As most chaser types already know, the Storm Prediction Center has issued a moderate risk for severe weather for Wednesday. A significant severe weather event is expected, but the kind of event is still uncertain. Will it be a wind and hail day with a few tornadoes? Or, will it be a full-scale tornado outbreak? Or, more likely, will it be somewhere in-between? It’s still too early to say, of course. And, as usual, many will dogmatically assert their favored position, but the truth is that we just don’t know yet. Here are a few reasons why Wednesday could be huge, and a few reasons why it could be a dud.
Reasons Why Wednesday Could Be Huge
- Strong upper-tropospheric trough entering the Plains – this will set the stage for robust moisture advection, strong low-level shear, and the transport of the elevated mixed layer off the High Plains (and associated steep lapse-rates).
- Strong instability – the elevated mixed-layer looks primed – as it has all season – to generate strong instability. Finally, rich moisture – almost 200 mb deep – will exist with it! You can see this when toggling between the forecast CAPE and the forecast mixed-layer CAPE: they’re virtually the same. Of course, this assumes that morning storms will not interfere with daytime heating.
- Incredible hodographs– in spite of the screwy nature of the upper system (from a pattern-recognition standpoint), the hodographs look bodacious. Very strong low-level shear should develop as early as noon, owing to the development of a 40 – 55 kt low-level jet. As a result, the classic “sickle-shaped” hodographs – associated with most major tornado outbreaks – are forecast to develop.
- Chaser-friendly storm motion – normally, storms move quite fast in 50 kt flow at 500 mb. However, the low-level flow will be quite backed, which will slow the storm motion some (read: quite chaseable!).
Reasons Why Wednesday Could Be Disappointing
- Weak cap / instability – the potential is there for some big CAPE, but will it materialize? The cap is forecast to be weak, which could lead to the development of too many storms and a significant reduction in instability.
- Poor phasing of parameters – generally, you look for negatively-tilted troughs for big tornado outbreaks, since the low-level shear is usually stronger in those cases (because of backed low-level flow) and the storm mode tends to be more favorable (because of the favorable orientation of the shear vector to the axis of the convecting boundary). This system will be positively-tilted, which is generally associated with lesser events. Will it matter?
- Unfavorable storm mode – will a weak cap allow storms to develop early, leading to the early development of a mesoscale convective system? Or will storms tend to “seed” each other, prompting the development of chaser-unfriendly high-precipitation storms? Or will storms find optimal spacing, allowing for the development of multiple classic tornadic supercells? These questions probably won’t be answered until the day the event occurs.
What do you think is the most likely outcome?