What is important to you when evaluating effects of early convection?

[Michael Wilkinson]

Convection occurring early in the day always complicates trying to forecast for later that evening. I just recently learned details about the April 27, 2011 outbreak. I was surprised to learn that there was so much convection prior to the big outbreak. I’ve thought of early convection as being a huge stabilizing and therefore limiting factor for storm development later in the day.

What obvious things let you know that early convection is going to cause a huge problem, and what obvious things let you know that you might still see good storm initiation late in the day?

My apologies if this question is way too broad for simplified discussions. I thought that I had a basic understanding of what early convection does. Now, I realize that I don’t know enough about it to know if I’m even asking a reasonable question.

 

[Andy Wehrle]

As you stated, the effects of overnight/morning convection are variable and not always as straightforward as they might seem.

The main things I look for are:

1.) Does it clear in time for several hours of sun/heating before the later round?
2.) It is an organized MCS that leaves a large cold pool? In this case, even clearing behind it may not be enough for airmass recovery in the area where it tracked, but wherever the outflow boundary sets up could become the new target (this occurred in northern Illinois on June 22, 2015 after a large midmorning MCS swept through the original tornado threat area of southern Wisconsin).

We had thunderstorms around 8 AM on the morning of the state-record tornado outbreak of August 18, 2005 which included the long-track F3 Stoughton tornado. However they were out of the area and skies were partly cloudy by early afternoon.

I'd seen morning convection screw up enough setups that I made a painful mistake on the Pilger day in 2014. I was heading west on US-20 in Iowa with a target of Sioux City by 3 PM. However, at a late morning data stop (this was before I had a smartphone and thus data on the fly) I noticed a region of ongoing rain over the Missouri River on the IA/NE border. At this I made a snap decision to retarget south to Omaha, which was south of the area being rained on and in clear skies.

The trek south on I-35 to Des Moines and west on I-80 put me just far enough away from the Pilger supercell to reach it blasting north on US-77 just as it produced the last-gasp bird fart tornado near Hubbard (seen from just north of Winnebago), then collapsed as it indeed hit the area stabilized by the rain I had been worried about earlier. The area where the storm had gone nuts just a little bit to the west had cleared out earlier, and destabilized to become an extremely volatile environment.

The morning QLCS in MS/AL on 4/27/11 was an unusual beast. It was clearly a very organized system, and produced enough tornadoes (including several long-track EF2-3) to more than verify a high risk even without the violent outbreak later in the day. However it was largely out of the area by or shortly after dawn, allowing for strong heating in its wake. It left an outflow boundary that served to enhance many of the most violent tornadoes that day, including Cullman and Hackleburg. However the conditions that day were so ideal that many other storms in the open warm sector were able to become cyclic, violent tornado machines.

Areas further to the north in middle/northern TN, KY, IN, OH were in fact stabilized by repeated rounds of convection through the midday, which may have been the only thing that prevented violent tornado activity there, thus being the only thing in my mind that prevents that day from being unquestionably equal to or worse than the 1974 Super Outbreak in every metric.

 

[Dan Robinson]

It is usually related to the timing of the upper wave. Whenever a strong jet max arrives, you're going to see convection breaking out in the target area in response to that lift. As Andy said, it's not always bad as long as the MCS moves away from the dryline quickly and doesn't act like a cold front pushing the moisture south and east. In most cases, there will still be a ribbon of moisture back on the dryline for storms later in the day.

Not only does an early wave arrival introduce convection, but it could mean that the jet max will be departing before evening, meaning you might not have the lift/deep-layer shear you need for supercells late in the afternoon. Watching the evolution of the upper level pattern can help you identify the potential for early day convection.

 

[Jeff Duda]

As you stated, the effects of overnight/morning convection are variable and not always as straightforward as they might seem.

The main things I look for are:

1.) Does it clear in time for several hours of sun/heating before the later round?
2.) It is an organized MCS that leaves a large cold pool? In this case, even clearing behind it may not be enough for airmass recovery in the area where it tracked, but wherever the outflow boundary sets up could become the new target (this occurred in northern Illinois on June 22, 2015 after a large midmorning MCS swept through the original tornado threat area of southern Wisconsin).

This entire post was pretty awesome, so props to you, Andy, for having several excellent examples of how this issue can impact a chase. But I want to emphasize these two points, because they are the singlemost important aspect of addressing early-day convection. It pretty much comes down to how much that convection will limit destabilization later in the day and if it will shift the warm sector to an area where the kinematics no longer favor organized storms. The best examples of this latter issue is when early day convection stabilizes the area under the best shear and forces the new warm sector so far south that it either remains capped or has insufficient shear for severe weather.

 

[Andy Wehrle]

This entire post was pretty awesome, so props to you, Andy, for having several excellent examples of how this issue can impact a chase. But I want to emphasize these two points, because they are the singlemost important aspect of addressing early-day convection. It pretty much comes down to how much that convection will limit destabilization later in the day and if it will shift the warm sector to an area where the kinematics no longer favor organized storms. The best examples of this latter issue is when early day convection stabilizes the area under the best shear and forces the new warm sector so far south that it either remains capped or has insufficient shear for severe weather.

Thanks, Jeff and I was thinking of that in the case I mentioned, 6/22/15 when the effective warm sector was shunted quite a bit further south, but not too much although it was probably close.

 

[James Wilson]

Morning convection can be bad for all the reasons shown but can also leave behind outflow boundaries that will or can be a huge factor in severe weather later in the day

 

[samuel stone]

If there's already a southward drifting frontal boundary in the area and models are wishy washy on how far south it will dive before stalling, strong night time convection in and near the boundary can strengthen the cool side resulting in a further south stall