville on January 29th and 30th was 66 degrees and 65 degrees, respectively.
The AO value hit zero around January 28th and about two weeks later the weather patterns were starting their transition toward a long term period of significantly warmer conditions.
Since late January the AO has remained consistently in positive territory, except for a very brief period around the middle of February when it dipped slightly into negative territory.
Even though the AO showed a generally cyclic pattern of temporary rises and falls from December through mid-February, the minimum in each successive cycle became less negative. This indicated that a change in the general weather patterns, with milder conditions for Nashville, might be in the making--and, indeed, a significant pattern change did eventually evolve after the first 10 days of February. Middle Tennessee weather came under the influence of a mean trough of low pressure along the West Coast and over the Gulf of Alaska, producing a milder west and southwesterly flow of air extending from California into the Tennessee Valley. Extended mild periods, like the ones we've experienced over recent weeks, haven't occured since last October and November. On February 20th the temperature at Nashville hit the 70s for the first time since November 25, 2010. The high of 82 degrees on March 18th was the warmest at Nashville since October 18, 2010!
After the little cool snap we experienced on March 10th, an extended spell of warmer-than-normal weather once again took shape for the Mid-State. Pleasant temperatures rounded out the remainder of the winter, and are forecast to continue through the first days of spring as well. Indeed, if you look at the plot of the most recent AO trend (below), you will notice that all except two of the forecast (red) trend lines remain positive through at least April 1. Even the two forecast lines that show a temporary crossing into negative territory late this month suggest potential for only a temporary minor cooling.
Following are CPC's latest 6 to10 day and 8 to 14 day outlooks, which show prevailing temperatures averaging about normal for the Mid State through April 1. Thus, as the bitter cold days of December and January fade quickly into our memory, warmer-than-normal temperatures forecast for the next four days now bid us a more pleasant departure from the winter and a warm introduction to springtime. The low pressure trough that has recently existed over the Gulf of Alaska and West Coast is forecast to remain locked in place through at least the latter part of this month, which means warmer weather for Middle Tennessee and highs probably pushing into the 70s--or even to 80 degrees--on an increasingly routine basis! The main exception will be the weekend of the 25th through 27th, when a brief cool snap will likely give us temperatures some 5 degrees below normal.
The long-lasting and strongly-negative AO, that extended from mid-December through early February, is not what we would typically expect during a La Nina, such as the one we are currently in. Even though a trough of low pressure often exists over central and eastern Canada during La Nina, it is not typically as strong or deep as it was earlier this winter. The strongly-negative AO became associated with a persistent and unusually deep trough that repeatedly pushed frigid Canadian air masses southward, not just into the Great Lakes and Ohio River Valley, but all the way to the Gulf Coast!
Following is a representation of what a typical La Nina winter weather pattern looks like:
The next map, showing observed weather on January 21, 2011, indicates the type of weather pattern that became common over the United States during the strongly-negative AO. It shows that the "typical" La Nina trough over eastern Canada and the north-central United States actually deepened this year--all the way south to New Orleans and Mobile! This left Middle Tennessee vulnerable to repeated onslaughts of frigid Canadian air (and recurrent snowstorms):
The next picture, which shows the type of weather pattern we experienced in early March, indicates some big changes compared to what we observed during the earlier part of the winter. Instead of northwest flow out of Canada, you will notice a westerly flow (represented by the green lines), from the warmer Pacific Ocean, across California, and straight into Middle Tennessee. This type of pattern has persisted for most of the time since February 11th, and is responsible for our much warmer temperatures (as well as our much wetter conditions). It represents the milder "variable Pacific jet stream" as shown in the figure above, labeled "Typical January-March Weather Anomalies".
Following is a map of one of the heaviest two-day rainfall events in Middle Tennessee that occurred following this pattern shift. These rainfall totals were recorded by the our Mid State team of CoCoRaHS volunteers. (Note: the acronym, CoCoRaHS refers to "Community Collaborative Rain, Hail & Snow Network.")
During the first part of the winter, CPC's 90-day outlook for the January through March period (issued Dec. 16, and shown below) revealed expectations of a more typical La Nina pattern--and suggested that temperatures would probably average near or above normal in Middle Tennessee. Those expectations failed to materialize for the largest part of the winter--but eventually came to fruition, for the last 38 days of the "astronomical winter" (defined as the period between the winter solstice to the spring equinox).
CPC's subsequent 90-day outlook, issued January 20, 2011, for February, March and April looked much the same:
The most recent 90-day outlook, issued February 17th, kept the same theme alive for March April and May:
CPC's 30-day outlook for March (issued Feb. 17, and shown below), suggested that the new warmer weather pattern--that began to develop after February 11th--would likely persist for much, if not most of the remainder of the winter. Based on the weather experienced so far this month, and the mild weather forecast for the next several weeks, it would seem that this particular forecast for March will likely verify quite well for Middle Tennessee:
The fact that average temperatures for the first 51 days of the winter were frequently below normal in Middle Tennessee, indicates the complications involved with climate forecasting. Climate temperatures respond to a number of different weather patterns whose effects are often intermingled. This winter season has given us a good example of this comingling...as the strongly negative AO eventually transitioned to a strongly positive AO.
Up until recently, the negative AO definitely held an overwhelming upper hand on the type of weather experienced in Middle Tennessee. Even though the AO went through temporary relaxation periods (toward more positive values) earlier in the winter, the AO--up through the first 11 days of February--had yet to establish a long-lasting pattern of positive (warmer) values. After the middle of February, however, a long-term trend toward milder conditions did, indeed, occur. The average daily temperature at Nashville for the last 17 days of February was 9.5 degrees above normal, and has averaged several degrees above normal so far this month.
It is a big challenge for climate forecasters to determine how long one pattern will last before another starts to exert its influence. The unusually strong negative AO pattern that set up during the last part of the fall and earliest days of the winter certainly offered a BIG challenge to climate forecasters trying to predict the type of weather we might expect for this winter--especially since its effects were contrary to what a La Nina winter typically looks like in Middle Tennessee.