The use of a Winter Precipitation Index to Assess Snowpack Changes in the Colorado Rockies and Oregeon Cascades

Freezing levels are rising in many areas of the world according to work by others. The potential impact of such change is enormous in regions such as the western United States, that rely heavily on snowpack as a water source. As freezing levels rise, more of the winter precipitation will be in the form of rain, or rain-on-snow events. Changes in snowpack will be observed first in sensitive areas and during sensitive months, defined as wherever and whenever the mean temperature is close to freezing. Thus, a month such as May, can be a sensitive month indicator in colder areas. This study analyses the state of winter precipitation for shifts that increase or decrease snowpack in the Colorado Rockies and the Oregon Cascades. Snowpack and total winter precipitation data for the past two decades are compared to the long term mean (about 1937-2000), on the basis of the winter as a whole, the first of each month, January 1 to May 1, and by El Nino-Southern Oscillation (ENSO) phase. A snowpack index which normalizes for changes in winter and monthly precipitation amounts, is used for this analysis. This index is the ratio of the percent of snowpack snow water equivalent (SWE) to the percent of cumulative total winter precipitation. Thus, a value of one indicates no change in the snowpack regime, a value of less than one indicates less of the winter precipitation is in the form of snowpack, and a value of greater than one indicates more winter precipitation is snowpack. This integrative index reflects changes in climatic factors such as temperature, wind, atmospheric moisture, and insolation. Thus, an index value of less than one, or a negative temporal trend, roughly suggests warming conditions, whereas a value greater than one, or a positive trend, suggests cooling during the 1981-2000 compared to the longer record. The northern and southern Colorado Rockies, and the Oregon Cascades are analyzed on the basis of slope (east versus west), and ENSO phases of warm (El Nino), cold (La Nina), and neutral. Preliminary results for northern Colorado show index values of less than one (warming) for the east and greater than one (cooling) on the west slope for the 1981-2000 winter seasons, and on the first of each month. These slope differences are significant seasonally (3.9% level), and monthly (0.00% level). Analysis by ENSO phase also shows the same pattern. Trend analysis shows the May 1 snowpack index slightly increasing (cooler) on both slopes (east: 0.015/year, sig. at 6.6% level; west: 0.016/year, sig. at 5.1% level). This is important because May could be a sensitive indicator month in this region. Trends for the other months are not significant. Results for Oregon show the east and west to be more similar to each other, and ENSO phase differences to be more variable, compared to northern Colorado. The mean data also suggests that more of the winter precipitation is in the snowpack on May 1, particularly during La Nina (both east and west), on the west slope in neutral years, and on the east slope during El Nino. This is consistent with the positive trend in ratio values for N. Colorado.