May 2020 Tornado Outlook

If you really wanted to, you could chase storms all year long.

No matter what month you choose, if you try hard enough you can find thunderstorms somewhere in the United States. Even in the winter months of January and February, thunderstorms occur (most typically along the Gulf Coast states). But it’s when the cold nights of winter give way to the warm afternoons of Spring, when the sunsets get later and the nights stay warmer, that severe thunderstorm and tornado season really comes to life. And there’s one month in particular which features more tornado activity than all of the others; the one month that storm chasers look forward to all year long – May. 

The month of May features the most tornadoes on average across the United States. A lot of that has to do with the evolution of the jet stream across the hemisphere during this time. As the seasons begin to change, transitioning from Winter to Summer, atmospheric ingredients necessary for severe thunderstorm development and tornado outbreaks sometimes come together in the Central United States, particularly in the area known as Tornado Alley.

Tornado outbreaks, in general, occur with an intricate alignment of moisture, instability, lift and wind shear. Typically, troughs of lower heights in the atmosphere dig into the Western United States, with a jet stream spreading into the Plains or Midwest. Warm and humid air flows northward from the Gulf of Mexico. This moisture aids in the development of instability, while the incoming disturbance from the Western US leads to favorable wind shear and lift. 

When these systems are timed just right, all of these ingredients can work together to aid in the development of some of the worlds most powerful thunderstorms – supercells – which form in these pristine atmospheric environments. These powerhouse storms can produce gigantic hail, flooding rainfall, damaging wind gusts, and of course tornadoes. Read more about these ingredients here.

So, how do we predict these systems in advance? Is there any way to tell if a certain May could feature more or less of them than others? The answer is yes, but the way we get there is complicated and the results are not always simple.

Long range forecasters look at a variety of factors to make a forecast. The process can vary from month to month, or even from year to year. For context, for this months tornado outlook my research included but was not limited to:

1. The current state of the atmosphere around the globe (including ENSO, Pacific Ocean sea surface temperatures, the stratospheric polar vortex, tropical forcing and the MJO)
2. The evolution of the pattern over the past several weeks and months (i.e, how did the global weather pattern evolve throughout the Winter months)
3. The suggested and sensical evolution of several major indices and factors such as global wind, the MJO, and the Pacific Jet stream
4. Analog years, or years in the past that are a loose match to general atmospheric conditions and progressions across the globe so far in 2020

As you might imagine, putting this all together to something digestible is quite complicated. A long range weather forecast is like a puzzle that always seems to need more pieces – meteorologists often are constantly adjusting and tweaking their processes. 

Let’s break this down more simply by starting in the present: Where do we stand today?

As of the time of this writing, we have just emerged out of an active pattern for severe weather and tornadoes, specifically in the Southeast US and parts of Dixie Alley. In the next 7 to 10 days, essentially during the entirety of the first half of May, forecast models suggest that the weather pattern will become extremely hostile for severe weather outbreaks or above normal tornado activity. 

Why? High latitude blocking and generally unfavorable tropical forcing regimes will aid in the development in a deep, anomalous trough over the Northeast United States and Great Lakes. Abnormally cold air will surge southwards from Canada into these areas, leaving the Plains with a dry, northwest flow. The pattern in the Pacific Ocean will also remain unfavorable during this time period, with disturbances riding over the top of a large ridge in the Western US – only enhancing the intensity of troughing in New England. 

From a more technical standpoint, the evolution of the MJO/Strong East Asian mountain torque event is leading to significant changes in the waveguide throughout the North Pacific. Final warming of the stratospheric polar vortex is also aiding in variability in the high latitudes, with ridging developing in both the NAO and EPO regions at times over the next two weeks.

As a result, the expectation is that the first 15 days of May will be rather quiet in terms of tornado potential. As this is occurring, forecast models are hinting that substantial changes will already be ongoing across key areas of the tropical Pacific ocean and North Pacific Ocean.

Ensemble guidance is suggesting that tropical convection which has been rooted over the Western part of North America will begin to shift. Convection near the Indian Ocean (IO) is expected to become more dominant during the latter half of May. While it remains unclear if this wave will propagate eastwards towards the Mid-Continent or Dateline, ensemble support is strong that IO convection will remain persistent. This signal (low frequency, IO dominant forcing) is historically associated with West Coast troughing with a 10-14 day lag time.

There will initially likely be some resistance as the tropical forcing evolves with a stubborn area of convection remaining further east closer to Western North America (interference). Still, confidence is rising in the eventual development of the mentioned IO convection, plus an extension of the North Pacific Jet stream and a gradually falling AAM/at least some removal of AAM from the overall NHEM budget closer to 5/20.

It is important to note that this evolution is not “classic” in terms of historical sequences associated with tornado outbreaks. For such an event, given the current atmospheric base state, we would be looking for a more amplified and coherent MJO propagation supporting GWO re-orbiting with amplification through the 8-1-2 space and a rapidly falling AAM (Weickmann, Berry 2008)

Some uncertainties still exist in regards to the nature of the split flow and behavior of the overall mid and upper level pattern during this time. In particular, it will be important to note and understand how the mid and upper level winds evolve out of the North Pacific Ocean over the next few weeks, and how the potential split flow behaves in Western North America. For instance, troughing and/or the most potent jet streaks could remain too far west for widespread severe weather in the plains, or a jet extension could help shift deeper troughing into the Southwest US.

Additionally, low frequency tropical forcing signals create inherent uncertainty as well. As my colleague Armando Salvadore has mentioned in our long range discussions this year, a sort of “dual-forcing” signal has emerged in the past week or so, where the low frequency nature is almost putting a damper on the tropical forcing propagation. Weickmann and Berry (2008) discuss how dateline forcing and IO suppression is in fact most helpful for a tropic –> ET wave train.

Still, it’s the current suggested tropical forcing progression that will actually eventually help lead to the notable pattern change. Orographic forcing and diabatic heating within the developmental processes of the IO standing wave that we discussed above, and the progression of tropical forcing throughout the hemisphere, will eventually lead to a substantially changed pattern across the Northern Hemisphere.

The forecast “playing field” starting around mid-month has plenty of potential for severe weather in the Central US, especially if and when the zonal extension of the Pacific jet stream occurs closer to the months third and fourth week. This would lead to gradually lowering heights on the Western US coast, with broad southwesterly flow approaching the Plains perhaps as early as May 20th, and a synoptic pattern generally more favorable for severe weather and tornado activity.

As a result, the May tornado forecast looks something like this:

May is has begun with below average tornado activity and a period of below average activity is expected to continue for the next several days (You Are Here). This will likely lead to below average tornado counts for the first 10 to 13 days of the month as the favorable hemispheric weather pattern continues. Later in the month, a substantial pattern change is anticipated that could lead to a notable uptick in tornado potential.

Both the third and fourth weeks of May could very well feature above average tornado activity, but it remains unclear exactly how the timing of the pattern change will evolve. There is also some building evidence that the active pattern could persist into the first few days of June, but the duration obviously remains an uncertainty at this time. The most important piece of this forecast, still, is a significant mid-month pattern change that will feature a notable uptick in severe weather and tornado potential across the Central United States.

If you are a resident of tornado alley, while we are still at least 7-10 days away from significant activity at the time of this post, now is a great time to review your severe weather plan for late May, as you always should. It’s peak tornado season, so you and your family should have a plan and a safe place to go if a severe weather event does unfold.  The National Weather Service has some great tips for developing a safety plan here.

If you are a storm chaser – take some time to rest up, and take heart in the likelihood of increased severe weather activity after mid month.

That’s all for now. See you guys out in the Plains soon!