U.S. climate normals: precipitation, snowfall, and snow depth

The 1981-2010 U.S. Climate Normals being released by NOAA's National Climatic Data Center during 2011 include a suite of descriptive statistics based on precipitation, snowfall, and snow depth measurements at nearly 8000 stations from across the United States and its Caribbean and Pacific territories. The statistics are computed from the Global Historical Climatology Network - Daily data set for stations from the U.S. Cooperative Observer Network, other stations operated by the National Weather Service, and some military sites. Three groups of statistics are provided: long-term averages, frequencies of occurrence, and percentiles. Long-term averages of month-to-date, year-to-date, monthly, seasonal, and annual totals of precipitation and snowfall serve as basic descriptors of a location's climate. Daily relative frequencies and average number of days per month for precipitation, snowfall, and snow depth exceeding various thresholds (e.g., precipitation >= 0.01inches) provide a starting point for estimating the present-day and future likelihood of those threshold exceedances. Finally, percentiles of both daily values and monthly totals offer distributional information that can be useful when placing a particular amount of precipitation or snow into historical perspective. Our presentation will provide a brief overview of the methodology used to calculate the various statistics and show examples of the applications to which the different quantities are best suited.