Louisiana Flood 2016

Reported by FloodList (12 DECEMBER, 2016): USA – Climate Warming Increased Louisiana Floods Rainfall by 20 Percent Says Study

AMS presentation recording (Jan 2019)

The catastrophic August 2016 flood in Louisiana was a result of intense precipitation produced by a slow-moving tropical low-pressure system interacting with an eastward traveling baroclinic trough to the north.  While tropical-midlatitude interactions of this nature are rare, they are not unprecedented.  

Our analyses point to the tendency for increased summer upper level troughs propagating out of the western U.S.; these then have an increasing potential to cross paths with low-pressure systems that form around the Gulf Coast.  Combined with the projected increase in precipitable water, resulting precipitation magnitude would increase.  

Large-ensemble modeling indicates that the prospect of future tropical-midlatitude interactions is a scenario that Louisiana will face in the future.  WRF downscaling simulations estimate that the post-1985 climate warming may have increased the event precipitation (11–14 August 2016) on the order of 20%. 

The challenge: 

The extreme amount of precipitation caused by the sub-synoptic scale low pressure system has proven difficult to forecast: Neither the GFS nor the CFS predicted the elevated precipitation amounts before August 9, giving rise to a forecast lead time of less than two days.  This lack of forecast skill is due to the weak large-scale forcing and prominent connection to seasonal modes, in addition to the random encountering of the weather systems.