You may have heard we are heading right into a La Nina this winter, which has some noticeable effects on our weather.
First off, typically a La Nina pattern brings warmer and drier than normal conditions through most of Winter.
Here’s how the CPC sees it for us this winter:
To set up the rest of the article…
A seasonal forecast of warmer/drier doesn’t mean we never get cold or that it never snows . People tend to take things too literal when it comes to climate prediction (much like with Climate Change science) as there will be variations even through the winter versus the average.
A prediction of warmer and drier simply means the most likely outcome if you took all the high temperatures and all the precipitation this winter into account, we’d average slightly warmer and drier than our average through the winter months.
That’s all to set up the next piece of this article…
So Warm Winter means big tornadoes?
So warm and dry means a nasty tornado season when it comes to Spring right?
Thus far, the CPC is forecasting this La Nina to be a weak one.
Historically that’s the most common type of La Nina as you can see on this chart:
As you can also see, this past Spring, 2017, was a weak La Nina as well.
So how do we fare when it comes to La Nina’s and tornadoes in Oklahoma?
Overall our average (regardless of ENSO cycle) looks like this:
- 48 Total Tornadoes (February – June)
- 1 – February
- 4 – March
- 12 – April
- 23 – May
- 8 – June
When you compare that to an average La Nina tornado season, you see a lot of familiar numbers.
- 48 Total Tornadoes (February – June)
- 1 – February
- 4 – March
- 15 – April
- 20 – May
- 7 – June
We’re Not that different after all
A typical La Nina Spring is about on par with average. Remember what we talked about earlier though! Average means that some Springs are going to be quiet and some will be busy. La Nina tornado seasons play this trend out.
The high for tornadoes in a La Nina was 130 tornadoes during the Strong La Nina of 1999.
The low for tornadoes in a La Nina was 16 tornadoes during the also Strong La Nina of 1989.
This points to a few things, the first is that La Nina isn’t the only driver of tornado activity from February – June if there’s that much variation — but we can work with averages to get a decent view of what a typical La Nina spring may point towards.
Overall, La Ninas tend to edge towards:
- About average February and March tornado totals.
- Slightly higher Aprils.
- Slightly lower Mays.
- Slightly lower Junes.
So What About A Weak la nina?
When you look at the numbers for weak La Ninas, you come up with a bit of a different picture than the overall. This is partially due to some very big years from strong and moderate La Ninas skewing the averages higher.
Here’s the numbers for weak La Ninas:
- 41 Total Tornadoes (February – June)
- 2 – February
- 3 – March
- 13 – April
- 18 – May
- 6 – June
Thus, the averages of weak La Ninas point towards:
- Lower than average tornado counts in Oklahoma.
- Slightly more active February (our most active February ever was from weak La Ninas)
- Less active March vs. average.
- Slightly above average April.
- Quieter May and June.
So we’re obviously going to have a much below normal spring right?
Again, that’s where you have to take averages as just that.
The high for weak La Ninas was 2017 with 72 total tornadoes.
The low count for weak La Ninas was 22 in 1972.
So again there’s a lot of variation there and there’s a wide range of results possible.
Why Tornado seasons are kinda hard to anticipate
The big reason for tornado season forecasts being exceptionally difficult is that a singular event can make or break a years tornado count. Thus, anticipating the total tornado count because of one thing (La Nina) makes this a very inexact exercise.
The big case in point is May 3, 1999. If you simply make that year’s May closer to the average strong La Nina tornado count for May you effect the data in a big way.
Suddenly May is well below normal throughout all La Nina years and the overall average is also clearly below normal for La Ninas.
One event moves the needle that much.
If you play with the data and remove the outliers both at the bottom and the top you come up with a different chart than the one above.
For the purposes of this chart I removed the highest and lowest tornado counts from the strong, weak, and moderate categories.
In all of these cases the results are ‘smoothed’ a bit towards the average by eliminating outliers.
The best way to put this is that ingredients will come together on a certain number of days every spring in Oklahoma for tornadoes (impossible to tabulate here). This is probably affected by big picture ENSO stuff by some order of magnitude.
However, due to smaller scale functions of the atmosphere, year to year the number of days the atmosphere fulfills its tornado potential vs. others can vary greatly. If the cap was a little stronger on May 3, 1999 or if winds didn’t locally back as much or if it were weaker on a few days in 1989, the data could be much different in each year but likely not as much when an average is taken into account.
So all that being said…
So A Weak La Nina Means…What for Tornadoes in OK?
On average (there’s that word again) a weak La Nina tends to see a 14-15% lower than average tornado count in Oklahoma from February – June.
Weak La Nina years trend towards average through April, but tend to be much lower than normal the back part of the season from May into June.
Its probably incorrect to say a weak La Nina season gets off to a fast start versus it just does what a typical season does until May, when the numbers tend to fade in the last half of the year.
There are exceptions to this within weak La Ninas though. This past Spring (2017) is the best example of this. April of 2017 was very quiet (10 tornadoes) while May was very active (57 tornadoes).
On the flip side of that tough, there are more years where the trend of a more active April versus May plays out within weak La Ninas. Some examples of that:
- 2009 (17 in April vs. 4 in May)
- 2006 (8 in April vs. 4 in May)
- 1985 (10 in April vs. 6 in May)
- 1984 (23 in April vs. 9 in May)
What is really interesting is that the most active weak La Nina years are all the ones that come through with bigger May counts.
- 2017 – 57 Tornadoes (72 Total)
- 1955 – 32 Tornadoes (70 Total)
- 1965 – 32 Tornadoes (64 Total)
As with any other cycle, Oklahoma’s tornado seasons seem to be made or broken typically by what May does.
So for storm season 2018, a weak La Nina tends to say that we’ll be about average through April before seeing a falloff of activity in the latter part of the season, ending below normal for the Spring.
Will that play out? We don’t 100% know, but the odds of a quieter than normal Spring in Oklahoma are probably slightly higher than a busy one if the weak La Nina does happen as forecast.