Over the last three years, Israel has been revising its civil defense plans, and how to deal with the growing arsenal of rockets and ballistic missiles aimed at it. The latest change is the announcement that the military is dispersing its stocks of supplies, equipment and spare parts to a larger number of (better protected) locations.
For the last two years, the government has been constantly updating and revising a war planning document. This plan assumes a future war with Syria, and gives the local officials an idea of what to expect. Currently, the Israelis estimate that there would be as many as 3,300 Israeli casualties (including up to 200 dead) if Syria tried to use its long range missiles against Israel. If the Syrians used chemical warheads, Israeli casualties could be as high as 16,000. Over 200,000 Israelis would be left homeless, and it's believed about a 100,000 would seek to leave the country.
Israel now assumes that Iran would fire some of its ballistic missiles as well, armed with conventional warheads. But the big danger is Syria, which is a client state of Iran. Syria has underground storage and launch facilities for its arsenal of over a thousand SCUD missiles. Armed with half ton high explosive and cluster bomb warheads, the missiles have ranges of 500-700 kilometers. Syria also has some 90 older Russian Frog-7 missiles (70 kilometer range, half ton warhead) and 210 more modern Russian SS-21 missiles (120 kilometer range, half ton warhead) operating with mobile launchers. There are also 60 mobile SCUD launchers. The Syrians have a large network of camouflaged launching sites for the mobile launchers. Iran and North Korea have helped Syria build underground SCUD manufacturing and maintenance facilities. The Syrian missiles are meant to hit Israeli airfields, missile launching sites and nuclear weapons sites, as well as population centers. Syria hopes to do enough damage with a missile strike to cripple Israeli combat capability.
Israel has long been aware of the Syrian capabilities and any war with Syria would probably result in some interesting attacks on the Syrian missile network. The SCUD is a liquid fuel missile and takes half an hour or more to fuel and ready for launch. So underground facilities are a major defensive measure against an alert and astute opponent like Israel.
But Syria has been adding a lot of solid fuel ballistic missiles to its inventory, and recently transferred over a hundred of these to Hezbollah, in Lebanon. Hezbollah and Syria are both clients (on the payroll) of Iran, and would likely coordinate an attack on Israel. Hamas, in Gaza, is a semi-client of Iran, and might be persuaded to join in as well.
No unclassified government planning documents have discussed what Israel would do in response to such an attack, but in the past, Israel has threatened to use nukes against anyone who fired chemical weapons at Israel (which does not have any chemical weapons). But current plans appear to try and keep it non-nuclear for as long as possible.