The most deadly and destructive tornado outbreaks in recent history in 1974 and 2011 were in La Niña years. Not all La Niña years have bad tornado seasons, however. Multiple conditions are required for a destructive tornado season. First, warmer than normal sea surface temperatures in the Gulf of Mexico and western north Atlantic ocean are essential. In La Niña years, the jet stream is contracted around the pole and in El Niño years the jet stream extends southwards towards the equator. The contracted jet stream in La Niña years brings warm Gulf air masses further north than normal into the midcontinent. These warm humid air masses are like fuel for severe storms, but the conditions have to be right to ignite the fuel. American and international models are in strong agreement that the conditions will be favorable this spring for igniting the unstable air masses into severe storms that may bring multiple destructive tornado outbreaks.
The IMME forecast map above shows a wet Pacific northwest (PNW), a wet Ohio river valley and dry warm weather over Florida, the southeast and desert southwest. This map implies a storm track that dives down from the PNW into the central Mississippi river valley. “Inside slider” storm systems that dive down into northern Nevada and Utah from the PNW can be powerfully destabilizing to warm air masses built up over the southeast into the midcontinent. That’s what happened last December when the very unusual strong tornado outbreak devastated towns in Kentucky and the Tennessee river valley. Multiple models are forecasting similar conditions this spring, starting in March.
There are modest differences between models but the strong general agreement is based on the physical conditions associated with La Niña and the continuous ongoing increase in heat in the upper ocean. Sea surface temperature patterns, after months of fall and winter storms, reflect heat patterns that extend hundreds of feet — over 100 meters — into the upper ocean. That’s why these different models have good agreement and why they may be useful in forecasting a season ahead.
The American CFS model predicts a jet stream pattern and a strong Bermuda surface high that will force a strong low level wind jet up the high plains on eastern side of the Rocky Mountains. Jet stream waves that whip down from the PNW into the central plains in this pattern will have powerful wind shear and abundant warm humid air in front of them.
The general agreement between models is quite stunning and disturbing. The details of April and May will depend on how the stratospheric polar vortex breaks down as it does every year in late spring. The vortex has had multiple stretching events this winter, and those stretching events led to cold air outbreaks in north America, but it stretched without breaking down. That may lead to an energetic early end warming in the stratosphere. That could impart additional momentum to the jet stream and that momentum might impart additional energy to storms in May.
The ocean heat and atmospheric circulation patterns that led to the unprecedented tornado outbreaks in December are forecast by multiple models to evolve into a similar situation that could lead to multiple catastrophic spring storms and a much more active tornado season than normal. Multiple models forecast the easterly displacement of the stormy region that might be called tornado alley.
Late Update: For more information why last year, a La Niña year was not a more destructive year for tornadoes there’s an excellent piece written on NOAA’s climate website. climate.gov
The worst La Niña years for tornadoes had a cold Pacific decadal oscillation (PDO) signature in the sea surface temperature and atmospheric circulation patterns. That signature is a cold tongue of water pushing down the west coast of Canada into the Pacific northwest. That’s what we see right now.