Extreme Value Snow Water Equivalent and Snowmelt for Infrastructure Design Over the Contiguous United States

Snowmelt‐driven floods result in large societal and economic impacts on local communities including infrastructure failures in the United States. However, the current U.S. government standard design precipitation maps are based on liquid precipitation data (e.g., National Oceanic and Atmospheric Administration's Precipitation‐Frequency Atlas 14; NOAA Atlas 14) with very limited guidance on snowmelt‐driven floods. In this study, we developed 25‐ and 100‐year return level design maps of snow water equivalent (SWE) and 1‐ and 7‐day snowmelt including precipitation events (e.g., rain‐on‐snow) using long‐term observation‐based gridded SWE developed by University of Arizona (UA) incorporating the national snow model product (SNOw Data Assimilation System; SNODAS) over the contiguous United States (CONUS). For the 44 U.S. states where the NOAA Atlas 14 maps are available, the design snowmelt values from this study exceed the standard design values in 23% of the total extent. The snowmelt values exceed the NOAA Atlas 14 design precipitation by up to 171 and 254 mm in the northeastern United States; 127 and 225 mm in the north central United States; and 191 and 425 mm in the western mountain United States for the 25‐ and 100‐year return levels, respectively. A comparison of 7‐day design snowmelt between with and without precipitation shows that including precipitation results in an average increase of 42 and 68 mm for 25‐ and 100‐year return levels, respectively, over snowmelt that do not include precipitation. The design snowmelt maps from this study complement the NOAA Atlas 14 design precipitation and provide additional guidance on infrastructure design for snowmelt‐driven floods in the CONUS. Plain Language Summary Snow meltwater is a dominant driver of severe spring flooding in the north central and northeastern United States. Recent snowmelt floods, including rain‐on‐snow events, have resulted in large societal and economic impacts on local communities including infrastructure failures. The current U.S. government standard design precipitation maps are based on liquid precipitation data (e.g., NOAA Atlas 14 Volumes) with very limited guidance on snowmelt‐driven floods. We developed 25‐ and 100‐year return level design snow water equivalent (SWE) and snowmelt maps using the long‐term observation‐based gridded SWE incorporating the national snow model product over the contiguous United States. The design snowmelt values from this study exceed the Atlas 14 stand