Case Study

A sample forecast: The Atlanta Case Study

Atlanta is one of the fastest-growing cities in the country. The metro area more than doubled in population to nearly 6 million in the last 20 years. Atlantans are not unfamiliar with flooding. The Chattahoochee River flows through the metro region, and regularly overtops its banks in response to the region’s mix of convective and frontal storm and occasional tropical storms.


In a traditional engineering study, a common starting point for estimating the 100 year rain storm is the 100 year 24-hour rainfall map in the USDA Soil Conservation Service’s Technical Paper 40 (TP40). See the image below for more detail. In Atlanta, the map reports the 100 year storm is approximately 8 inches. That is, the map is saying that there is a 1% chance that the biggest storm in Atlanta in a given year will drop 8 or more inches of rain.

Traditionally, the starting point for finding the 100 year rain storm is to go to the 100 year 24-hour rainfall map in USDA Soil Conservation Service’s Technical Paper 40. This map gives the maximum amount of rainfall that might be expected to fall in a 24 hour period in any given year with 1% probability. In Atlanta the amount is around 8 inches. A key question about this map, and this approach, is that it assumes the 100 year storm is stationary – i.e. it doesn’t change in time. Many studies have shown stationarity is not true. StormCaster is helping quantify to what degree the 100 year storm is changing.

A key assumption of the TP40 map – or rather the way we use it – is that the 100 year storm is stationary, or doesn’t change over time; that the 100 year storm in 1960 is the same as the 100 year storm in 2010. Many studies have shown this not to be the case (NOAA, 2010). The StormCaster was designed to help quantify to what degree the 100 year storm is changing.

Using a rain site at Monroe, GA – about 40 miles east of downtown Atlanta – the StormCaster was used to create a forecast for the region. GCM grid cells are shown in the location map below. They cover the Atlanta region and extend to the Northeast, stretching as far as Charlotte, NC.


The forecast is based at Monroe, GA – about 40 miles from downtown Atlanta.

The forecast of 100-year storm level (below) ranges from relatively unchanged (blue and green lines) to a clear upward trend (red line), with the 100 year storm increasing from 8.5 inches to 10.3 inches over the 21st century.

The red line is the ensemble prediction from the four GCMs used for the A1B scenario. This scenario was set by the U.N. Intergovernmental Panel on Climate Change (IPCC). They describe the scenario as follows:

“The A1 storyline and scenario family describes a future world of very rapid economic growth, global population that peaks in mid-century and declines thereafter, and the rapid introduction of new and more efficient technologies. Major underlying themes are convergence among regions, capacity building and increased cultural and social interactions, with a substantial reduction in regional differences in per capita income. The A1 scenario family develops into three groups that describe alternative directions of technological change in the energy system. The three A1 groups are distinguished by their technological emphasis: fossil intensive (A1FI), non-fossil energy sources (A1T), or a balance across all sources (A1B) (where balanced is defined as not relying too heavily on one particular energy source, on the assumption that similar improvement rates apply to all energy supply and end-use technologies).”

The mix of trends seen in the forecast implies disagreement among the models in terms of how large and frequent future storms will be. Indeed, if we look at the uncertainty in the A1B scenario (dashed lines), an unchanging forecast like the other two scenarios is possible. Less uncertain, of course, is that Atlanta needs to be prepared for changes in their storm regime in the future.


StormCaster’s ensemble forecast indicates the 100-year storm may range from remaining relatively unchanged (blue and green lines) to increasing by almost 2 inches (red line) depending on which green house gas scenario is used.


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To produce a storm forecast that includes climate change impacts, StormCaster uses two sources of data.

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