Residents of Israel begin reciting "Barech Alenu" in the Amida prayer, in place of "Barechenu," on 7 Marheshvan, whereas in the Diaspora we do not begin reciting "Barech Alenu" until December 4th. This gives rise to the question of which text one should recite if he travels to Israel from the Diaspora during the interim period, from 7 Marheshvan until December 4th. Many people from our community, for example, travel to Israel during the final week of November. During their stay in Israel, should they recite the text of "Barech Alenu," in accordance with the practice in Israel, or should continue reciting "Barechenu" as they had been doing back home? Furthermore, if Halacha requires these travelers to recite "Barech Alenu" during their stay in Israel, which text do they recite upon their return home, if they return before December 4th? Should they continue reciting "Barech Alenu" since they had already begun doing so during their trip, or should they return to their community’s custom of reciting "Barechenu"?
Hacham David Yosef addresses this question in his work Halacha Berura (Siman 117, p. 187), and he writes that visitors to Israel during this interim period must follow the practice of Israel and recite "Barech Alenu" instead of "Barechenu." Hacham David cites this ruling from a number of Aharonim, including the Maharash Vital and the Havot Yair. This is also the position of the Mishna Berura (117:5), Be’er Moshe and Mishneh Halachot.
Hacham David then mentions that if the traveler returns to the Diaspora before December 4th, then he changes again to the text of "Barechenu." Since he is no longer in Israel, he should resume reciting the text recited by Diaspora communities during this period. Nevertheless, Hacham David adds, it is preferable for an individual in this case to add the words, "Ve’ten Tal U’matar Li’bracha" – the prayer for rain – in the Shema Kolenu blessing of the Amida. This means that in Shema Kolenu, one should recite, "Ki Kel Shome’a Tefila Ve’tahanunim Ata Ve’ten Tal U’matar Li’bracha…" One should add a prayer for rain in the Amida because it may otherwise seem absurd that he began praying for rain during his trip to Israel and has now stopped.
This is also the ruling of Hacham Ovadia Yosef, in his work Yehave Da’at (vol. 1, end of Siman 73).
Summary: Somebody who visits Israel at any point during the period from 7 Marheshvan until December 4th must recite "Barech Alenu" instead of "Barechenu" during the Amida prayer while he is in Israel. If he returns to the Diaspora before December 4th, he switches back to "Barechenu," but should preferably add the words "Ve’ten Tal U’matar Li’bracha" in the Shema Kolenu section of the Amida.
