The lack of a tornado season in 2012 explained
Drought has covered much of the nation.With the changes in weather patterns that created this drought, it also eliminated much of the tornado ingredients across the country. With that in mind, tornado counts are much lower than previous years in 2012. I will briefly touch on this year compared to the last few and discuss a few probable reasons for differences.
The main source of much of the rain across the central US is the convective season during the months of April to August. This is especially true for Tornado Alley where the bulk of storms come through during clashing arctic cold and tropical warm air masses in the months of April through June. This period, the focus for thunderstorm activity is also the main time-frame of tornadoes for which the region was named for. This year, without thunderstorms/rain, there weren’t many tornadoes either. Here is the tornado map for this year to date:
Now, looking at the tornado graph in the big image above, examine the lack of tornadoes in the month of May (we will focus on this month since it is near the typical peak). Compare that to the total rainfall for the month (top right) and associated deficits for each state within the central states. The lack of rain can be connected to the lack of cold air plunges southward in mid-to-late spring that help develop strong systems. We can see the lack of cold air (top left) in the average temperatures for each state as well with above normal temps for the majority of the Plains reflecting the stagnant air mass.
Now compare these to 2011′s maps for May below. There were many more cold air intrusions that helped temperatures average lower than normal and precipitation values in excess for many Plains states. This is reflected in more thunderstorm activity in the central US as well that helped in the development in more tornadic activity in the month of May.
The lack of competing air masses and cold air intrusions in the month of May is connected to the upper level pattern. A large ridge sat over the central states that prevented any large troughs from pushing eastward across the Rockies. These are the main tornado makers in the Tornado Alley that permit cold arctic air to surge southward. The upper level patterns were touched on here:
As the jetstream begins to shift farther south with the nearing winter, troughs across the US will become more commonplace and cold air will surge southward (shown above). If the Gulf can remain warm, which seems likely, and strong southerly flow can develop ahead of the trough, clashing air masses could occur this fall. This is typical with a small “second season” of activity and we could see a few more episodes of tornadoes across the central states. This could also relieve the drought in places, but at this time, it is to far out to tell if this can occur.
One final consideration is how this year, the typical seasons seem to be off somewhat. Each season seems to have started up to two-three months ahead of when the typical meteorological seasons occur. For instance, warm temperatures arrived very early for much of the US in March and April. Winter almost seemed to never have occurred and much of the summer-like temperatures/patterns were already happening across the states by May. This was reflected in the tornado count as well with an active early spring/late winter and a very quiet late spring and early summer. You can see these trends on the following chart.
The 2012 tornado count is trending below most year’s tornado counts and is actually getting dangerously close to the minimum. This is thanks in part to a very quiet storm pattern starting about mid-May which has basically taken out the last half of tornado season.