As far as I know, Mumbai is just another coastal city, with the same sticky humid almost constant temperature weather all-year round. If at all, the December factor will be more enjoyable, with low humidity and crisper fresher air.
Kihim beach, being a beach, and just 100 out of Mumbai, is expected to be the same weather.
Delhi guys need not worry about any warm clothes at all, if they bring them along, it will just be more weight to haul in their cars/trains.
Bangalore guys are used to some cold, so this will be easier. Hyd temp swings wider than Mumbai's so they should also be OK.
We already had a storm passing over near Mumbai, and that was the first in 43 years so probably we need not worry now, as lightening does not strike the same place twice. Chances of another storm are that low.
So, in all, we could not have a better place to converge than Mumbai for our national meet.
Convenient, eh? 
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