Heavy and Extreme Precipitation Trends in the United States

Severe weather and climate events have gained attention due to their financial impacts and loss of human life. It may seem like we’re experiencing more severe and intense weather today than a few decades ago, but what does the science say about precipitation trends?

Total annual precipitation has increased globally by 2.2 percent since 1901 and the contiguous United States has experienced a five percent increase over the last century. The intensity of precipitation has also increased in the United States. Some studies describe precipitation according to its intensity in the following categories: moderately heavy precipitation (daily totals between 0.5-1 inch (12.7-25.4 mm)); heavy precipitation (daily totals between 1-3 inches (25.4-76.2 mm)); very heavy precipitation (daily totals over 3 inches (76.2 mm)); and extreme precipitation (daily totals over 6 inches (154.9 mm)). Scientific studies can vary in the metrics they use to categorize precipitation, but overall they are showing an increase in frequency of heavy and extreme precipitation events for parts of the United States. Heavy and extreme precipitation totals are also contributing to an increased portion of total annual precipitation in the United States.

Dr. Kenneth Kunkel, Lead Scientist for Assessments, Cooperative Institute for Climate and Satellites at NOAA’s National Climatic Data Center, and Research Professor, North Carolina State University, specializes in climatic variability and change, especially as they relate to extreme events. In a recent conversation, Dr. Kunkel said that “there is a strong signal in our studies that heavy and extreme precipitation have increased.” So what do the regional trends look like? He confirms that there is an increase in heavy and extreme precipitation in the Central and Eastern United States, whereas the Western United States shows no trend. It may seem odd to not see a trend in the Southwest due to the severe drought the area is currently experiencing, but Dr. Kunkel explained that “when you look at the precipitation records for the Southwest, there have been dry and wet periods. The last decade has been dry, the 80s and 90s were wet, the 20s, 50s and 70s were dry, and the early 20th century was wet.” The mix of dry and wet years makes it difficult to identify a trend. Dr. Kunkel also explained “in general, the Midwest and the Northeast have the higher trends in heavy and extreme precipitation.”  When it comes to the Southeast region, “there is no strong trend in total annual precipitation; it has been a mix of dry and wet periods. But there has been an increase in heavy precipitation”

A shift towards more frequent and intense precipitation concerns the scientific community. Heavy and extreme precipitation events have resulted in damaging floods. An example is the Mississippi River flooding in spring of 2011, where nearly 300 percent of normal precipitation combined with melting snowpack in the Ohio Valley caused flooding of the Mississippi River and its tributaries, affecting states downstream including Arkansas, Tennessee and Louisiana. This severe weather event cost over three billion dollars, accounting for damage to crops and infrastructure, and flood fighting efforts in Louisiana.

(Source: Kunkel, K.E. (2014, March 5) Telephone Interview. and Groisman, P.Y., R.W. Knight, and T.R. Karl. 2012. Changes in Intense Precipitation over the Central United States. Journal of Hydrometeorology. DOI: 10.1175/JHM-D-11-039.1 and Kunkel, K.E., D.R. Easterling, K. Redmond, K. Hubbard. 2003. Temporal Variations of Extreme Precipitation Events in the United States: 1895-2000. Geophysical Research Letters. doi:10.1029/2003GL018052. and Easterling, D.R., J.L. Evans, P.Ya. Groisman, T.R. Karl, K.E. Kunkel, P. Ambeje. 2000. Observed Variability and Trends in Extreme Climate Events: A Brief Overview. Bulletin of the American Meteorological Society 81(3):417-425. and Kunkel, K., K. Andsager, D.R. Easterling. 1999. Long-term Trends in Extreme Precipitation Events over the Conterminous United States and Canada. Journal of Climate 12(8). and National Oceanic and Atmospheric Administration. Billion Dollar Weather/Climate Disasters. Accessed online 6 March 2014. http://www.ncdc.noaa.gov/billions/events)

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