The tempo of storms has changed with global warming. A single storm that might drop X amount of water across a zone one thousand miles in length and hundreds of miles wide may now drop that same amount of water over a zone that is only a few hundred miles in length. Major floods in Calgary, Boulder, Southeastern Minnesota, Duluth, and other very wet rainfall events are now on record as examples of this, and the cause is quasi-resonant Rosbey waves.
Why? How? Read here for a reasonably recent review of the phenomenon. It has to do with changes in the way heat built up at the equator makes its way across the surface of the globe. The change is caused by the Earth warming more at the poles than at the equator. It seems that the rate of Arctic warming has exceeded expectations of a couple of decades ago, and this higher degree of warming at the poles caused a previously suspected but generally not appreciated phenomenon, the formation of giant wave in the jet stream, that in turn stall out weather systems and make the effects of global warming worse than otherwise likely. So this is an example of both a worsening and an unexpected worsening.
Not all the stalls are in the form of major rainfall events. Droughts can also occur. The so-called “ridiculously resilient ridge” that caused the recently ended California drought was one of these stalled systems.
And now, we are starting to see stalled hurricanes, for, probably, similar reasons.
In this recent Yale Climate Connections video by Peter Sinclair, Angel Adames-Corraliza, Jeff Barardelli, James Kossin, Jeff Masters, Allison Wing, Kerry Emanuel, and Michael Mann, talk about hurricanes moving more slowly, meandering, or just plain stalling, and thus causing much more damage and death and other bad things because of the enormous amounts of rainfall this phenomenon brings. Witness recent such hurricanes Florence, Harvey, Dorian.