Climatically driven changes in snow characteristics (snowfall, snowpack, and snowmelt) will affect hydrologic and ecological systems in Alaska over the coming century, yet there exist no projections of downscaled future snow pack metrics for the state of Alaska. We updated historical and projected snow day fraction (PSF, the fraction of days with precipitation falling as snow) from McAfee et al. We developed modeled snowfall equivalent (SFE) derived from the product of snow-day fraction (PSF) and existing gridded precipitation for Alaska from Scenarios Network for Alaska and Arctic Planning (SNAP). We validated the assumption that modeled SFE approximates historical decadally averaged snow water equivalent (SWE) observations from snowcourse and Snow Telemetry (SNOTEL) sites. We present analyses of future downscaled PSF and two new products, October–March SFE and ratio of snow fall equivalent to precipitation (SFE:P) based on bias-corrected statistically downscaled projections of Coupled Model Intercomparison Project 5 (CMIP5) Global Climate Model (GCM) temperature and precipitation for the state of Alaska. We analyzed mid-century (2040–2069) and late-century (2070–2099) changes in PSF, SFE, and SFE:P relative to historical (1970–1999) mean temperature and present results for Alaska climate divisions and 12-digit Hydrologic Unit Code (HUC12) watersheds. Overall, estimated historical the SFE is reasonably well related to the observed SWE, with correlations over 0.75 in all decades, and correlations exceeding 0.9 in the 1960s and 1970s. In absolute terms, SFE is generally biased low compared to the observed SWE. PSF and SFE:P decrease universally across Alaska under both Representative Concentration Pathway (RCP) 4.5 and RCP 8.5 emissions scenarios, with the smallest changes for RCP 4.5 in 2040–2069 and the largest for RCP 8.5 in 2070–2099. The timing and magnitude of maximum decreases in PSF vary considerably with regional average temperature, with the largest changes in months at the beginning and end of the snow season. Mean SFE changes vary widely among climate divisions, ranging from decreases between −17 and −58% for late twenty-first century in southeast, southcentral, west coast and southwest Alaska to increases up to 21% on the North Slope. SFE increases most at highest elevations and latitudes and decreases most in coastal southern Alaska. SFE:P ratios indicate a broad switch from snow-dominated to transitional annual hydrology across most of southern Alaska by mid-century, and from transitional to rain-dominated watersheds in low elevation parts of southeast Alaska by the late twenty-first century.

Numerous studies have evaluated precipitation trends in Alaska and come to different conclusions. These studies differ in analysis period and methodology and do not address the issue of temporal homogeneity. To reconcile these conflicting results, we selected 29 stations with largely complete monthly records, screened them for homogeneity, and then evaluated trend over two analysis periods (1950–2010 and 1980–2010) using three methods: least absolute deviation regression, ordinary least squares regression (with and without transformation), and Mann‐Kendall trend testing following removal of first‐order autocorrelation. We found that differences in analytical period had a significant impact on trends and that the presence of inhomogeneities or step changes also posed a substantial challenge in detecting reliable long‐term trends in precipitation over Alaska, particularly in the southern part of the state. Although some of these inhomogeneities occur in the mid‐1970s and could be associated with well‐documented changes in the Pacific Ocean and the Aleutian Low at that time, many of the inhomogeneities co‐occur with changes in station location, instrumentation, or operation. These operationally induced changes make it difficult to accurately detect the impact of decadal to multidecadal climate variability on precipitation amounts and to assess historical precipitation trends in Alaska.

There is a great deal of interest in whether and how Alaska's precipitation is changing but little agreement in the existing peer-reviewed literature. To provide insight on this question, we have selected three commonly used 0.5° resolution gridded precipitation products that have long-term monthly data coverage (Climatic Research Unit TS3.10.1, Global Precipitation Climatology Centre Full Data Reanalysis version 5, and University of Delaware version 2.01) and evaluated their homogeneity and trends with multiple methods over two periods, 1950–2008 and 1980–2008. All three data sets displayed common broadscale features of Alaska's precipitation climatology, but there were substantial differences between them in terms of average precipitation amount and interannual variability. Temporal inhomogeneity was a significant concern over Alaska in gridded precipitation products, as it was in the state's coastal weather stations. Although underlying station inhomogeneities were inherited to some extent by all of the gridded data sets, differences in data set construction contributed to dissimilarities in inhomogeneity, as well. There were contrasts in trends between the two time periods, and some minor discrepancies occurred as a function of the trend detection method, but the main disparities stemmed from choice of data set. Indeed, there were large areas where these data sets disagreed on both the sign and significance of precipitation trends. Until further analysis can resolve these differences, researchers using gridded precipitation data or evaluating studies based on such data should interpret results with extreme caution.

Across the western United States, seasonal-scale El Niño-Southern Oscillation (ENSO) teleconnections have been extensively described. Less attention has been paid to spatial and temporal details of these relationships that might be particularly important to managers and decision makers. Here, we explore intra-seasonal relationships between the Southern Oscillation Index (SOI) and average temperature, wet-day temperature, overall precipitation amount, precipitation frequency, and rain/snow partitioning at 138 Global Historical Climatology Network weather stations across the Pacific Northwest of the United States. Results of this study indicate that ENSO influences were generally more pronounced along the coast and in the northern Idaho/western Montana region than in the lee of the Cascades. There was a distinct seasonality in response, with stronger correlations in the late winter (February–March) than in the autumn (October–November) or mid-winter (December–January). The fraction of stations demonstrating correlations to SOI has changed over time, with a trend towards greater spring teleconnections in recent decades.