Attributing Extreme Déjà Vu Events: Hurricane Harvey and Louisiana Flood

The southern Great Plains suffered from two extreme events in 2016 and 2017: Hurricane Harvey in Texas and a stalled low-pressure system over Louisiana, respectively. Despite the difference in wind intensity, both storms were stalled leading to the production of 4+ days of heavy rainfall over a populated area: Harvey's intense rainfall lasting several days was reminiscent of the Louisiana flood that occurred a year earlier over the city of Baton Rouge. The two events bear a striking resemblance in terms of the time of year that they occurred (August) as well as in their geographical location (neighboring states and near-coastal cities). Both systems had stalled due to a blocking high pressure that prevented movement further inland followed by interactions with a frontal system to the north (see figure). While the Gulf Coast is no stranger to tropical storms, any one of them stalling for days over a major metropolitan area is a recipe for disaster.

The present study conducted a synoptic attribution on the pattern and long-term trend of the heavy precipitation associated with hurricane Harvey and the Louisiana low pressure, through regional simulations forced by various sets of lateral boundary conditions including those with the long-term trends altered. The resultant attribution simulation, with the climate trends removed from the input data, suggest that post-1980 warming in both the ocean and atmosphere resulted in an estimated 20% increase of the accumulated event precipitation for Harvey (with an interquartile range of 13%−37%), as well as for southern Louisiana. Additional earth system modeling analysis signifies an increasing trend in the number of cases that have similar synoptic patterns and associated stationarity to that of hurricane Harvey and the Louisiana low pressure. Precipitation associated with these stalled weather systems was projected to increase as well, due to the increased precipitable water over and near the Gulf of Mexico.

Given that attributing convectively-driven extreme precipitation events is challenging, this study calls for a careful reevaluation of the attribution of convective-driven systems that may become more stalled in the future and produce more rainfall.