Attended a very informative seminar regarding El Nino/PDO and the central Pacific low-frequency variability today.
It turns out there are two types of El Ninos - the traditional/classic/"canonical", and a type that is more limited to the central Pacific, called a "central Pacific warming" type of El Nino. The current pattern of SST anomalies across the Pacific is highly correlated with the typical pattern of a CPW El Nino, so we are in that kind of El Nino. I don't think the traditional ENSO index measures this type of El Nino very well, which may in part explain why the current El Nino is so "weak" (although I don't know that even an index modified to measure this type would give a much stronger value in the current scenario). It turns out that CPWs are closely linked to a positive PDO phase as well as a positive NPO, which results in high pressures over the Aleutians/Gulf of Alaska where there would typically be a semi-permanent Aleutian low.
The CPW is also a lower frequency oscillation than the canonical ENSO - this is the kicker. What this means to me is that it isn't going to change much anytime soon. This means that the planetary scale pattern that is driving the synoptic scale pattern in the CONUS will probably not change in a mean sense anytime soon.
Bottom line: it will likely be another down year in terms of total tornado count. Don't expect to see many big time troughs/open waves swinging off the Pacific and across the Rockies or the related widespread major severe weather outbreaks this year. That doesn't mean there won't be outbreaks and it doesn't mean there won't be tornadoes. Many tornadoes and localized outbreaks occur when mesoscale setups are especially conducive for severe weather (regardless of the synoptic scale setup). It just means there won't be many instances where a bigtime outbreak will show up days ahead of time in the models. Forecasts will be more challenging because events will be harder to see coming since they probably won't be apparent until the day before or the day of.
Also, since CPW has a frequency of 7-10 years, it also probably explains the recent lean years of activity (the speaker actually explicitly mentioned that the wintertime patterns of 2013/2014 and the California drought have a significant causal contribution from the current CPW event). It was hypothesized in earlier posts that this may be climate change showing itself. I think this type of event further shows that this is not related to climate change, but rather a decadal scale oscillation. There are others at play. Just know that there are a handful or more of seasonal-annual-decadal-multidecadal oscillations around the world that are all doing their thing. The total signal (the weather that we experience) is associated with the sum of all of these oscillations. Sometimes they add up in such a way as to make tornadoes happen more frequently in the US; somtimes they add up in such a way as to make tornadoes happen less frequently. It appears that for the past few years, all these entities have added up in such a way as to make tornadoes less frequent. That's just how it is.
