A recent study into the August floods in Louisiana says that the extreme rainfall was brought about by an unusual set of conditions that are likely to increase in frequency and strength in the future.
Furthermore, the study suggests that climate warming since 1985 could have increased the event precipitation (during 11–14 August 2016) by 20%.
The study also finds that such weather conditions appear to have slipped through the net when it comes to triggering warnings. Forecast systems now need to adapt warning criteria to take into account weak topical disturbances like the Louisiana “unnamed” low-pressure system.
The study, led by Dr. Simon Wang of Utah State University, and recently published in the journal Geophysical Research Letters, concludes that the intense precipitation that caused the catastrophic flood was produced by so-called tropical-midlatitude interactions between the slow-moving low pressure system and an eastward-traveling short wave to the north.
The study adds that tropical-midlatitude interactions are likely to increase in frequency and possibly also in strength in the future in the USA.
“The analysis presented points toward the tendency for more and perhaps stronger upper level troughs propagating out of the western U.S. in late summer; these then have an increasing potential to cross paths with low-pressure systems that form around the Gulf Coast.”
Increased Rainfall and Climate Change
Soon after the devastating Louisiana flood, a NOAA-led study concluded that anthropogenic climate warming played a role in the increased probability of the return time of a similar extreme event happening in the future.
The Utah State University research also connects the Louisiana floods with climate change and directly links an estimated 20% increase in the event precipitation to the contribution of climate warming since 1985.
“…simulations suggest that the warming trend since 1985 could have increased the event precipitation (during 11–14 August 2016) by 20%. Future work should include a coordinated, large attribution effort and perhaps the utilization of convection-permitting models to more explicitly represent extreme precipitation during the warm season.”
Extreme Rainfall Warnings
The findings of the study have implications for potential forecast of non hurricane-related summer extreme precipitation.
Tropical-midlatitude interactions of this nature are random and difficult to forecast. Existing climate projections did not highlight Louisiana as a hotspot for a significant increase in extreme precipitation.
Historically, most extreme flood events in the Gulf Coast have been a result of tropical cyclones. US extreme weather warning systems are more tuned to the threats of cyclones or hurricanes. Official warnings are normally posted in advance of impending tropical cyclones or hurricanes that enable resource preparation thereby reducing concomitant risks.
However, the study says that weak topical disturbances like the Louisiana “unnamed” low-pressure system currently do not merit comparable warning criteria, this despite an ever increasing potential for future significant precipitation incidents.
The Utah State University research points towards the tendency for more and perhaps stronger upper level troughs propagating out of the western U.S. in summer; 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. These new findings call for potential forecast and warning of non-hurricane related summer extreme precipitation in the Gulf Coast states to enable adaptation.
See more about the study here: Synoptic and quantitative attributions of the extreme precipitation leading to the August 2016 Louisiana flood
B - East Baton Rouge
C - East Feliciana
D - Iberia
E - Iberville
F - Livingston
G - St. Helena
H - St. James
I - St. Tammany
J - Tangipahoa
K - Washington
L - Lafayette
M - St. Martin
N - West Baton Rouge
Livingston - August 12 to August 13, 2016
Baton Rouge - August 12 to August 13, 2016
New Iberia - August 12 to August 13, 2016
Lafayette - August 12 to August 13, 2016
August 12 to August 16, 2016
East Baton Rouge Parish – 5 Tangipahoa Parish – 3 St. Helena Parish – 2 Livingston Parish - 2 Rapides Parish - 1