A Different Perspective on Atmospheric River Events: Condensational Heating and its Role on Rapid Snowmelt and Flooding across Northern Nevada in February 2017
Paul Frisbie, NOAA/NWS Elko, NV
Clair Ketchum, NOAA/NWS Elko, NV
Greg Barnhart, NOAA/NWS Elko, NV

Long duration flood events are rare for northern Nevada, especially over a large area. Historically when such flooding occurred, like in 1962 and 1983, a snowpack containing high snow water equivalent covered the lower elevations. A similar scenario existed in February 2017 when damaging life threatening floods occurred. Antecedent conditions leading up to the floods include numerous atmospheric river events that led to significant above normal precipitation in October, December 2016, January and February 2017. Snow water equivalent was very high throughout northern Nevada, including the lower elevations below 6000 feet per the national snow analyses. A sudden air mass change from modified arctic air to a moist warm regime contributed to a fast evolving flood event. Rapid snowmelt triggered major overland flooding, a breach of the earthen dam at 23 Mile Reservoir, and closed U.S. Highway 93 between Twin Falls, Idaho and Wells, Nevada. There are many factors that contributed to the rapid snowmelt, but one cause that is often not considered are thermodynamic factors such as condensational heating. When looking at atmospheric river events, rising dew point temperature above 0℃ over a snowpack will lead to rapid snowmelt. Recognizing such a scenario is a key factor when messaging a rare but highly impactful wintertime flood event.