dogmelon wrote:Thank you for the explaination. I'm interested as to why the convergence happened here and why it doesn't do it more often? We get NEers ahead of Southerlies a lot and nothing happens, why was today different?
Thanks for the question. I will have to give you a quick and simple answer but I hope it tells you something. The convergence associated with a southerly change is quite common in Canterbury but a number of things need to come together in order to get significant thunderstorms. The most essential are low temperatures at upper levels and high temperatures and dewpoints near the surface making the atmosphere unstable. The energy associated with the warmth and moisture at low levels can be released if something triggers convection, the air rises and moisture condenses and releases heat.
Things that can help the convection to get going are upper troughs, upper level divergence and of course convergence at the surface. The better these things come together the stronger the storms.
It seems that the storms did not get more intense than they did today due to the lack of a strong upper trough, insufficient cooling aloft, and the fact that there were areas of cloud this afternoon that limited surface heating. By the time the southerly change had reached Christchurch it was late afternoon and not warm enough at the surface to provide energy for strong storms. If the change had reached Christchurch a bit earlier and if there had been less cloud earlier today I believe these storms would have been a great deal more energetic. With a strong upper trough in addition to all that they could have been severe.