The National Oceanic and Atmospheric Administration's Storm Prediction Center (NOAA SPC) provides one- to eight-day severe weather forecast for the U.S. that includes tornado watch. To extend the current severe weather forecast beyond eight days to 2 - 4 weeks (i.e., subseasonal time scale), many studies have looked into the Madden-Julian Oscillation (MJO), a patch of tropical thunderstorm that forms across the Indian Ocean and moves slowly eastward across the Pacific Ocean with 30 - 90 days period. Several studies found that U.S. tornado activity increases when the moving thunderstorm (MJO) passes across certain longitude bands in the tropical Pacific. However, the relationship between U.S. tornado activity and the MJO shown in these studies is largely statistical. A team of scientists at NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) and the Cooperative Institute for Marine and Atmospheric Studies (CIMAS) explored the physical mechanism underlying the relationship between U.S. tornado activity and the MJO, which is not yet fully understood. In a study recently published in the Journal of Climate (Kim et al., 2020), they showed that a series of key atmosphere-ocean processes are involved in the remote impact of MJO on U.S. tornado activity. Specifically, when the moving thunderstorm is passing across Indonesia to the Pacific, the air across Central America becomes very dry due to changes in the Walker circulation that moves upper-level air westward and lower-level air eastward across the tropical Pacific. The drying air across Central America redirects the low-level air flow to enhance the supply of warm and moist air into the U.S. east of the Rockies. This enhanced supply of warm and moist air to the U.S. destabilizes the atmosphere to produce a favorable condition for tornado activity. The study showed that this link is applicable only during May-June-July since the MJO does not produce a drying condition across Central America during other seasons. The research team is currently exploring to apply the identified link between a drying condition across Central America and U.S. tornado activity to develop a subseasonal (2 - 4 weeks) forecast model for U.S. tornado activity targeting May-June-July season.
Kim, D., S.-K. Lee and H. Lopez, 2020: Madden-Julian Oscillation-induced suppression of northeast Pacific convection increases U.S. tornadogenesis. J. Climate. 33, 4927-4939. https://doi.org/10.1175/JCLI-D-19-0992.1.