What is an atmospheric river? A large and narrow stream of high intensity moisture extending from tropical influences pushed by the jet stream across mid-latitude land masses. The North Pacific atmospheric river is the most famous (and effective). What causes an atmospheric river to form? The best way to create an atmospheric river is the phasing (or connection) between the storm track propelled by the subtropical jet stream and increased heavy convection in the deep tropics enhanced by presence of large areas of strong vertical currents known as the positive (or convection) phase of the Madden Julian oscillation (MJO).

As promised yesterday, a robust convection phase of the Madden Julian oscillation (MJO) shifts into the tropical West Pacific just-after mid-month causing a warming influence to develop on the U.S. climate. Snow cover slows the warm-up in the medium range but note the mega-cluster ensemble “most likely” upper air forecast dissipates the polar vortex and creates high pressure across much of the U.S. in 15 days.

Emergence of a marine heat wave (MHW) east of Australia extending to the east of New Zealand coupled with another elongated MHW across the central and southeast Indian Ocean are well-correlated to high pressure ridge areas. In-between the ridge areas, a low-pressure area has spawned over the central coast of South Australia. The upper trough combined with the warmer SSTA off the East Coast is causing excessive rainfall in Eastern Australia so far during meteorological summer 2023-24.