After two rare high-risk days in March, as defined by the U.S. Storm Prediction Centre, what's in store through the remainder of spring, and for North America's peak tornado season?
We're racing towards the climatological climax of tornadic activity in the U.S. while hanging on to a weak and waning La Niña. But, as history tells us, the effects of La Niña can persist well into the storm season.
Take the 2008 and 2011 U.S. storm season, for example:
As of early April, over 200 tornadoes have been reported across the United States; most of them have fallen outside of the typical 'tornado alley' in the U.S. Plains.
We'll look at a few factors that go into deciphering the tornado risk across the next several months, starting with moisture, which is a very significant one.
A large portion of North America is under severe drought conditions. Nineteen states and a wide swath of the Canadian Prairies are facing significant moisture deficits.
Moisture is a critical ingredient for thunderstorms, and these surface dew point values become paramount in developing a thunderstorm risk.
A higher tornado risk generally exists when a parcel of air only has to rise a few hundred metres before reaching a relative humidity of 100 per cent.
The southern U.S. has an abundance of moisture sourced from the Gulf of Mexico, but severe thunderstorm setups in Canada are a little fussier.
If the upper-level environment isn't favoured to move moist air from the Gulf, evapotranspiration – the sum of moisture evaporated from the landscape and transpired (similar to sweating in humans) from plants – becomes a key player in developing a moisture-laden boundary layer.
Canadian crops play a significant role in carving out a severe thunderstorm season in Canada. Check out the spatial density of corn.
Why is it important? One hectare of corn can emanate over 40,000 litres of water into the atmosphere! Holy corn sweat.
Say you were concocting a continent from scratch. You couldn't have designed a more robust tornado-making continental machine: moist low-level air from the Gulf of Mexico, a hot, dry elevated air mass compliments of Mexico – plus cold, Canadian air masses to the north and west.
Let's dig into the air mass from Mexico a bit more. This unique North American feature makes the continent a global hot spot for tornado activity.
Meet the elevated mixed layer. You might have heard the term 'cap' used in the lexicon for thunderstorm inhibition, which features warming temperatures with increasing height.
The Mexican Plateau is a different beast. It provides another ingredient besides warm temperatures: low dew points as well.
Extrapolating the drought risk into Mexico, the Mexican Plateau is a heightened topographic region between the Sierra Madre Occidental and Oriental mountain ranges. The dry, dusty environment averages 2000 metres above sea level. A prevailing southwest flow acts as a conduit to deliver this once surface-based environment well above the elevation of the U.S. Plains.
Only the most prolific thunderstorm setups can overcome drought conditions and an enhanced elevated mixed layer in 2021.
The U.S. National Weather Service has an excellent set of resources to analyze the different ENSO states – that's El Niño Southern Oscillation – and the impact on tornado watches.
Which regions see heightened alerts during La Niña vs. El Niño?
The heart of the April 2021 storm season appears to be unusually docile, particularly when you look at the upper-level forecasts over the next couple of weeks for severe weather.
The setups will likely feature more subtle dryline nuances and subtle mesoscale features – not the classic bowling ball upper troughs over the four corners of the continental U.S.
Where is the deck stacked in May? As you can see, the risk favours where the moisture source is, with regions north of the Gulf of Mexico likely to see some heightened tornado activity through May.
SEA SURFACE TEMPERATURES
This factor is likely to play a small role in the severe weather setups over the next few months.
Let's face it, even a couple degree temperature anomaly in the Gulf of Mexico is still relatively warm and moist. Perhaps that limits the amount of warm, moist air that's effectively transported into the Canadian Prairies.
A chillier sea surface anomaly has plagued the Pacific Northwest and coastal California. The dominant driver appears to be the developing drought conditions in providing ample feedback to build persistent ridging over California.