What caused the Great Plains precipitation pattern change?
How to judge medium-range forecast models (especially pattern change)
Discussion: During May an eastward shift of the Madden Julian oscillation across the equatorial Pacific shut down trade winds which prevented up-welling of cool subsurface water allowing the surface temperatures to warm. Weak El Nino gained some strength. As always the warmer waters in the deep tropics fueled a large mass of thunderstorm activity. The mid-latitude storm track interacted with the tropical moisture to the south which delivered 17-18 in. of rain to Kansas in May (Fig. 1).
The wet pattern shifted to Texas in early June and the Great Plains has largely turned drier (Fig. 2). The dry spring in the Southeast will turn very wet over the next week or so. The catalyst to the early June precipitation pattern change was Tropical Disturbance 91L which consolidated heavy rainfall to the Gulf States.
Weather pattern change and especially excessive rainfall events are almost always (80% of the time) driven by presence of the Madden Julian oscillation in the tropical longitudes of the Americas or tropical cyclone activity.
The MJO is in the Indian Ocean now and tropical cyclone activity is not expected over the next 2 weeks. Therefore a return to persistence (Fig. 3) is the most likely forecast for Central U.S. precipitation in the medium-range.
Fig. 1: The 30-day U.S. percent of normal precipitation pattern indicates enhanced wet weather in the Central Great Plains with additional drying in the Southeast.
Fig. 2: The 7-day U.S. percent of normal precipitation pattern indicates a CHANGE. The catalyst to the change was a tropical system.
Fig. 3: The 90-day U.S. precipitation anomaly pattern represents persistence and is the likely pattern to return in the absence of any tropical cyclones or MJO influence.