from Daniel Swain Weather West Blog May 3 2017
…An unusually “wavy” atmospheric pattern is currently evolving over the entire Northern Hemisphere, with significant implications for California weather over the next couple of weeks. The development of a pronounced and remarkably well-defined “wavenumber 6” pattern in the mid-latitudes has been a source of substantial interest in the meteorological community in recent days, and for good reason: this kind of atmospheric arrangement is strongly associated with slow-moving and sometimes extreme weather conditions as high-energy “kinks” in the jet stream become “stuck in place” across certain regions. For those of you wondering what the heck “wavenumber 6” even means, the explanation is actually rather straightforward: it literally refers to the number of complete atmospheric “ridge-trough” cycles across a particular latitude band. In other words, we know that a “wavenumber 6” pattern is in place just by counting the number of blue troughs (or red ridges) in the adjacent figure….
…“Wavy” atmospheric patterns tend to displace cold, dry polar airmasses toward the equator and warm, moist tropical airmasses toward the poles–and the strong thermal contrasts this creates are conducive to the development of alternating zones of heatwaves, cold outbreaks, and flooding rains (and that’s exactly what is expected to occur in the coming days across the Northern Hemisphere). Once in place, these patterns tend to remain “quasi-stationary”–exhibiting a high degree of persistence and moving very slowly (if at all). Recent evidence suggests that the frequency of high-wavenumber patterns has increased during the Northern Hemisphere warm season, which may be linked to rapid Arctic warming and the subsequent loss of sea ice. The Potsdam Institute has created an excellent, broadly accessible video on this topic for those interested in learning more.