“Look to the past, to see the future”….I cannot tell you how many times I heard that phrase in a variety of ways in my meteorology and climatology courses in college. Understanding and studying past patterns can provide clues to predicting future weather patterns. When attempting to do some long range forecasting, the same rules apply. But, when you single out one season, in this case winter, you have to keep in mind that no 2 winters are alike. Some have similar patterns, but each is different. So as we approach the 2009-2010 Winter, we are looking for a common pattern trait and in this case it’s El Nino.
El Nino is the periodic warming of the Pacific equatorial waters off the west coast of South America. It’s gone on for centuries and received its name (”El Nino” - “The Boy” in spanish) because the warming occurred around Christmas time. For Peruvian fisherman, the warming would cause poor coastal fishing. Over the past few decades, we have found that this regional warming in the Pacific can lead to changes in global weather patterns. In North America, El Nino, can lead to a more active southern branch of the jet stream. This can lead to above normal rainfall values along the Gulf coast during the winter. This pattern can also increase wind speeds in hurricane source regions which can limit tropical activity. We already saw signs of this during this year’s hurricane season. The latest outlooks from the Climate Prediction Center project that El Nino could persist through the winter months and even strengthen a bit.
See the image below as to how El Nino can enhance the southern jet stream. Notice the very wet pattern along the coast, but drier over the Ohio Valley. The Tennessee Valley being right in between or just near normal. Temperatures over the northern Plains being above normal. Click Here for a larger view of this map.
As for the Tennessee Valley, El Nino can have some similar patterns during the winter months. I researched 5 El Nino episodes since 1980. I posted my findings in my blog update from September 22nd.
The El Nino years researched were:
1982-1983
1986-1987
1991-1992
1994-1995
2002-2003
Here are the results of the comparison of those year sets: (Winter Stats)
4 of the 5 El Nino Episodes had near normal rainfall locally
5 of the 5 El Nino Episodes had “measurable” snowfall (trace & above)
4 of the 5 had snowfall of 1″ or more (snowiest 1986-1987 8.5″)
1 of the 5 had below normal rainfall
Some ice events were common in each of the year sets
As I stated above, none of these winters were exactly alike, but the similarities were noted above. And, as I made the forecast for the 2009-2010 winter, I had to keep that in mind.
SNOWFALL OUTLOOK: I said this repeatedly last year that our lack of snow was not due to a lack cold air, but a lack of moisture with the storm tracks steering away from our region. So, as we approach this winter the possibility exists for the storm tracks to flow along the Gulf Coast. This would keep the heavier rain along the coast, but close enough for us to have a little more rain than the drier regions north. Most importantly, the persistent storm tracks to the south could provide us with some better chances for ice/snow than previous winters. Notice how each of the past 5 El Nino seasons had at least some measurable snow, with 4 of 5 having snowfalls of 1″ or more. Ice events were also common during those years. So, for the upcoming winter, I am expecting a slightly higher than normal chance of the Chattanooga area to having ice/snow. Our normal yearly snowfall average is around 4.5″ which we have not achieved in years. I think that we have a decent chance of seeing “normal” this winter. I am also concerned about an ice event or two that has been common during similar winter seasons. I am a Tennessee Valley native, so I like snow just as much as most of you ….hopefully I am not being too biased here in the outlook 
TEMPERATURE OUTLOOK: Temperatures should be near to a little below normal for the upcoming winter. This is a byproduct of enhanced cloud cover and some rain due to the increased coastal activity. December could even start the season with some very cold temperatures. El Nino seasons can sometimes limit Arctic intrusions to the southern US (but not totally). This explains the above normal temperatures over the Plains that are common during El Nino winters. Still, there will be some arctic air at times spilling south. Will there be large swings from warm to cold this winter? Yes, colder periods will be often times offset by warmer periods. That is common around here anyway.
PRECIPITATION OUTLOOK: Precipitation (total rainfall) looks to be near normal. I am a little concerned that it could turn slightly below normal as many of the past El Nino events through the decades have exhibited below normal values over the Ohio and Tennessee Valley regions. Remember, it doesn’t take much moisture for snow or ice. Just 0.10″ of precipitation equals 1″ of snow when cold enough. So, what is typically a day with “light rain” of 0.10″ turns into a wintry day when cold enough.
WHAT ABOUT A WHITE CHRISTMAS?
That is the question that I have been asked a lot lately. My kids even asked me that almost every day. Ever since Bing Crosby made the song “White Christmas” famous, it has become a scene that everyone dreams about. We do have some White Christmas’s in our past. The greatest snowfall total ever recorded in Chattanooga on Christmas Day was in 1969 when just over 5 inches of snow fell. I was 2 years old at the time, and we have pictures of that snowfall from our front yard in Chickamauga, GA. Since that time, just over a trace of snow has been the greatest total.
Here is a list of snowfall on Christmas Day from 1879 to 2008 (Source: National Weather Service, Morristown, TN):
Snowfall on Christmas
Trace 2007 (flurries & sleet)
Trace 2002
Trace 1993
0.2″ 1989
Trace 1976
Trace 1970
5.1″ 1969
Trace 1962
Trace 1961
Trace 1956
Trace 1953
Trace 1935
Trace 1915
0.1″ 1913
1.2″ 1902
0.1″ 1899
Trace 1879
So, based on these stats, our yearly average chance of snow on Christmas is just under 10%. Hey, it’s better than no chance at all!…Of course this far out, it either will of won’t, so the 50/50 odds could apply!
Well, there you have it! Only time will tell what will transpire over the next 3 1/2 months. As I have done every year, we will check back on this forecast in mid March and give it a grade. I enjoy doing this whether it is right or wrong! And, I’ll have another winter forecast this time next year. Thanks for reading!
David Glenn
StormTrack 9 Chief Meteorologist
