La Nina Effects on the Tennessee Valley

I have used the term "La Nina" in several past posts especially relating to the tropical season. With NOAA making it official that a La Nina pattern should persist for at least the next 3 months, I thought I would do a little research on the effects a La Nina pattern could have on our local weather.

La Nina is a global pattern that results from the cooling of the ocean water off the coast of South America. It is just the opposite of the El Nino pattern of warm water in the same region. The upwelling of water in the equatorial region off the South American coast leads to this effect. Now, El Nino and La Nina episodes are nothing new, both have been occurring at alternate periods for centuries. However, with each episode we are learning more about the global ramifications that take place. In simplest terms, a La Nina pattern usually equals mild and dry winters in the U.S. and increased hurricane activity in the western Atlantic. El Nino episodes usually relate to wetter patterns over the southern U.S and slightly decreased hurricane activity in the tropical breeding grounds of the Atlantic. Now, that is a broad scale view of the two patterns. With a developing La Nina, here is some research on how our local weather has been effected by past La Nina episodes.

I took a look at La Nina episodes that have occurred since 1980 and there were 5 of them. The most recent was in 2000-2001. Other years include, in order: '83-'84, '88-'89, '95-'96, '98-'99 and '00-'01. Each set of years had some differences as would be expected. But, there were some similarities. I will start with tropical weather. With the exception of '83-'84, each La Nina set of years had above normal tropical activity late in the season (mid Sept-November). The average was 4 September storms, 3 October storms and 1 November storm. There was also an average of at least 2 major hurricanes during the late part of the season. As for our local conditions during the time frame of October-March, here is what I found. Octobers were warmer and drier than normal. Novembers had near normal temperatures and above normal rainfall (yipee!). The December's during these years were quite interesting. Decembers average below normal for temperatures and slightly above normal for rainfall/snowfall. Snowfalls of 2.3 inches (2000) and 3.5 inches (1998) occurred. The months of January and February both had above normal temperatures with at or above normal rainfall. Some light snows occurred in the January years, but not as much in February. One month that really stood out was the month of March. In each La Nina year, March was cooler and drier than normal.

Only time will tell what transpires over the next 6 months. But, based on the averages, we could be looking at a milder, but slightly wetter than normal winter (December could be the cold month), followed by a cooler March. At the very least, the prospects of rain are looking a little better than last winter. We shall see how 2007-2008 turns out!