There is a site on the city web page that tells you lot's of info about flooding potential zones in PA. It is
You can look-up any address and see if it is in a special flood zone which requires flood insurance or in a zone with the potential for floooding, but not likely to happen less than every 100 years and does not need flood insurance.
Also, some areas are in flood zones because of creeks, and some because of the bay. The creek flooding is more likely to happen, but generally less devasting. If the bay floods, which is a more remote possibility, it is potentially more serious. Most of the creeks in south palo alto have been laid with cement so those areas no longer require flood insurance unless they are subject to the bay flooding. San Francisquito Creek has not been flood proofed, and was the one that caused all the trouble in the El Nino Flood in 1998.
FEMA sets the catagories for flooding potential. If you are in catagory A or AH you need flood insurance, and in X you don't.
There is an ongoing battle over San Francisquito Creek with some people wanting to keep it in its natural state and others wanting to flood proof it. There is also a never ending battle over storm drain systems which are old and are even more likely to cause a flood than the creeks or bay.
If a home is in a special flood zone, A or AH, then any new construction, or any major remodeling requires the home to be elevated above the flood level set by FEMA. Also, you can not build a basement in a special flood zone in Palo Alto.
The areas in PA that are subject to San Francisquito Creek Flooding are north University and Crescent Park. South Palo Alto, east of Ross is subject to bay flooding. there are a few other areas, but thoose are the major ones.