Chase season predictors: urban legends or useful?

[Dan Robinson]

Thanks for that Greg, I will try that sometime.

The other factor to this question that comes to mind is that good tornado seasons in the High Plains and the Texas panhandle are relatively infrequent to begin with. We really haven't seen a season like 1995 that was so intense and focused in those areas in the modern era. Even in the 90s, most of the bigger events were closer to the center of Kansas/Oklahoma (thinking of April 26, 1991 and May 3, 1999). I wonder if there is a bit of a bias in chaser expectations for those type of seasons vs how relatively rare they are.

It would be interesting to look at drought data for the 1990s, but the UNL archives only go back to 2000. Anyone know of a source for older data? EDIT: I found this site, I'll try pulling some of this data.

https://www.ncdc.noaa.gov/temp-and-precip/drought/historical-palmers/

 

[Dan Robinson]

I added a second graphic showing Palmer Index drought maps and tornado reports from 1980-1999:

https://www.dropbox.com/s/4bivagn8tbaia66/drought-1990-v1.jpg?dl=0

If I can find some time this weekend, I'll go back and add a column to the original graphic with the Palmer maps just to see how they compare to the UNL drought monitors.

 

[Warren Faidley]

Excellent discussion. I for one wish I had more time to do the kind of data gathering everyone has done here. A lot of my chasing observations over the last 30+ years are done so from memory as I am terrible about keeping actual records. I will say there has been a shift of tornado activity in the western parts of TA over the last 10+ years. This is mainly because it's one of my favorite regions to chase given the visibility and road networks. The biggest change in tornado activity has to be in the LBB to MAF areas. I remember that was once an area I would travel to at least twice a year. Now it's rare to go once a year. I would love to see someone put together a moving map of tornado activity over the last 50 years to see if indeed the active regions have changed. I attended a lecture by Ted Fujita once where he theorized tornado activity shifted in circle every so many years.

 

[Tim Supinie]

I'm kind of interested in where this leads, but I'd like to point out some issues I see with Dan's analysis. Brett and Greg have touched on these a little bit, but I'd like to add my take.

One problem I see is with the subjective nature of what we're trying to look at. A "good" chase season is subjective, and one can have a good chase season with few opportunities or have a bad chase season with many opportunities. Additionally, an active Southeast season and a dead Plains season has fewer opportunities for me (based in Oklahoma), but way more opportunities for someone based in Alabama. You could try to use tornado reports as a quantifiable proxy, but lots of tornadoes doesn't mean lots of good chase opportunities (as you've alluded to). Maybe daytime tornado days within x miles of a point is a better proxy for a "good" chase season? I'm not sure. It won't account for visibility, but that's going to be really hard to objectively evaluate, especially before WSR-88D's.

Speaking of the time before WSR-88D's, another issue with taking this analysis back to 1980 is the tornado record. You'll probably want to consider only F1+ tornadoes because I think the F0 tornadoes are the most susceptible to being undercounted in the tornado record. Looking at only F1+ tornadoes definitely doesn't solve all your problems, but it's something.

Another item: the CPC outlooks I'm not sure are useful here. My understanding is that the CPC outlooks aren't incredibly skillful over a 3-month period, even when that period starts at the time of forecast issuance. And even then, "above normal precip" doesn't necessarily translate to "good chasing." I'm open to data, though.

Full disclosure: I'm of the opinion that, while drought may have a small effect on how the spring goes, the other large-scale-pattern-related factors tend to overwhelm the drought signal. Though, again, I'm open to data (which I'm not sure exists in this case).

 

[Paul Knightley]

This is great information - my thoughts have been that decent influxes of moisture from the Gulf won't really 'care' if the ground is dry or not. Perhaps thoughts about drought years being poor have also come from other kinds of thoughts - e.g. lots of dust making the viability poor, and hence giving the notion of a 'bad' year. Of course, if a link were found, then maybe it would have more to do with the prevailing long wave pattern which caused drought through the winter persisting into the spring and being unfavourable for severe weather. However, I do think there is something of an urban myth about it.

Personally speaking, as someone who has to pick their chasecation months in advance, the immediate synoptic set-up of those two weeks is much more important than anything else - I'll wager for many other people who aren't always on tap to chase, this is true too.

 

[Dan Robinson]

I updated the original graphics to include Palmer Drought Index maps for April and May as well as the Oceanic Niño Index plot through the years shown on the graphic (shows El Niño and La Niña state). The ONI plot is continuous (start of each year is at the bottom of the cell, end of the year at the top) so chase season is roughly 1/3 to 1/2 of the way from the bottom of each row.

These are much larger graphics that exceed the limits of JPEG compression - to preserve clarity, the export is in PNG format -

2000-2017 graphic full 6.5MB image is here:

https://www.dropbox.com/s/szlzrb2cv5oytvy/season-prediction-v3.png?dl=0

a lower-colorspace version is here, about 2MB:

https://www.dropbox.com/s/znv3udsg8x9klat/season-prediction-v3-2.png?dl=0

1980-1999 graphic full:

https://www.dropbox.com/s/dmcmo2v8uaf8rlm/seasons-1990s-v2.png?dl=0

lower colorspace version:

https://www.dropbox.com/s/imjej3c24j170fx/seasons-1990s-v2-2.png?dl=0

I also edited the title of the thread since this is evolving more into a general "chase season prediction" exercise.

 

[JF Massicotte]

I updated the original graphic for 2000-2017 to include Palmer Drought Index maps for April and May as well as the Oceanic Niño Index plot through the years shown on the graphic (shows El Niño and La Niña state). The ONI plot is continuous (start of each year is at the bottom of the cell, end of the year at the top) so chase season is roughly 1/3 to 1/2 of the way from the bottom of each row.

This is a much larger graphic that exceeds the limits of JPEG compression to preserve clarity, so the export is in PNG format - full 6.5MB image is here:

https://www.dropbox.com/s/szlzrb2cv5oytvy/season-prediction-v3.png?dl=0

a lower-colorspace version is here, about 2MB:

https://www.dropbox.com/s/znv3udsg8x9klat/season-prediction-v3-2.png?dl=0

I also edited the title of the thread since this is evolving more into a general "chase season prediction" exercise.

Not sure that I like what we see if we try to make analogues out of the oceanic nino index. However, the A-M-J CPC outlook this year is very similar to the 2013 one.

Current:




2013:




So many contradicting signals. I guess that we'll have a better idea in one month!