Lake Pontchartrain remains elevated today, more than three days since Hurricane Isaac made landfall in southeast Louisiana. Although the lake level has slowly been falling since early Thursday, the level was still about three feet above normal earlier this morning, according to the NOAA Tides and Currents gauge at New Canal Station on Lake Pontchartain. The water level at this location was at least four feet above normal for approximately 60 hours, beginning at around 10PM on Aug 28th until around 10AM on Aug 31st.
According to preliminary levels provided by NOAA Tides and Currents, the maximum level at New Canal Station was around 6.2 feet above normal. A USGS site at Pass Manchac, between Lake Pontchartrain and Lake Maurepas, reported a preliminary level of around 7 feet.
The prolonged storm surge event in Lake Pontchartain may have contributed to enhanced flooding from heavy rainfall around the lake. Flooding occurred in several communities near the lake, including Laplace, Mandeville and Slidell. It's likely that heavy rainfall could not drain as quickly as normal due to the storm surge inundation at the lake.
A similar situation happened in June near Tampa, Florida, when heavy rainfall runoff from Tropical Storm Debby was impeded by elevated water levels in Tampa Bay due to storm surge. Also, respondents to the Gulf Coast Climate Information Needs Assessment survey in the Houston/ Galveston area noted that heavy rainfall runs off slower when the level of Galveston Bay is elevated. (link: http://www.southernclimate.org/publications/Gulf_Coast_Assessment_Final.pdf). It appears that this pattern is widespread and most likely applies to some of the flooding caused by Hurricane Isaac in Louisiana.