2021-03-17 EVENT: AR/LA/MS/TN/AL/GA

[JamesCaruso]

It is very difficult to get a genuine "high risk" day without two factors, one of which will surprise some readers...

• Negatively tilted system. This was the case with April 27, 2011. With a closed low like we had last week? Almost no way.
• Convection in the morning. After having done a synoptic climatology of numerous (I think it was something like 50) F-4 and F-5 tornadoes, more than 90% occur with convection earlier in the day. There seems to be something about thunderstorms/reload/supercells that produces "the big ones." Of course, a major squall line with QLCS tornadoes occurred the morning of 4-27-11.

Interesting observation about “convection in the morning,” but, correlation not being causation, couldn’t convection in the morning just be a byproduct of the parameter space that is in place on a big day? On the bigger days, moisture is already in place the day before, then you have the approaching strong dynamics (and/or a lifting warm front) and you get morning convection. Then the atmosphere is worked over, so you need a lull if the instability is to be reloaded. With the more powerful systems, you have multiple short waves coming through, so you get more lift coming in at the right time later in the day. So the morning storms may have nothing to do with *producing* “the big ones“ - after all, morning convection has ruined many otherwise promising chase days. Instead, the parameters of a big day are simply more likely to produce morning convection?

 

[Patrick K]

g causation, couldn’t convection in the morning just be a byproduct of the parameter space that is in place on a big day? On the bigger days, moisture is already in place the day before, then you have the approaching strong dynamics (and/or a lifting warm front) and you get morning convection. Then the atmosphere is worked over, so you need a lull if the instability is to be reloaded. With the more powerful systems, you have multiple short waves coming through, so you get more lift coming in at the right time later in the day. So the morning storms may have nothing to do with *prod

This was my thought too. EF4/EF5s generally require a very favorable environment, and morning convection can act in lieu of a cap by washing out instability and allowing it to rebuild for a more favorable time, for example with a shortwave out ahead of the main system. These potent systems often reload the atmosphere rather quickly with multiple rounds of tornadic storms, where I agree the earliest convection of the day is either non-tornadic or less tornadic, like with 4/27/11.

Morning convection can also produce a whole network of outflow boundaries which can in some cases increase severity of the storms that are able to latch on later in the day.

Of course, morning convection can also wash out the entire day and ruin instability. However, those days likely aren't high end EF4/5 producers anyway.

 

[Mike Smith]

Interesting observation about “convection in the morning,” but, correlation not being causation, couldn’t convection in the morning just be a byproduct of the parameter space that is in place on a big day? On the bigger days, moisture is already in place the day before, then you have the approaching strong dynamics (and/or a lifting warm front) and you get morning convection. Then the atmosphere is worked over, so you need a lull if the instability is to be reloaded. With the more powerful systems, you have multiple short waves coming through, so you get more lift coming in at the right time later in the day. So the morning storms may have nothing to do with *producing* “the big ones“ - after all, morning convection has ruined many otherwise promising chase days. Instead, the parameters of a big day are simply more likely to produce morning convection?

Good morning, Jim and Patrick,

You may be correct. As stated, I was listing "two factors" not "two causes."

For example, it was foggy and rainy until mid-afternoon for both Great Plains (a/k/a "Woodward") Tri-State Tornado (1947) and the Topeka Tornado (1966). Then, it cleared up for a couple of hours. The supercell then developed and the rest is unfortunate history.

Mike

 

[rdale]

With yesterday's tornado surveys, the 45% tornado contour has verified which means the High verified...

 

[Ethan Schisler]

With the latest surveys out now, it brings Alabama's 3/17/21 tornado count to 25 which makes it their 6th largest tornado outbreak on record. Most all tornadoes were less than EF2 with a few EF2 and one long track tornado from Southwest MS to Putnam, AL which was rated an EF2 (a few chasers got this one and it was fairly visible at times). I'd say from a high risk point of view, it probably verified over Alabama, but the Mississippi portion of the risk certainly did not. Shift that 45% hatched about 60 miles SE and you have perfect verification, but its not a perfect world we live in...so it comes down to perspective really. If you were living in Central/Southern AL and got hit by that train of tornadic supercells from mid day to evening, it probably felt like a long day to you and the forecast certainly panned out. If you were back west toward the Delta and was waiting on that second wave of tornadic storms in the afternoon/evening, you were probably like "what is all the fuss about?".

All about where you are and perspective....I should note that in my first post I never called it a "bust", but said it underwhelmed and I think from a strong-violent tornado perspective on what one would expect with such a risk caliber...it indeed did underwhelm quite a bit (many short tracked, brief, weaker tornadoes in that zone). That isn't always a bad thing though. Nobody wants to see EF3-EF5 tornadoes blasting through communities and homes, and destroying lives. Because to be frank, if you are getting an EF4 or 5 rating, that is what it probably took. So on that note, I'm thankful! Hopefully we get lucky on Thursday and its the same sort of deal, minimum amount of folks affected!

 

[Jeff House]

Morning Rain Afternoon Pain is some of both correlation and causation. Typically a short-wave coming out of the main trough is responsible for the morning rain. Same main trough is responsible for the evening severe as another short-wave ejects.

Causation part is outflow boundaries. Morning rain will lay down the outflow boundaries we chasers seek. OFBs can have SRH similar to that of the warm front; and, the OFB may have greater instability farther south. These boundaries from morning rain are part of causation. Also other variables like rate of destabilization may be more impactful than just a snap shot of it.

I generally prefer to chase OFB over WF. They both have plenty of SRH. Tie goes to the OFB which can have greater instability with which to work. Especially when it's cool on the north side of the WF, I prefer the OFB. There are times the OFB may be capped; and, I might prefer the WF. If that's late in the season there's probably plenty of instability near the WF. Depends on the situation but I normally go for the OFB.

Alabama tornadoes on March 17 indeed favored the intersection of the outflow boundary and a pre-frontal trough approaching. South pre-frontal trough is to Plains dry line. Intersection with outflow boundary (and warm front) are areas of interest.

Big days they both go anyway. Just wanted to tie together a common chase strategy OFBs with the morning rain discussion.

 

[JamesCaruso]

other variables like rate of destabilization may be more impactful than just a snap shot of it.

That’s an interesting thought Jeff. Does this suggest that “scenario A” with rapid destabilization can be more favorable than a “scenario B” where the destabilization was in place for a longer time, even if scenarios A and B both have equivalent CAPE when the source of lift arrives?

 

[Mike Smith]

Yes! There seems to be something about the rate of destabilization that is in play.

That’s an interesting thought Jeff. Does this suggest that “scenario A” with rapid destabilization can be more favorable than a “scenario B” where the destabilization was in place for a longer time, even if scenarios A and B both have equivalent CAPE when the source of lift arrives?

 

[Bill Barlow]

Mike, your second condition occurred in the early morning hours of April 26,1991 when a nocturnal MCS developed over south central Kansas and north central Oklahoma. This same general area was the focus for severe thunderstorm development later that afternoon that went on to produce four F4/5 tornadoes, with the most noteworthy the Andover storm. The winds seemed more backed in this area as the cold pool rapidly warmed during the afternoon and may have enhanced the low level hodographs with larger helicity values near this mesoscale boundary. I chased that day in north central Oklahoma and remember watching convection rapidly develop to my north in Kansas, but i stayed where I was as the storm movements that day were to the northeast at 30-35 mph and I felt I couldn't catch up to those northern storms. I eventually got situated to watch the Red Rock F4 tornado develop near Billings then move across I-35 and northeast from there. Back in those days, there was no mobil internet/radar displays or cell phones. You made your forecast/target area then watched the sky for clues. This year will be the 30th anniversary of this outbreak.

 

[Andy Wehrle]

Mike, your second condition occurred in the early morning hours of April 26,1991 when a nocturnal MCS developed over south central Kansas and north central Oklahoma. This same general area was the focus for severe thunderstorm development later that afternoon that went on to produce four F4/5 tornadoes, with the most noteworthy the Andover storm. The winds seemed more backed in this area as the cold pool rapidly warmed during the afternoon and may have enhanced the low level hodographs with larger helicity values near this mesoscale boundary. I chased that day in north central Oklahoma and remember watching convection rapidly develop to my north in Kansas, but i stayed where I was as the storm movements that day were to the northeast at 30-35 mph and I felt I couldn't catch up to those northern storms. I eventually got situated to watch the Red Rock F4 tornado develop near Billings then move across I-35 and northeast from there. Back in those days, there was no mobil internet/radar displays or cell phones. You made your forecast/target area then watched the sky for clues. This year will be the 30th anniversary of this outbreak.

4/26/91 was one of the events that got me interested in severe weather although I didn't experience it firsthand; growing up in the '90s it was the focus of several Weather Channel documentaries such as Enemy Wind and got extensive discussion in Grazulis' The Tornado: Nature's Ultimate Windstorm as well as @Warren Faidley's Storm Chaser: In Pursuit of Untamed Skies (Red Rock was his first tornado if I recall correctly!), among others.

 

[Mike Smith]

We had large hail at our home in northeast Wichita ~7 and the PNC area was under a tornado warning that Friday morning.

NSSFC did a great job that day! Not only did they issue forceful outlooks, they also issued only the second-ever PDS tornado watch. Yet, some local meteorologists seemed -- for a time -- to be mislead by the morning rain.

If memory serves, the sky was completely clear around 10am. The cumulus field didn't redevelop until about noon to 1pm along with a few cirrus.

There are violent tornadoes (Greensburg) that occur without morning rain in threat area but they seem to be a minority.

Mike, your second condition occurred in the early morning hours of April 26,1991 when a nocturnal MCS developed over south central Kansas and north central Oklahoma. This same general area was the focus for severe thunderstorm development later that afternoon that went on to produce four F4/5 tornadoes, with the most noteworthy the Andover storm. The winds seemed more backed in this area as the cold pool rapidly warmed during the afternoon and may have enhanced the low level hodographs with larger helicity values near this mesoscale boundary. I chased that day in north central Oklahoma and remember watching convection rapidly develop to my north in Kansas, but i stayed where I was as the storm movements that day were to the northeast at 30-35 mph and I felt I couldn't catch up to those northern storms. I eventually got situated to watch the Red Rock F4 tornado develop near Billings then move across I-35 and northeast from there. Back in those days, there was no mobil internet/radar displays or cell phones. You made your forecast/target area then watched the sky for clues. This year will be the 30th anniversary of this outbreak.

 

[Lanny Dean]

While we certainly have case studies (some peer reviewed) reflecting early day convection in regards to tornado outbreaks/events, I am not overly convinced that early morning convection is always (or even sometimes) truly representative of the possibility or severity of tornadoes. I have experienced both early day convection which preceded tornadoes and experienced many events that were not.
Obviously, I think we all would/could agree that rapid re-destabilization is key. A possible lesser known event that came on the heels post convection is the Tulsa, OK 30 March 2016 event. Early morning convection with severe lasted through much of the day prior to clearing towards central OK and ultimate the DL bulge. Less than roughly two hours after clearing, we had severe and tornado warnings for the Tulsa metro area with a strong tornado hitting the northern side of the metro and adjacent communities.

However, I have experienced/documented many tornadoes that did not have the same type of pre convection environments. I know I have read a few peer reviewed papers regarding the latter (apologies for not locating them and sharing prior to posting) might be interesting and give a little more insight if someone could locate and share accordingly.
-Lanny

 

[Mike Smith]

[1:30am Thursday]

We will have the opportunity to check the observation that morning rain usually precedes violent tornadoes today.

As everyone, I'm sure, knows by now SPC is forecasting a high risk. Radar as of 1:22am shows showers and thunderstorms overspreading the the threatened region. The 05Z HRRR shows significant to heavy rain falling by 17Z over the entire high and moderate risk areas.

Will there be violent tornadoes as SPC is forecasting? If so, it will have rained during the morning hours.

If anyone is going to chase this situation, please be careful. These storms will likely move faster than last week's. The road network plus hills/trees will make it more difficult to chase today's storms.

 

[Jeff Duda]

With yesterday's tornado surveys, the 45% tornado contour has verified which means the High verified...

What's your source? Because according to this one, only a small 30% verified.

 

[rdale]

Sam Lillo -

https://twitter.com/x/status/1373990129535123460

 

[Jeff Duda]

I guess it depends on how you define the coverage then. My understanding is that if, for example, there are 10 tornado reports within the same 40 km grid box, the grid box still only counts for 1.

 

[Mike Smith]

With today being the anniversary of the April 3 outbreak, I wondered if there had been "morning rain." Turns out there was.