Discussion: The primary contributor to the historic wet pattern across the U.S. dating back 1 year is a persistent contribution of tropical/subtropical moisture from the East Pacific and western North Atlantic best-defined by tracking the location of the Madden Julian oscillation. The MJO is an intra-seasonal massive area of heavy convection in the tropics born in the Indian Ocean and shifting east transiting the global equatorial region usually in 6-8 weeks. However, much of the past year the MJO has stalled and best most intense at the longitudes of the Americas. The most recent MJO episode was the strongest of the past 12 months stalling in May in the East Pacific/Atlantic tropics and the catalyst to the Central U.S. flooding and severe weather. The MJO is forecast to shift east toward the Indian Ocean the next 2 weeks which should ease the extremeness of heavy rainfall events in the Central U.S. the first half of June.
Fig. 1: A complex chart but very helpful identifying the cause of persistent wet weather patterns across the U.S. and in particular the severe weather regime of May across the U.S. The blue zone represents vast areas of thunderstorms in the tropics mostly east of the Dateline in the East Pacific to the Atlantic equatorial region much of the time since December. Presence of this convection in this zone enables the U.S. storm track to entrain tropical moisture making storms able to produce enhanced precipitation. This pattern has been in-place dating back 1 year and explains the 12-month wettest climate in U.S. history. Note that the most recent (blue) plots are strongest therefore the tropical moisture contribution to the May U.S. weather pattern has been further enhanced helping to fuel more excessive rain and severe weather.