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Snowpack is critical for water supply in the western U.S. but challenging to physically Ìýmodel. The snowline, the average elevation at which snow will persist in a given region, is of interest because it enables us to identify changes in both water supply timing and in habitats for snow-adapted species like the Canadian lynx. Here, we focus on a hydrometeorological approach to map the snowline. In this approach, a series of snow-rain Ìýthresholds are tested using surface air temperature observations, validated against in situ Ìýobservations of snow. Given snowfall dependencies on temperature and relative humidity, Ìýwe expect that the rain-snow partition threshold will vary regionally. Prior analyses have Ìýsought to map the rain-snow partition such as Jennings et al. 2018, Sims & Liu, 2015, and Klos et al., 2014. However, their data included limited observations at low surface Ìýpressures (i.e. high elevation), warranting further investigation. We therefore assess Ìýwhether rain-snow temperature thresholds can be more accurately characterized by using a more extensive network of in-situ observations. Ìý
This study uses the NRCS SNOw TELemetry (SNOTEL) automated observational network’s Ìýdata from 1990-2020, from 947 sites across the western U.S. We use historical records of Ìýdaily precipitation, snow-water equivalent (SWE), and air temperature to align actual SWE Ìýagainst the SWE estimated using the amount of precipitation that accumulates below each Ìýtested temperature threshold. Here, we identify the optimal temperature threshold which Ìýminimizes errors between observed and estimated SWE. Preliminary results find that the Ìýaverage partition temperature is greater than 4.5 °C across most of Colorado, with a Ìýmedian of 5.5 °C for all years. We can also delineate by wet and dry years (wet being those Ìýwith peak SWE above the long-term mean, and dry those below). For example, a band of Ìýhigh (>4 °C) thresholds in the Northern Rockies encompasses more stations across wet Ìýyears than across dry. We aim to identify the cause of this, and di`erences by basin, Ìýelevation, and wet / dry years. We anticipate that these findings will help validate further Ìýanalyses of the snowline, and aid in the understanding of the evolving habitats in the Ìýmountainous U.S.