State of the Chase Season 2021

Warren Faidley

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May 7, 2006
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It's important to consider there are multiple chase seasons, not just "one size fits all."

I break them down into: "Winter season," generally in the SE states in January February and early March. "Early spring season" starting in late March to Mid-April. "Mid-spring" (the traditional chase season) from late April though early June. "Late-spring and early summer" running through mid-July. There are isolated events for the remaining months and one could argue for a "Canadian season" since things seems to be shifting further north every year.

Each season has it's own climatology and "x-factors," complicating "broad-brush" forecasts. I've seen seemingly "bust" seasons saved by robust Colorado-sets up in June. I've also seen insane set-ups in Western Texas killed repeatedly by strong caps. Trying to forecast, for example, when and if the Hudson Bay Low will form or recovery times for Gulf RH is impossible this far out.

2021 is likely going to be another year where you have to remain flexible if you can, picking out the few early gems (e.g., Midland last year), and rolling the dice for an active week in May or June. Or, move to Canada.
 

Jeff Duda

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I disagree with the notion that there are distinct seasons for severe weather. There is a distinct peak of severe weather/tornado activity in the spring months across the US with a monotonic increase and decrease leading to and going away from that time. The "fall hump" is statistically barely distinguishable from the rest of the year's annual cycle. What actually changes other than the frequency of severe weather is where it occurs. If you want to argue that seasonality is implied by region, then fine. But the transitions are smooth, statistically speaking.
 
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Oct 10, 2004
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Off-topic, but I see AccuWeather's seasonal forecasts as akin to throwing shit against the wall and seeing what sticks.

I don't think there's any reliable way to predict activity level of the spring tornado season beyond aggregate statistics, but we're heading into spring with massive drought in the southwest (although, parts of NW TX, W OK, and W KS have escaped the bad stuff), so that can't be great. Could be looking at strong caps.

Worst part is the synoptic pattern, though. That's what hosed 2018 and 2020. I don't see how that can be predicted at this length.
Perhaps that strong EML, while resulting in a "too strong" cap for the Southern Plains, could result in a more "ideal" EML further north/east (AKA MBY and parts of IA/IL/MN) where a more common failure mode is the lack of a sufficient one to prevent early initiation/MCS mode. But, that still requires we not flip right from "extended winter" in April, to the jet being in Canada.
 
Oct 10, 2004
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Regardless of any positive signs, man the climate models for AMJ are just resoundingly bad. Not even going to bother posting them, but the consensus from the ECMWF/NMME/CANSIPS/etc. is for another very dead season with a poleward retracted jet (even starting in April this time) along with some serious drought problems given torching temperatures and a lack of precip.
Andy, do you think these models were perhaps not taking into account the current winter storm unfolding across the region? After all, it is quite unusual for the Southern Plains to go into spring with any sort of snowpack on the ground. When I think of a bad La Nina chase season my mind immediately leaps to 2012 when it was very mild and dry the preceding winter with very little snowpack anywhere, and the entire Plains and Midwest basically started to bake in March and never stopped until late summer/early fall.

This year so far is behaving more like 2008 or 2011, with a quite active mid-late winter east of the Rockies. Still, it's interesting and somewhat disconcerting that the models would be in such unanimous agreement that flies in the face of the favorable indicators that have been discussed on this and other forums.
 
Oct 10, 2004
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Again, the general thinking going into this year is that things look rather ominous for Dixie Alley, but more uncertain for the Plains/Midwest particularly in "prime" chase season of May into June (as is to be expected with a Nina). The climate models Andy B. posted about kind of throw a wrench into things, but of course they could easily be wrong at this range.

Although, like I said, I'm starting to think they might have been out to lunch since especially with the multiple rounds of widespread winter storms we just had, this year is behaving much more like a "wet" La Nina for the central CONUS than a "dry" one. Spring river flooding is already looming large as an upcoming issue in the Midwest, as it did in active springs like 2008 and 2011.
 

adlyons

EF1
Feb 16, 2014
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Linking a presentation that some of you may find interesting on the correlation between positive phase of the Trans-Nino index and large us tornado outbreaks. The presentation comes from Dr. Sang-ki Lee of NOAA AOML

Heres the skinny: "The decay phase of la nina (A positive trans-nino value) has been associated with a higher than average number of tornado outbreak and violent tornadoes days."

I updated some of my spreadsheets on ENSO and we have flipped from negative/neutral to a positive TNI. The last time this occurred was spring of 2017. Our values are weaker than past analogs so I still am hesitant to make direct comparisons to 2011 2008 ect. This is a weaker la nina transition, but there is still some evidence that the large scale pattern could favor some enhanced activity early to mid spring. Not sure how the ongoing drought will play into this, but here's some reading material.

1613748622281.png
 

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Oct 10, 2004
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Hmmmm...2017 wasn't a great year for chasing but not the worst of the late 2010s (that distinction goes to the following year). A lot of setups that busted or underperformed including the late April sequence which had some tasty model runs but of which the Canton tornadoes ended up being the only major event; the mid-May high risk and the late May moderate risk (which was primarily issued for a big derecho but with the potential of a tornadic supercell or two on the tail end in northeast OK which never really materialized).
 

Warren Faidley

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May 7, 2006
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Linking a presentation that some of you may find interesting on the correlation between positive phase of the Trans-Nino index and large us tornado outbreaks. The presentation comes from Dr. Sang-ki Lee of NOAA AOML

Heres the skinny: "The decay phase of la nina (A positive trans-nino value) has been associated with a higher than average number of tornado outbreak and violent tornadoes days."

I updated some of my spreadsheets on ENSO and we have flipped from negative/neutral to a positive TNI. The last time this occurred was spring of 2017. Our values are weaker than past analogs so I still am hesitant to make direct comparisons to 2011 2008 ect. This is a weaker la nina transition, but there is still some evidence that the large scale pattern could favor some enhanced activity early to mid spring. Not sure how the ongoing drought will play into this, but here's some reading material.
Do you know if anyone has broken down the data by regions? I seem to remember a similar study where a positive trans-nino was associated with more activity in the eastern / SE US as opposed to the Central Plains, e.g., the 2011 Super Outbreak? Thanks.
 

adlyons

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Do you know if anyone has broken down the data by regions? I seem to remember a similar study where a positive trans-nino was associated with more activity in the eastern / SE US as opposed to the Central Plains, e.g., the 2011 Super Outbreak? Thanks.
Warren, there is a paper by the same author delving into a bit more detail on the topic, but other than that I am not aware of anything else on regionalizing the results. The presentation did hint at more activity across the southeastern US and the Midwest similar to what you said but it is a bit broad/vague.
 

adlyons

EF1
Feb 16, 2014
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Hmmmm...2017 wasn't a great year for chasing but not the worst of the late 2010s (that distinction goes to the following year). A lot of setups that busted or underperformed including the late April sequence which had some tasty model runs but of which the Canton tornadoes ended up being the only major event; the mid-May high risk and the late May moderate risk (which was primarily issued for a big derecho but with the potential of a tornadic supercell or two on the tail end in northeast OK which never really materialized).
I would definitely caution using 2017 as an analog as it was a substantially weaker La Nina. But, like you said, it was certainly not the worst year ever. I only include that as a reference to the last time we had even remotely similar conditions. Prior to that, 2015 also had positive TNI values but temporally displaced from the method(Single year la nina decay) mentioned in the presentation.
 

James Gustina

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The Euro has been hinting at our first run at a limited severe weather threat on the Southern Plains early next week. Looks to have all the classic problems of an early March setup between cloud cover and uncertain warm front placement but something that may bear watching.

Both the GFS and Euro seem to be pointing to the Southern Plains and Dixie getting some Baja cutoffs mixed in with the overall west coast troughing through hour 240 from the 00Z runs last night/the GFS' 12Z run this morning. Might be interesting to see how our mid-late March shapes up should that type of pattern persist.