Nev wrote:Yes, I can't see what the problem is with 'thundery showers is'?
It's a term used by meteorologists worldwide in their forecasts when text may be limited to a strict word count. It simply means 'Showers, some heavy with the risk of a few embedded thunderstorms'...

I think Nev is on to it here.
We make a judgement as to when to use the phrase thundery showers. It is usually used when fast moving showers, some of which may be thundery, deliver brief heavy showers and for which there are unlikely to be any serious consequences (apart from an unfortunate hit from lightning). We also use it in forecasts that are already pretty wordy, but only if the situation is how I have just explained.
The full term 'thunderstorm' is used when we want to place more emphasis on a potentially more serious situation. I think using the word in full does convey a more serious meaning. Then, if we think the situation warrants more emphasis, we add terms like 'locally heavy rain' or 'downpours' or 'torrential rain' (for slow moving situations) 'with large hail' or 'with strong wind gusts to XXkph" etc.
As NZstorm mentioned, any thunderstorm does denote deep instability and has the potential for say, heavy rainfall (and other weather), but what is also important is its speed of movement. Storms with high speeds deliver only brief heavy rainfall but usually produce big wind gusts, when the speed is low the rainfall over an area can be high and the wind gusts often less severe.
Paul