As winter begins to draw to a close and leaves us with all of those memories of Alaska being hit by snowstorm after snowstorm while the rest of the United States wondered when winter was going to arrive, I find myself wondering about what this spring may hold in store after such an abnormal winter. Below is an image produced using data from Rutger’s Global Snow Lab which was featured in the January 2012 State of the Climate issued by NOAA.

The image depicts the January snow cover extent anomaly in thousands of square miles based on the 1981 to 2010 average across the contiguous United States. Unfortunately the years are off by one, as the first year on the right should read 2012 as we are all aware this year was well below normal for snowfall and the previous two years were well above normal. This simple mistake aside, I wondered how might this lack of snowfall affect the forthcoming severe weather season if we were to do a simple test, science aside, and compare other recent years with below average snowfall, like 2006.

The image above shows the last seven years of tornado reports as well as the 2005 to 2011 average and this year’s reports so far. Taking a look at 2006, we see that it was the third highest year for number of tornadoes up to this date with only this year and 2008, a year with above average areal snowfall coverage, having more tornadoes reported. However, we see that 2006 ended up being the second lowest total for the number of reported tornadoes out of these years. While snow cover may not have any direct implications to severe weather likelihood, there may be an argument in how it affects the amount of moisture available and the temperature gradient needed for severe weather. The temperature gradient argument is the stronger of the two, stating that with less snowfall in the north, temperatures would be warmer, typically creating for a weaker temperature gradient and thus less severe weather outbreaks. Taking a look at this logic, we see that 2010 and 2011, years with above normal areal snow coverage, both ended up with higher numbers of tornado reports but they achieved these numbers in very different fashions, with 2010 recording most of its tornadoes much later than 2011. So maybe the below average snowfall this winter has no predictive power on the intensity of this year’s severe weather season. But there are other factors we should also consider.

The ENSO cycle has long been considered to potentially hold some power in predicting severe storms. It is common knowledge among meteorologists that a La Nina tends to slightly favor more storms in the spring, especially across the Southeast, and an El Nino tends to slightly favor more storms in the summer months. Current conditions from the Weekly ENSO Update show that we are currently in a La Nina type pattern which is expected to continue into spring. While this particular La Nina pattern is not nearly as strong as last year’s La Nina, which lasted from the summer of 2010 into the spring of 2011, could it bring typical La Nina associated severe weather patterns to the United States for this spring?

With last year’s record outbreaks occurring during a La Nina, it would be easy to jump to the conclusion that this year would result in more extreme severe weather outbreaks, especially with the pace the season has got off to. However, we must remember that this year’s La Nina conditions are not as bad as last year’s conditions and in fact, what we are currently in has not even been classified as an official La Nina. An official La Nina must have sea surface temperatures in the Pacific region used to measure the ENSO cycle at or below 0.5 degree Celsius below normal for at least 5 consecutive 3 month averages. This means the La Nina conditions would need to persist through the JFM (January, February, March) cycle which they are modeled to do but even so, as stated, it would not be at the intensity of last year’s La Nina. That being said, the ENSO cycle would lead one to believe that we could be looking at a strong severe weather season but this is far from a set in stone prediction.
Returning to temperature gradients and moisture now, the above image depicts the current sea surface temperature anomalies across the Caribbean. Of interest here is the warmer than normal temperatures in the Gulf of Mexico, the source of moisture and strong southerly wind flow during severe weather season. With warmer than usual temperatures in the Gulf, the temperature gradient would be stronger creating for stronger winds and therefore increased chances of severe weather. These warmer temperatures are due to the fact that cold air has struggled to enter the United States this year, allowing the Gulf to remain relatively warm. Temperatures in the Gulf were also above average last year though we did not see this anomaly start occurring until March and by April, sea surface temperatures were well above normal as depicted in the image below for April 28, 2011.
In fact, this time period is exactly when we started to see severe weather reports really take off last year. Investigating this further, it appears that a warmer than normal Gulf does show some correlation with increased storm reports. Monitoring of this area of sea surface temperatures should be continued into severe weather season. If the temperatures continue to remain above normal and even become increasingly above normal, we could be seeing an indicator for strong severe weather outbreaks to occur.
One last measure I will look at is the NAO which tends to be more positive during severe weather outbreaks. Once again, this generalization does not always hold true but it can be useful to look at for short term consideration of severe weather probability. As can be seen in the models below, the NAO is forecasted to remain at least weakly positive through about the end of February, meaning that the track of the jet stream will remain farther north than normal over the Atlantic region. The models do not forecast out farther than this but taking a look at the history of the NAO over most of this winter, and assuming that this trend may continue, we can expect that severe weather outbreaks would be favored in this pattern.
So what can we expect this spring in terms of severe weather?
Well, early indications would point to another active severe weather season thanks to La Nina conditions, the NAO continuing to remain positive, and warmer than normal sea surface temperatures across the Gulf of Mexico. However, it is important to note that severe weather cannot occur without the correct synoptic setup and none of what I have looked at has taken a true look at this due to its inability to be predicted accurately in the long term. Therefore, the predictions above may not pan out this spring if synoptic conditions conducive for severe weather do not form or form often enough. It will be important to remain up to date on the latest conditions of the variables mentioned in this article as well as the synoptic scale conditions across the United States as we continue to inch our way into this year’s severe weather season.