The Climatological Relation Between Precipitable Water Vapor (PWV) and Extreme Precipitation for the September 2013 Colorado Flooding Event

Precipitable Water Vapor, defined as the amount of liquid water that would be produced if all of the water vapor in an atmospheric column were condensed, is a very useful parameter for forecasters. PWV is critical for determining the occurrence of extreme weather events; it can help determine atmospheric stability and the probability of convection and severe weather. The IPCC 4th Assessment found that changes in the frequency of extreme events, such as droughts, heat waves, and floods, are expected to increase. The frequency of extreme (99% percentile) PWV events are predicted to increase by a factor of 5-50 between 2000-2025 and 2075-2100, depending on season and region, which may lead to more extreme flooding.

This paper describes a cumulative sum method used to examine the predictability of extreme flooding events in Colorado, using PWV as the predictor. The method was developed using ground-based measurements of PWV from the Suomi-Net GPS sensor in Boulder, Colorado in September 2013. The derivative of the cumulative sum is used to define a threshold for identifying extreme PWV events, which also lead to extreme precipitation events. The insensitivity to the background climatology used in the method is demonstrated by use of quite different NCEP and ERA-interim reanalysis.

Current satellite observations have the potential to aid in extreme PWV detection, which could help in nowcasting for extreme flooding events. This paper investigates the potential application of the cumulative sum method to satellite observations for identifying conditions for which extreme precipitation will occur. Furthermore, Global Climate Model (GCM) output will be utilized to illustrate the long-range predicted increased in PWV in Colorado and the implication for the frequency of extreme flooding events along the Front Range.