What it means to call weather event `100-year flood'

Along the flooded banks of the Mississippi River, weather-watchers view the slowly moving disaster that has engulfed parts of the upper Midwest and talk of a 100-year flood.

Could the same area see another such flood in a year or two, even though this year's rising waters are billed as a once-in-a-century event? Yes, although it's not likely, and here's why.

Meteorologists and hydrologists express the probability of a weather event in terms of its ``return period.'' This phrase refers to the likelihood of an event, such as a major flood, repeating itself. This year's floods stem from a rare mix of conditions. Hence, meteorologists may refer to the event as a ``100-year flood.''

But this term is only a product of the statistical likelihood of occurrence. It does not mean an area could not have a 100-year flood two years in a row. Instead, a better way to look at a 100-year event is to say that it has a 1 percent probability of occurring in any given year.

That doesn't mean the Midwest is likely to get another 100-year flood next year. This spring has presented the perfect meteorological recipe for disaster.

First, a number of storms in early April caused rivers in the northern Plains to rise. A blast of arctic air brought a foot or two of snow across the Dakotas. A warm ridge of high pressure over the Plains melted the snow. Then came several days of heavy thunderstorms.

Voila! Nature created the recent floods along the upper Mississippi River that inundated riverbank towns in Minnesota, Wisconsin, Iowa and Illinois.

Most spring flooding in the United States follows a similar pattern, with rapid snowmelt adding to already rising rivers. This pattern can be further complicated in some northern states, where ice from lakes and rivers breaks away and creates dams that back up the rising waters behind them. These ice dams also can break up quickly, causing flash-flooding downstream.

Fortunately, all these factors seldom come together in any one place. Those trying to ward off the rising Mississippi have good reason to hope that they won't see another such flood any time soon.

Q  I found your Web page on ``Names of Winds'' (http://ggweather.com/winds.html) in trying to find whether the names of cars produced by Volkswagen are named after winds. The Bora, Sirocco and Passat -- which is a wind not mentioned on your Web page -- are all cars in the VW line. My question is whether you know of winds that may give names to other VW models such as Polo, Golf, Lupo and Sharan.  John Seth - Edinburgh, Scotland
A  I had never thought about it, but your supposition is absolutely right. Volkswagen of America officials have confirmed that they do indeed try to use wind names. Bora comes from the cold downslope winds that blow from Hungary and into the Adriatic Sea, the Sirocco is named for the warm desert winds that reach the Mediterranean from Africa, and Passat is a German term for a trade wind. VWA officials told me that the Golf name originated with the Gulf Stream -- an ocean current, not a wind -- while the Lupo is from the Italian word for wolf. The origins of Polo and Sharan are uncertain.

Q  I have long been puzzled by the terminology of showers vs. rain in weather reports. What is the difference? Viviana Patterson - Newark
A  The general definition for rain is any liquid precipitation falling from the sky. Specifically, it is used to describe widespread and steady rainfall. Showers, on the other hand, refer to precipitation (liquid or frozen) that is characterized by rapid changes in intensity and by being very localized. Usually rain is associated with weather fronts that move through the Bay Area, while showers are in the cold air behind a front.

Q Why is there so much foggy afternoon weather along the beaches, especially in summer? Jean Myer - Mountain View
A  The chief cause is the same cold water that turns your toes blue along the coast of Northern and central California. This chilly water is the result of the upwelling of cold subsurface water along the West Coast during the summer months. As a persistent sea breeze blows across this water and onto the shore, its moisture condenses into tiny droplets that accumulate into fog or low clouds.

Q  Do you know of a site where I find out about the general climate in specific California locations? I want to move out of Silicon Valley and am open to several areas. Mean temperatures are important to me. Linda -  Cupertino
Q I am moving to the San Jose area, possibly this summer. Are you aware of a precipitation map available online that shows annual seasonal rainfall for various parts of the San Jose area? I think that microclimate data ought to be included in my search of a good neighborhood. Mark Steelquist - 

A  Both of these questions are common at the Weather Corner. Climate is a very important factor to many people when they choose a location to live. Fortunately, in the Bay Area it is possible to choose from a wide variety of microclimates within an hour's drive. (OK, maybe two hours during the commute.)

As a resource for Bay Area climate, start with San Francisco Bay Area Climate pages at http://ggweather.com/climate/ and for the rest of California try www.nws.mbay.net/ca_clima.html. For those interested in the climate for other parts of the United States, check out http://ggweather.com/ccd/. And to compare different cities across the country, there is a really neat site from Yahoo at http://verticals.yahoo.com/cities/. The site includes comparison data on a wide range of topics, including weather.