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Severe Weather 2021 (19 Viewers)

Fred Gossage

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I will just post the blog article I wrote for our website so that I don't have to type all that out again, and so that I don't have to worry about hosting multiple pictures on here. I will then add to it below...

Back toward the latter part of summer, I did some local area digging in our region here for past seasons where a significantly -PDO, a moderate or stronger La Niña, and a positive TNI phase were all in place simultaneously. I used a PDO threshold of -1.5 (November of this year is already down to -1.77 as we talked about in the 2020 severe thread), an ENSO peak threshold of -1.0 (we've already hit -1.2 as a trimonthly peak, and weekly readings have been lower) and then a positive TNI number (ENSO 4 is already colder than ENSO 1+2, making the TNI now positive, and that will only increase in magnitude). I also only looked for first-year cold ENSO events because, as we are seeing now, these are less prone to the subtropical ridge dominating the weather all winter. That is an important distinction that has to be taken into consideration. These are the seasons, since 1950, where all those things were in place simultaneously:

2010-2011, 2007-2008, 1998-1999, 1973-1974, 1970-1971, 1955-1956

Every. Single. One. of those... not one single failure... had at least one F4+ tornado specifically in the area of AL/MS/TN between I-20/U.S. 80 and I-40 (JAN-MGM to BNA). THERE WAS NOT ONE SINGLE FAILURE. And even in cool seasons where the numbers were close but didn't quite meet those specific thresholds, there were STILL violent tornadoes in the spring in that specific area of AL/MS/TN. It has been almost ten years since the northern half of Alabama has had an officially violent rated tornado, and it has been since 2008 since the southern TN counties of my viewing area have had one. With the exception of far west Alabama being clipped on 4/28/2014, it has been that long since the same area has had an SPC High Risk. With respect to both things there, we are in rather uncharted territory.

The current large scale pattern in place that continues to further assemble itself is what is in place when large scale bad things happen in Dixie Alley. It doesn't always mean they WILL happen every time a pattern similar to this is in place, but this seems to be the specific set of large scale conditions that are in place when major, bad events here do happen. This is the first time since 2010-2011 we have had the combination of a significant first-year Niña in place, a significant -PDO, a +TNI, and cool-season pattern suppression that is keeping the subtropical ridging from taking over early. By no remote stretch of the imagination am I calling for a repeat of 2011, 2008, 1974, 1932, or any other such event... but maybe I am saying that this might mean that the door isn't quite as locked shut for something like that as it would be in any other year with different large scale conditions in place.
 
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andyhb

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Well that’s an... unsettling combination of years.

Great write-up Fred, as usual. The drought in the SW is another factor that I feel can’t be overlooked given the EML potential.
 

andyhb

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2020-06-01T00:00:00Z-0.08
2020-07-01T00:00:00Z-0.38
2020-08-01T00:00:00Z-0.28
2020-09-01T00:00:00Z-0.7
2020-10-01T00:00:00Z-0.69
2020-11-01T00:00:00Z-1.12

This is the first time in over 7 years that the Nate Mantua/JISAO monthly PDO calculation has dropped below -1.
 

Fred Gossage

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unknown.png


The +TNI is now officially observed to be in place. This would be a TNI value of +0.7, getting not far off from a significantly positive TNI already, and this will only strengthen over the next few months as the cooler water shifts into western areas as the La Nina decays.

https://www.spc.noaa.gov/misc/CSWW-2015/afternoon1/002_csw_workshop_mar_11_2015_version2.0.pdf

https://journals.ametsoc.org/view/journals/clim/26/5/jcli-d-12-00128.1.xml?tab_body=fulltext-display

Here are the links to explain why this is important...
 

MattW

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Fred, are they running the "Probability of US regional tornado outbreaks" or is that sill an experimental proposal?
 

Fred Gossage

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Fred, are they running the "Probability of US regional tornado outbreaks" or is that sill an experimental proposal?
Are you talking about that ERTAF medium range forecasting project with Victor Gensini or some product from the TNI publication that I overlooked?
 

MattW

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From your first link, in the summary it says: “Probability of US regional tornado outbreaks” is proposed as a new tornado index, and applied to measure tornado activity under 4 dominant phases of springtime ENSO evolution.
Did that become something that's ongoing?
 

Fred Gossage

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From your first link, in the summary it says: “Probability of US regional tornado outbreaks” is proposed as a new tornado index, and applied to measure tornado activity under 4 dominant phases of springtime ENSO evolution.
Did that become something that's ongoing?
If it did, I was never made aware that it went public.
 
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Meanwhile, the pattern continues quite benign across the CONUS. From SPC:

..DISCUSSION...
Medium-range models continue to indicate increasing amplification
within branching flow, downstream of a persistent strong and zonal
mid/upper jet across the western and central, southern mid-latitude
Pacific. It appears that this will include building mid-level
ridging to the west of the U.S./Canadian Pacific coast by early next
week, though downstream short wave developments are more uncertain.
Regardless, there currently appears little opportunity for
substantive, deep Gulf of Mexico boundary-layer moistening and
inland return flow, which is expected to contribute to the
maintenance of generally low convective potential through the
period.

@Fred Gossage, do you have a rough timeframe for when you expect things to pick up? Or is it anyone's guess as to when the wave train slows down and allows for more robust moistening between systems?
 

Fred Gossage

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Meanwhile, the pattern continues quite benign across the CONUS. From SPC:



@Fred Gossage, do you have a rough timeframe for when you expect things to pick up? Or is it anyone's guess as to when the wave train slows down and allows for more robust moistening between systems?
The more I look at things, the more I think this may be similar to 2011 in timing of the main action. Extended forecasts keep wanting to take the MJO through the Pacific during January, February, and even early March... not for extended periods, but tours through there. That favors the type of suppressed cooler pattern we have seen thus far. But even if it doesn't formally amplify through the Pacific, we're still staying in this cooler pattern now because the MJO is staying very low amplitude and not really shaking up the ongoing pattern. That's probably going to give those of us in Dixie and the TN Valley an increased winter storm risk once we get a little deeper into winter, especially compared to what is stereotypical for a La Niña. With the exception of maybe a couple of events prior to March, I think the main action waits until classic spring season to start, but I also think the same factors driving that cause the polar jet to be suppressed south at a lower amplitude and latitude so that it stays pointed at Dixie Alley deep into April.
 
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MattW

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If it did, I was never made aware that it went public.
That's too bad, it seems like it would be very useful. I've actually considered something like a "Tornado Probability Index" but based on the forecast models rather than long-term indices. But I'm just a software developer nowhere near meteorology except as a hobby :p
 

Fred Gossage

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Trade winds are about to be screaming over the western ENSO regions. That should result in cooling of at least Region 4, if not some of Region 3.4 as well. I also wouldn't be shocked if that reversal of the trades in the eastern ENSO regions is associated with some warming in Region 1+2 again. That should help to strengthen the +TNI pattern. Note that the coming trade wind burst being shown by the Euro ensembles there is a good bit stronger than the one around Thanksgiving, and that one had been the strongest one of this La Niña so far...
 

Fred Gossage

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The latest Euro weeklies show this current -NAO/AO configuration lasting into at least mid-February. I get the idea we may not see major changes to the large scale pattern until we're heading right into spring. I'm getting the idea more and more that the main polar jet may be pointed right at Dixie in the heart of April. Combining that with all the other things in place with the background state, and even Stevie Wonder would be able to see what's setting up. By no remote stretch of the imagination am I suggesting a repeat of any historic events of our past, but this is easily the most favorable large-scale background state we've had for Dixie Alley severe in the past 10 years, as well as the most dangerous. While the bold highlighted statement is completely true, the door will also be much more unlocked for that potential than it otherwise would be in most other years.
 

Richardjacks

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The latest Euro weeklies show this current -NAO/AO configuration lasting into at least mid-February. I get the idea we may not see major changes to the large scale pattern until we're heading right into spring. I'm getting the idea more and more that the main polar jet may be pointed right at Dixie in the heart of April. Combining that with all the other things in place with the background state, and even Stevie Wonder would be able to see what's setting up. By no remote stretch of the imagination am I suggesting a repeat of any historic events of our past, but this is easily the most favorable large-scale background state we've had for Dixie Alley severe in the past 10 years, as well as the most dangerous. While the bold highlighted statement is completely true, the door will also be much more unlocked for that potential than it otherwise would be in most other years.
Fred, you are correct. The low sun angle this time of the year typically does little for instability compared to March and later. The dry air advecting in from the east should keep any instability elevated. While it looks scary on the sounding for shear, any rain will keep the cooler/drier air locked in place. This should keep the threat south of Birmingham and Tuscaloosa and probably Auburn too.
 

andyhb

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Continued shift towards west-based La Niña/+TNI over the last week with strong trades and the Niña standing wave centered near the dateline. Nino 1+2 has warmed and Nino 4 concurrently cooling.
 
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SPC says zzzzzzzzzz continues for the foreseeable future:

Valid 111200Z - 161200Z

...DISCUSSION...
A weakening shortwave trough will move from TX across the Southeast
during the Monday/D4 to Tuesday/D5 period, with a weak surface low
moving from the northern Gulf Coast at 12Z Monday/D4 to just off the
Outer Banks of NC by 00Z Wednesday/D6. Given the cool/dry air mass
in place ahead of this system, little if any surface-based
instability is forecast to develop.

For the remainder of the period Wednesday/D6 to Friday/D8, models
show a stable pattern will continue with several days of offshore
flow across the Gulf of Mexico and East Coast
. While models disagree
on whether there will be a closed upper low or a large upper trough
over the eastern CONUS, very low heights and strong northwest flow
over the Plains will aid in prolonging the cool and stable pattern.


..Jewell.. 01/08/2021

The winter of 2007-08 was thrown out there as an analog for this winter quite a bit. That featured a localized tornado outbreak in the Midwest (primarily N IL to SE WI) on January 7th followed by Super Tuesday, probably the most significant Lower MS/Dixie Alley event of the new millennium until 4/27. No sign of anything like either in the pipeline.
 

Fred Gossage

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SPC says zzzzzzzzzz continues for the foreseeable future:
I think that's going to be the way it is for a while, except for occasional highly localized threats or nickel 'n dime type stuff. You'll be wishing for those come April...

1610156228275.png 1610156247301.png
1610156266272.png 1610156281064.png

That circled baroclinic zone on the last image would be a PERFECT low pressure track for Dixie Alley tornado threats...
 

Richardjacks

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I think that's going to be the way it is for a while, except for occasional highly localized threats or nickel 'n dime type stuff. You'll be wishing for those come April...

View attachment 5562 View attachment 5563
View attachment 5564 View attachment 5565

That circled baroclinic zone on the last image would be a PERFECT low pressure track for Dixie Alley tornado threats...
Fred seems to me that at some point i the next couple/few months the severe weather season will rapidly come to life.
 

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