Rain, all by itself, is probably pretty harmless. At least in normal amounts it is. Your locale is used to having what is called the seasonal average rainfall. There are other rain or flood benchmarks that should be of interest to you. The 10 Year Average, 30 Year Average and 100 Year Average. During a normal year your area handles the rain quite well. It has had millions of years to adjust the soil and terrain.
When a 100 Year Flood hits, you have had enough with rain. In California there was El Nino - that dropped over 3 times the normal amount of rain in what is considered to be a coastal desert area. Flooding was everywhere and frequent. On a humorous note, we had what we called the 2 inch drip from one of our ceilings. It required mother nature to dump in excess of two inches of rain in a 24 hour period for it to leak. This was proven several times over a 3 month period. Under 2", no leak... 2" or more, it leaked. This is in an area where the normal annual rainfall is 9 inches! That year we had 33 inches!
All by itself, there should be little or no disruption of your business. Coupled with other acts of nature, such as high winds (hurricanes, tornados and the like), rain can wreak havoc on your business. The roof gets peeled up and the rain comes in, shorting out electrical wiring, soaking manufacturing machines and goods and ruining your computer systems.
Mix rain with an explosion or fire and you have an entirely different but debilitating mess. Try having all that caused by an earthquake and... well if you don't have a plan in place - your plan will be to liquidate what you can and close the company. If you don't have a plan... a Disaster Recovery Plan, call us to get you started on one.
Wind is a much milder form of Hurricanes and Tornadoes. (See Tornado for more)
Your public library or book store are good resources. So is the internet. Here are a few selected URL links for your interest:
http://www.worldclimate.com/ Temps and Rainfall from around the world
http://www.ncdc.noaa.gov/ol/climate/severeweather/rainfall.html NOAA Weather Extremes