http://revach.net/article.php?id=1169

Section: Halacha   Category: Tshuvos
Rav Shlomo Zalman Auerbach: V'Sain Tal Umatar - Between Eretz Yisroel And Chutz La'Aretz, What Should Travelers Say?

If someone from Eretz Yisroel leaves to Chutz La'Aretz after they had already begun to say V'Sain Tal U'Matar, he should continue saying it in Chutz La'Aretz if his children and wife remain in Eretz Yisroel. If he is leaving permanently then he should not continue saying V'Sain Tal Umatar.

If he leaves Eretz Yisroel to Chutz La'Aretz before they start saying it, then when they start he should continue saying V'Sain Bracha and add V'Sain Tal U"Matar in middle of Shema Koleinu.

If in any of the above scenarios you forget to say V'Sain Tal U'Matar where appropriate you do not need to repeat Shemone Esrei since there are poskim that hold you do not need to say it at all (Minchas Yitzchak) .

If you are from Chutz La'Aretz and visit Eretz Yisroel between when they start saying it before they begin in Chutz La'Aretz, if you plan on staying until after Chutz La'Aretz begins, you should say V'Sain Tal U'Matar in Bareich Aleinu. If you plan on leaving before they will begin, you should add it in Shema Koleinu.

Important Note: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.