Thoughts about the upcoming 2015 chase season

[Paul Knightley]

We're still in a strong +ve PDO phase and this won't change any time soon, I don't think. Also, there is a 'Modoki' El Nino in progress at the moment (although there is much debate about whether this relatively 'new' feature is actually different from a more traditional El Nino!). One can look at the years when both were present and try to formulate an opinion - the most concentrated set of years when both were present appears to be 1986-1988. Of course, as the tropical Pacific pattern evolves the linkage becomes more tricky to follow as there may not be this 'version' of El Nino by May. One thing which is of interest is the MJO is very active at the moment - there are several tropical storms/disturbances in the Oceania region at the moment, indicative of a lot of tropical activity. If it continues on it's phase cycle it might favour a collapsing Pacific jet in around 15-20 days time.

 

[Andrew Pritchard]

Oh gosh darn it, you guys. The early April outbreak is gone!

 

[Brett Roberts]

We're still in a strong +ve PDO phase and this won't change any time soon, I don't think. Also, there is a 'Modoki' El Nino in progress at the moment (although there is much debate about whether this relatively 'new' feature is actually different from a more traditional El Nino!). One can look at the years when both were present and try to formulate an opinion - the most concentrated set of years when both were present appears to be 1986-1988. Of course, as the tropical Pacific pattern evolves the linkage becomes more tricky to follow as there may not be this 'version' of El Nino by May.

I agree with the general pessimism related to the strong +PDO, and believe it would be extra bad news if El Nino strengthens heading into May/June. While some associate El Nino with wetter conditions more favorable for High Plains chasing, looking at the past 50 years or so, it seems as though most such years featured a Nino that peaked during the winter and subsided by the heart of chase season (most recently 2007 and 2010).

While 1987 has some concerning similarities to this year, it is probably the ugliest analog (not hard to do, of course, since it's one of the worst chase years on record -- only 1988 is unequivocally worse). A more optimistic analog would be 2003, which also had a Modoki and strong +PDO alongside somewhat similar wintertime height anomalies. I think 1981, 1986, and (unfortunately) 2014 can also be thrown into the mix.

 

[Marcus Diaz]

The GFS is finally showing signs of something interesting. One of those pinched turds in the Southwest looks to eject over a moist airmass in Central/South TX. I'm cautiously optimistic for now...

 

[James Gustina]

The GFS is finally showing signs of something interesting. One of those pinched turds in the Southwest looks to eject over a moist airmass in Central/South TX. I'm cautiously optimistic for now...

Are you looking around next Wednesday, roughly?

 

[Chris Frost]

Are you looking around next Wednesday, roughly?

I'm seeing,per the latest GFS, some SW flow AOA 50-60kts overlaying a moist airmass (60ish dews) in Central TX in the Wednesday timeframe. Regardless if the troughing verfies, that Gulf looks to be open for business after Tuesday and that's obviously nice to see.

 

[Jeff Duda]

I don't really see much worth getting excited over next week regarding any one particular system. However, I do see two general things that I like:

1) Finally getting a nice rich moisture tongue reaching perhaps north of 35 N and persisting for several days
2) Increased chances of precipitation throughout the southern plains over the next two weeks.

I can handle waiting until April if that first setup is preceded by a couple weeks of replenishing rains and increased moisture. We need to get that land surface warmed and moistened. This week has been excellent for that across the central US. Snow pack everywhere is disappearing! That allows the soil to warm and moisten which will help moderate any further polar/arctic air masses that come screaming out of Canada. The medium-range models all tend to show another big cold surge coming through about two weeks out.

 

[Jeff Duda]

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.

 

[David Pauley]

The GFS is finally showing signs of something interesting. One of those pinched turds in the Southwest looks to eject over a moist airmass in Central/South TX. I'm cautiously optimistic for now...

I'm optimistic about it too for Tuesday evening/Wednesday afternoon. I don't expect too much, but some convection around here would sure be nice to see for once.

 

[Jason Boggs]

That's cool if the forecasts are more challenging. I can go with that since the forecasting is half the fun! Maybe even more isolated storm days since the strong troughs and energy may not be as apparent.

 

[Andy Berrington]

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.

Are there any previous analog periods for which this CPW El Nino cycle of 7-10 years would apply to? I'd like to do some comparisons.

What's interesting to me is that even in this relatively quiet period overall, we haven't really seen a shortage of high impact events and EF4+ tornadoes, with 9 and 7 of the latter in 2013 and 2014 respectively. Both of those counts are higher than such years as 2004 (5) and 2007 (5) and the 2013 total exceeds 1998 (8). These years as a whole were far more active, with 2004 and 1998 being among the busiest on record.

Ratio of violent tornadoes to total tornadoes in:
1998 -> 8/1424 ~ 0.56%
2004 -> 5/1817 ~ 0.28%
2013 -> 9/903 ~ 1.0%
2014 -> 7/823 ~ 0.85%

Of course these calculations don't factor in the percentage of the tornadoes in 1998 and 2004 that were spawned by tropical cyclones (especially considering Hurricane Frances and Ivan each produced >100 in 2004), which are far less likely to produce tornadoes of >E/F3 strength. They also don't account for tornadoes that didn't hit structures to validate a stronger rating (although this is somewhat nullified by the fact that all four years have this occurring). Other quiet years overall also saw a drop in violent tornado count, such as the 1986-1988 period.

 

[Jeff Duda]

Are there any previous analog periods for which this CPW El Nino cycle of 7-10 years would apply to? I'd like to do some comparisons.

Check the early 1990s, specifically centered around 1990-1991.

 

[Andy Berrington]

Check the early 1990s, specifically centered around 1990-1991.

I see that the 1991 data is omitted from the OSPO charts given the contamination from the Pinatubo eruption, but 1990 does follow that general SSTA distribution in the equatorial Pacific.

Also totally anecdotal, but 1990 and 1991 were two of the best years of the 90s for chasers, with '91 of course being perhaps the greatest ever.

I'm a bit unclear about the CPW Nino and +PDO/+NPO link. Is it implying that it is generally coincident with a +PDO/+NPO or is it an indicator of a positive tendency in the PDO/NPO in the future (or both)?

 

[Paul Knightley]

I agree with the general pessimism related to the strong +PDO, and believe it would be extra bad news if El Nino strengthens heading into May/June. While some associate El Nino with wetter conditions more favorable for High Plains chasing, looking at the past 50 years or so, it seems as though most such years featured a Nino that peaked during the winter and subsided by the heart of chase season (most recently 2007 and 2010).

While 1987 has some concerning similarities to this year, it is probably the ugliest analog (not hard to do, of course, since it's one of the worst chase years on record -- only 1988 is unequivocally worse). A more optimistic analog would be 2003, which also had a Modoki and strong +PDO alongside somewhat similar wintertime height anomalies. I think 1981, 1986, and (unfortunately) 2014 can also be thrown into the mix.

It should be noted that several analogues (and indeed some climate models) show a wet/cool pattern in the spring/summer for much of the Plains. It's always interesting to see 'wet' in seasonal forecasts for spring in the Plains but of course it doesn't mean 'severe' - it can mean widespread showers/storms with weak flow regimes aloft. As others have mentioned, though, sub-seasonal effects (e.g. the MJO) and other things will still lead to periods of active severe weather from time to time. I guess chasers will judge their own season according to whether an uptick in severe weather over a fairly short spell corresponds with their chasecation!

Perhaps a few lean years are required to see who's really into chasing - although that's another annual discussion I think!

 

[Dan Robinson]

One of the signs I look for in the long-range models every March/April is a transition away from the constant reloading of the eastern US trough/Hudson Bay vortex into a more progressive pattern, which seems to finally be showing consistently in the long ranges. This doesn't necessarily translate to good chase setups right away, but it at least "clears" things out for western troughs to start digging and swinging through.