Rashi on Devorim 11 (14), “I will give the rain of your land in its season” says this will be on Shabbos nights when everyone will be found in their houses. How does this fit with the institution of the “Sholom Zochor”?
A shalom zachor is certainly the exception rather than the rule. And even given that, if I were the one instituting the practice (pardon my convenient historical abstraction) I think I would have no problem doing so on a night on which people's only other preoccupation is staying dry. Better to walk in the rain and be able to eat the rest of the week than stay dry and starve. That is what makes it a b'racha!
The main problem with rain is not when one has to travel from house to house, but from city to city, where one has to travel long distance and through mud, etc.