
Given the huge interest in today's weather possibilities, this in-depth forecast is only for Wednesday. Summery for the forecast is rain approaching from the west, possibly heavier burst's with a small-ish risk of thunderstorms.
Temperatures by day, 19-20°C at best. Rain clearing by evening/overnight. Overnight low's feeling fresher, 11-12°C.
There is some rain across north Surrey and directly west of London at the time of writing this and given the 500hPa wind vector's (the height of the wind that "steer's" rain), it's expected to move directly east.
This would suggest that currently, areas on a line from roughly Royal Tunbridge Wells / Ashford and northwards can expect the southern part of this cell to move across in the next few hours. During the rest of the afternoon and evening, southern Kent should also see some rain.
Browsing across different weather based forum's and Website's, there is also quite a bit of indepth discussion relating to what is known as a "triple point" setup, which simply means a cold front overtakes a warm front ahead of it and both of the edges curve inwards towards a low pressure system.
This raises the chance of some thunderstorms breaking out this afternoon and there is the basic instability requirements being met, and a SLIGHT risk of a tornado - although there has not been much discussion as to the area most at risk.
Both
TORRO (TORnadic and storm Research Organization - UK), and
ESTOFEX (European forecast agency) are very quiet on any possible weather relating to this.