Monday, 24 January 2011
Washington State Convention Center
Several heavy precipitation events that occurred over Louisiana, northeast Texas, and southwest Arkansas were examined to determine common environmental parameters associated with heavy rainfall. Six significant heavy rainfall events in 2006, 2008, 2009, and 2010 were analyzed using the National Weather Service's Weather Event Simulator. Five of the six cases occurred over Louisiana, with the remaining one case occurring over northeast Texas and southwest Arkansas. Five of the six cases were associated with upper level troughs or cyclones, while the other case was associated with Hurricane Gustav (2008), which made landfall near Cocodrie, Louisiana, and tracked over Louisiana and south-central Arkansas. These extreme rainfall events were responsible for significant disruption of normal day-to-day activities due to flash flooding over large parts of Louisiana. The synoptic and mesoscale features that contributed to the excessive rainfall in each case were identified. Although the overall synoptic patterns were slightly different in each case, a strong upper level trough, a deep atmospheric moisture profile from the surface to 500 mb (with corresponding high precipitable water values), and the interaction with the subtropical jet stream were found to be common parameters that contributed to heavy rainfall. Low level features such as veering winds in the surface to 850 mb layer, a strong 850 mb jet, a consistent moisture source from the Gulf of Mexico, a slow moving surface boundary and its associated strong moisture convergence, all proved instrumental in the development of flash flood-producing rainfall in a pre-existing unstable environment. Results from this analysis were used to create a flash flood decision tree for use by National Weather Service forecasters to determine if a Flash Flood Watch should be issued. Other environmental and soil factors were also included as helpful guidance in the issuance of a Flash Flood Watch.
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