The meteorological spring 2021 is here, and so is the new tornado season in the United States. This year, colder than normal Pacific could lead to increased severe weather events and above-average tornado activity. The recent cold delayed the season, but the activity will soon ramp up.
So far, we are seeing a quite slow start of the storm season. But the severe weather activity has started earlier than usual over the recent years. And what the weather models are now trending, this year’s tornado season could again be an above-average this spring.
The above-normal sea temperatures over the Gulf of Mexico play a very important role each year, as they provide one of the most important ingredients, the fuel for the storms. The other ingredient needed is the jet stream, winds in the upper-level parts of the atmosphere. Those are forecast to remain stronger than normal through the coming months.
A combination of very moist and warm lowest levels of the atmosphere and the enhanced jet stream aloft, bring these ingredients together and the result is organized severe storms, including tornadoes. Those normally increase during the spring months, from March through June, this is the heart of the tornado season.
As of March 1st this year (the chart above), there were 24 tornadoes reported across the United States so far. Including two destructive tornadoes with 4 fatalities (one in Alabama on Jan 25th and three in North Carolina on Feb 15th). This is indeed a very slow start, about 20 percent of a typical ~116 tornado reports during January and February together.
But thanks to the below-average sea temperature across the tropical central Pacific waters, the spring months could trigger above-average and a significant tornado activity this year. These changing Pacific waters, known as ENSO (El Niño–Southern Oscillation) is a natural cycle and is known as one of the main drivers for the weather dynamics around the Earth.
Such weather pattern across the Pacific Ocean and North America continues as we’ve just ended the harsh meteorological winter across a large part of the United States. After the Polar Vortex collapse in early January, the winter events have ramped up across the United States. And lead to a historic and deadly cold outbreak across the southern Plains through mid-February.
The global weather models are lately trending that similar pattern activity will persist through March and April this spring. Which fits well into the general effect of the cooler Pacific leading to a more robust tornado season and a higher frequency of severe events in the US.
Both well-known areas, the Tornado Alley and the Dixie Alley tend to get more dangerous severe weather when cooler Pacific waters are present. And is such years, a tornado season starts also brings about 4-6 weeks earlier activity and tends to be more active than normal.
Indeed, we have to be aware that no seasons are alike and their activity can vary significantly from year to year. Some years started early, some started late. As we have seen above, 2021 might have a very slow start, but that could change soon. The latest model guidance hints at an increased chance for severe weather next week, as a typical pattern establishes across the Contiguous US.
The attached animation hints at a more progressive pattern around March 10th, with some troughs and frontal emerging into the Great Plains and southern portions of the country from the west. Normally, this brings enhanced threat for dangerous severe weather.
Dixie Alley and the low-mid Mississippi Valley could be one area to focus on this year and with the cooler Pacific, these areas receive more tornadoes than during a normal year.
What are the odds that the historic cold outbreak, putting a large part of the country into a deep freeze last week, will play a role in the future patterns through early tornado season activity this year? Let’s first see what are the main ingredients for these dangerous weather events.
MAIN INGREDIENTS: INSTABILITY, JET STREAM AND SHEAR
Thunderstorms form when surface heating results in upward rising warm and moist air mass, typically along with some kind of the existing frontal boundary(e.g. cold front, etc.). When air parcels rise, the air condenses and creates precipitation. If you hear thunder and lightning, a thunderstorm is born.
When a more robust environment sets, the ingredients result in more volatile storms and therefore also potentially dangerous weather events. That’s when severe storms come into play. The most dangerous storms are supercell thunderstorms. Supercells are the most severe and often lead to flooding, destructive hail, winds, and tornadoes.
The main ingredients determining the type or intensity of the potential severe weather (storms) are usually the available energy, known as instability, and the winds. Both low-level and upper-level winds, and the wind shear.
To simplify things: the higher the instability and winds/shear are, the more robust and organized/dangerous storms will develop.
When we talk about the instability, we have in mind the potentially available energy the environment has to fuel the storms. The more energy is in place, the more vigorous convection and storms can develop. Sometimes even explosive development of the storms where huge amounts of energy are present.
This is where the seawater temperatures of the Gulf of Mexico are the important factor. Those have a direct influence on the tornado season activity across the South and Southeast parts of the US due to much higher energy building up. The southward region is known as the Dixie Alley.
The source of a very moist air mass actually comes from the deep south tropical region, as far as from the seawater of the Caribbean region. During February this past winter, the water temperatures over the region were more than 1 degree Celsius above the average.
This is quite a concerning and important signal for the upcoming tornado season as this high moisture is then gradually spread into the United States mainland during the spring months, starting in March and increasing through May and June. So the energy could get very extreme once the stronger warming starts through March and April.
Let’s remember the fact: The higher the moisture and temperature are present, the higher energy will accumulate. In other words, this directly translates into more robust convection and severe type of thunderstorms.
Jet stream and wind shear
The second ingredient needed for storms to get going and organize is the wind throughout the whole atmosphere. From the low-levels to the upper-level parts. The winds in the higher levels are known as the jet stream.
The jet stream brings strong winds and also enhances the wind shear. Shear is the change of wind speed and its direction with height. Wind shear is, besides instability, a very important key in severe thunderstorm environments. Changing winds with height lead to a rotating updraft of the storms, helping them to sustain and live longer. Supercell storms form in those conditions.
Every year, as the winter transitions to the spring season, these upper-level winds begin their typical retreat towards the north. This year, the jet stream stronger than usual. It also amplifies the winds closer to the ground, which will help to pump the moist and very warm air mass from the Gulf of Mexico farther north.
The chart above indicates the typical late winter/early spring climatological activity of the severe weather across the US. With the jet stream amplified over the CONUS, the highest frequency across the South and Southeast parts of the country typically increase.
According to the studies of the tornado environments across the Tornado Alley, these upper-level winds tend to be stronger in early spring, in March, and April when the Pacific waters are cooler than normal. And that’s why these patterns have such an important role in predictions of what to expect during a tornado season this year.
COOLER PACIFIC WATERS, SIMILAR TO 2011
As mentioned earlier, spring severe weather and tornado seasons do vary from year to year. So we also need to monitor earlier activity during the winter months to get the first idea. For example, if we compare 2021 and last year, there was no significant cold outbreak in the late winter while we had a very significant, long-lasting cold blast lately.
But there is one interesting, and potentially dangerous fact to keep in mind; the tropical Pacific waters are similarly cool if we compare those with the infamous year 2011. Remember, April 2011 was very extreme with deadly tornado outbreaks which spawned more than 700 tornadoes in this single month.
The so-called “2011 Super Outbreak” (April 25–28th, 2011) was the largest, the costliest, and the most expensive meteorological disaster for the US. It was also one of the deadliest tornado outbreaks in American history on record. Leaving catastrophic destruction across the Southern, Midwestern, and Northeastern United States. In just a four-day span, violent supercell storms spawned three EF-5 tornadoes in addition to 12 EF-4 and 21 EF-3 tornadoes.
You surely have heard about April 27th, 2011. A day we all want it has never happened. This day remains entitled as the deadliest day for tornadoes since the “Tri-State” outbreak almost 100 years ago, March 18th, 1925. Leaving 316 people dead on this single day.
So, quite similar to the meteorological spring of 2011, the cooler Pacific waters are about to remain through at least April or possibly also during May this year. So persisting right through the core timeframe of the US tornado season. If we compare both years, even more, there are some additional interesting facts.
Both 2011 and 2021 have had a rather slow start (remember the chart we attached earlier). And we see what April exactly 10 years ago did then – by the end of the month, there were already more than 1200 tornadoes reported in the United States. 2011 kicked-off extreme in April.
TORNADO SEASON SLOW START, BUT RAMPS UP SOON
To be honest, we can blame the frigid and destructive cold outbreak for the bad weather in the recent weeks, but it might be a lifesaver at one stage as well. As there were strong dynamic weather patterns across North America since mid-late January, convective activity and frontal systems were lacking rich moisture and severe weather.
The consecutive widespread Arctic cold outbreaks have pushed the frontal systems and storm tracks much farther south than usual, so most of their activity was limited to the Gulf, the Gulf Coast, and Florida.
Statistics are clear and show that the tornado season has so far produced *only* around 20 % of the usual tornado reports across the United States. The states of Alabama, Georgia, Florida, and North Carolina have seen a few, unfortunately also severely damaging and deadly.
But we could soon be in for a significant ramp-up once the conditions improve and the environment warms up. If you are a storm chaser, your time is coming soon.
The interaction of the cold air masses coming from the north with the much warmer air mass farther south, lead to frontal systems and convective weather. When we have a transition from late winter into the early spring season, both pressure and temperature gradients between the north and south are the most pronounced.
As we know from the past observations, the effect of the cooler tropical Pacific Ocean posting from winter into spring hints at a strong blocking high-pressure system in the North Pacific that results in the lower pressure to its east, over Canada, and over the United States. This is exactly what has happened this year like the pattern wants to follow the book.
Lately, there was a high-pressure system (marked with H letter) over the North Pacific and another one over the southern US, while a large low-pressure system (L) is over Canada and the northern tier of the United States. In between, a powerful jet stream is blasting high-speed winds. A textbook example of a such pattern over the North American continent.
It seems that the more progressive pattern coming up next week will bring the severe weather and tornado season ramping up through mid-March. With also still a favorable pattern for cold air intrusions south into portions of the US.
Later, trends are turning that season gets much more active as April month approaches. As soon as the cold pools will leave, strong warming will make its role for sure. We can just remember how remarkable warming came over Texas was through the final days of February. Compared to the peak of the deep freeze a week earlier, temperatures jumped for more than 80 degrees F (45 °C) in just a one-week timeframe!
For example, the city of Dallas, TX plummet to -2F (-18.9 °C) ob Feb 15th and rose very fast to an impressive 80F (26.7 °C) on Tuesday last week. The exceptional comparison chart above is provided by Scott Duncan.
ACTIVE TORNADO SEASON LIKELY
If we imagine, warming has never been an issue during the spring months. And if we then think about the ongoing stronger jet stream caused by cooler than normal Pacific waters, we do have a hint at what’s coming up ahead for this tornado season. It is obviously not that simple, of course. But we are likely coming into a very tornado-active spring this year. Might just continue the streak of the April months from recent years.
As the pattern seems to also continue and extend into May, so the tornado season will keep storm chasers very busy this year.
Recent long-range model trends also suggest that the main activity this upcoming tornado season might be well off the traditional Tornado Alley – the area covering the southern and central United States into the Midwest with an annual high frequency of severe weather hazards and tornadoes.
The main activity this tornado season this year might well be shifted into the low-mid Mississippi Valley and the central Gulf Coast. Which, of course, doesn’t mean the traditional Tornado Alley will not see tornadoes but it might keep the numbers below average.
With the gradual northward retreat of the upper-level jet stream above the strong temperature contrast below, the states around the Mississippi river up towards the Tennessee valleys and east to Alabama might be in higher threat through the end of April.
Basically, the area being known as the Dixie Alley, often experiencing deadly tornado outbreaks in the past.
Going deeper into the month of May and June, the typical northward shift of the jet stream will occur. But the winds will likely remain stronger than normal, so the tornado season will probably continue as above-average during the peak season.
The potential area with the highest number of tornadoes this year is a very unfriendly area for the local residents when tornadoes occur. The Mississippi Valley is full of trees and high humidity most of the time, so tornadoes are even more dangerous while residents have a lack of good visibility e.g. on the flat open fields of the Great Plains (Tornado Alley).
Similar activity was also seen in the spring 2011 season, as the Mississippi and Tennessee Valleys were also badly hit with destructive tornadoes.
DROUGHT COULD INFLUENCE THE SEASON
One of the negative factors needed to be monitored is severe to extreme drought conditions across the western portions of the United States. It could influence the upcoming tornado season as the La Nina pattern extends into this spring, so it will likely worsen the drought.
According to the latest US Drought Monitor data, exceptional drought conditions are observed across parts of Colorado, Utah, Nevada, Arizona, New Mexico, and far West Texas. Further below-average precipitation in the upcoming months will also increase fire threats, as well as increase possible early intense heat waves.
So, the ongoing drought could be one of the reasons why the eastward shift off of the traditional Tornado Alley activity might happen this spring. Drier air is an obvious limiting factor for widespread and volatile thunderstorm environments.
As we learned earlier, the high moisture is, besides the high upper-level winds, one of the key factors for potential instability and violent storms to occur.
***The images used in this article were provided by NOAA, Tropical Tidbits, and Wxcharts.