But do not be surprised if it is warm and sunny. When you register you will get detailed information on what to pack. Consider a gamma globulin shot as a precaution, but no shots are required to travel to Israel. The water in Israel is safe to drink, but its high mineral content might irritate your system. Bottled water is readily available.
Washed produce is safe.
Pack all medicines in your carry-on. All U.S. citizens need a valid passport to enter Israel. Local county courthouses, many state and probate courts, and some post offices accept passport applications. It takes time for the passport to be issued, so apply soon. When in Israel, you should keep your passport with you at all times.