Hi,
Weather in Israel in the summer is hot and dry. It does not rain, so you can leave your umbrella at home. For general guidelines on what to pack and the weather, take a look at: http://goisrael.com/Tourism_Eng/Tourist+Information/Planning+your+trip/What+to+Pack.htm
and: http://goisrael.com/Tourism_Eng/Tourist+Information/Planning+your+trip/Weather.htm
Make sure you bring at least one outfit that is more modest for visiting holy places - that is a short sleeved shirt or a jacket as well as a skirt or long pair of pants for women and a pair of slacks for men. Most important are comfortable walking shoes or sandals and a hat, which is a must when you are out in the sun. Don't forget to pack plenty of sunscreen and be ready to have a great time.