USA severe storm season off with a bang and how La Nina could super-charge it this spring
Blog by Nick Finnis
Issued: 19th March 2021 11:24
Updated: 19th March 2021 15:14

USA severe storm season off with a bang and how La Nina could super-charge it this spring

The UK weather often becomes increasingly benign as we head through the Spring months, as the polar vortex which drives our often turbulent winter and early Spring weather begins to weaken and correspondingly the jet stream driven by it weakens and begins to meander more, bringing more chance of high pressure establishing – like this week.

However, across the pond in the U.S.A, spring is the beginning of the severe storm season across large swathes of central, southern and eastern U.S.A The severe storm season tends to start off across the southern Plains and Deep South in March and April before extending further north across the central and northern Plains through May and June. This year could see more severe weather and tornado activity than usual due to La Nina.

The Texas Panhandle in the High Plains of NW Texas saw a number of strong tornadoes last Saturday, at least 8 tornadoes touching down, two rated EF-2 on the Enhanced Fujita scale - packing winds of up to 120 mph. Fortunately the tornadoes rampaged across rural areas, with no deaths and only a few injuries were reported, though power poles were snapped, some homes were damaged and a few lorries caught out were flipped over.

Wednesday this week saw a more widespread and dangerous life-threatening severe weather risk across the Deep South, with a tornado outbreak across parts of the states of Mississippi and Alabama. It prompted the U.S National Weather Service’s Storm Prediciton Center (SPC) to issue a rare ‘High Risk’ of severe weather, the first March high risk since 2012. A rare 45% probability of tornadoes (probability of a tornado within 25 miles of a point) was issued for central eastern Mississippi and central eastern Alabama. This is only the 6th time in the history of the convective forecasts of the Storm Predicition Center that a 45% has been issued for tornadoes. The previous five occasions were on April 7 2006, April 7 2011, May 24 2011, April 14 2012 and May 20 2019.

23 tornadoes were recorded in the SPC storm reports, mostly across Mississippi and Alabama. The twisters, some violent, tracked across often wooded terrain in these states, flattening numerous trees, snapping power poles, downing power lines and damaging some homes and farm buildings. Some injuries were reported and some properties were flattened. Fortunately no deaths have been reported, with people generally taking heed of tornado warnings and seeking shelter. Unfortunately these high risk severe events do lead to fatalities, particularly when a strong tornado hits populated areas and people are caught out. Fortunately Wednesday didn't turn out as bad as feared.

Severe thunderstorms (and tornadoes) can occur at any time of year, but it’s spring that produces more severe storms in the U.S than other seasons. This is due to the differing temperature contrasts at different times of year. Atmospheric instability, where warm air at the surface is under much colder air aloft, which leads to surface-based thunderstorm development and tornadoes, is greatest in spring. This is because the surface is warming as the sun gets stronger through spring, but it’s still cold aloft. The opposite is true in autumn – when it gets progressively cooler at the surface but it’s still warm aloft.

Although severe thunderstorms in the U.S. can bring damaging large hail up to the size of baseballs, flash flooding and damaging wind gusts over 70mph, it’s the tornadoes that can produce that cause most damage and deaths. Tornadoes can occur at any time of year, but they are more common during a distinct season that begins in early spring for southern states along the Gulf of Mexico. The severe storm season follows the  jet stream as it swings farther north, so does tornado activity. May generally has more tornadoes than any other month, but April's twisters are sometimes more violent. Farther north, tornadoes tend to be more common later in summer.

Despite the current outlook suggesting no severe storms for the next 3 days or so, severe storms season and the chance of tornadoes tends to peak from late April through to June across the Plains of the USA. In addition, there is talk that this year's tornado season in the USA could be more severe than usual - due to La Nina..

Studies have found that a fairly strong La Nina can bring more tornadoes over parts of Tornado Alley in the Plains, namely Texas, Oklahoma and Kansas, but also other southern states. 

The climate pattern of La Nina is a natural cycle marked by cooler-than-average ocean water in the central Pacific Ocean and is one of the main drivers of weather around the world, especially during the late autumn, winter and early spring. La Nina tends to concentrate warm and humid air across the southern states of the USA in Spring, helped by a greater tendency for a trough setting up over western USA and high pressure in eastern USA pulling north warm and humd air from the Gulf of Mexico. This sets up strong north-south temperature gradient, which in turn increases severe weather.

The temperature of ocean waters in the central and eastern Pacific at the begining of March has been similar to the La Nina pattern in early March 2011, that year is notorius as being one of the most destructive and deadly tornado seasons ever. There were six EF-5 tornadoes confirmed that year, the highest on the Enhanced Fujita Scale, with wind gusts over 200 mph. The annual number of deaths attributed to tornadoes was 551, an outbreak between 22-27 May saw 180 tornadoes in the central U.S. A deadly EF-5 tornado hit Joplin, Missouri during that outbreak, killing 158 people alone. There were also 3 other tornado outbreaks in April. 

A quiet period of weather for the USA over the weekend and into early next week - with no severe weather forecast, but being early Spring, severe weather will no doubt return soon and with it a risk of tornadoes - particularly across the Plains over the U.S.A, tornado season tends to peak form late April through to June, so a long road ahead.

Tags: World Weather  Severe Weather

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