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John Holmes, the senior forecaster at Netweather and who worked at the Met Office for many years has kindly sent a forecast. I will keep this updated as we approach the race.
Weather Forecast for London Marathon Updated 11.00 a.m. 12.04.13
The overall idea shown for some days on the 500mb upper air charts has been for an upper ridge fairly close to the south of the UK. Its shape is open to some doubt, most of the time it shows as a ridge type on others (the minority) as a flattish westerly pattern with higher contours close by the south.
Putting this in terms of the surface pattern is never easy but I suggest that the idea shown this morning of a surface high over the southern half of the UK, probably more as a flattish sort of ridge on Sunday than the centre shown for Saturday.
The wind flow is not clear and could be a light to moderate flow from an easterly or a westerly point. The position of the ridge being the key. My own feeling is that the position will give a westerly component, actual speed is not possible to give this far out, not calm anyway!
This implies a dry day, perhaps a misty start with some low cloud at first but any of this clearing as the sun comes out with sunny spells by lunchtime.
Lows of somewhere between 3 and 6C and afternoon values up to 10-12C. So not all that warm which should help?