Rio Grande National Forest Historic Climate Assessment
Total Page:16
File Type:pdf, Size:1020Kb
RIO GRANDE NATIONAL FOREST HISTORIC CLIMATE ASSESSMENT Prepared for the United States Department of Agriculture Forest Service and Rocky Mountain Research Station by the Colorado Climate Center Peter Goble Nolan J. Doesken Linda Joyce 1 EXECUTIVE SUMMARY Located in south-central Colorado, the Rio Grande National Forest covers 2906 square miles of federally protected land. The climatic conditions experienced within the Rio Grande National Forest are heavily influenced by its landlocked, continental positioning, combined with its mid- latitude location. Most importantly, the climate of the area varies greatly over short distances due to complex topography and large differences in elevation. The Forest ranges in elevation from 7600 to 14335 feet above sea level (USDA 2016). In this report the recent recorded history of the Forest’s climate is explored. Using the data available, the mean state, variability, and historic trends of the Forest’s climate are summarized with an emphasis on temperatures, precipitation, and snowpack. The historic climate summary is rounded out with information on several of the Forest’s extreme weather events, physical descriptions of known mountain weather patterns that apply to the Forest, and an assessment of some of the largest needs for future weather observation. Data used to complete this report were primarily collected by, and obtained from, the following organizations: the National Weather Service, Oregon State University, the National Aeronautics and Space Administration, and the United States Department of Agriculture. Data are analyzed from four of the National Weather Service’s Cooperative Observing Network (COOP). The COOP sites used in this report were Crestone, Hermit, Saguache, and Wolf Creek Pass. The Del Norte Station was consistently used to represent the approximate climate of low mountain valleys within the Rio Grande National Forest. The Hermit station is used to approximate high mountain valleys, and the Wolf Creek Pass/Wolf Creek Summit stations are used to approximate mountain pass-like weather. Precipitation and snowpack data are utilized from both manual snow course sites and automated Snowpack Telemetry (SNOTEL) sites. The SNOTEL sites used are Wolf Creek Summit and Slumgullion. Manual snow core measurements came from the Wolf Creek Summit, Santa Maria, Pool Table Mountain, and Cochetopa Pass sites. Wind observations are obtained from the Remote Automated Weather Station (RAWS) network, and include the Blue Park, Bighorn, and Great Sand Dunes stations. Modeled data are used for a complete spatial representation of the Forest. Model data used come from North American Regional Reanalysis (NARR), and the Parameter-elevation Relationships on Independent Slopes (PRISM). Temperature and Precipitation Averages across the Forest: Using spatially gridded climate data, the mean annual temperature across the Forest was found to be cool, between 28 and 40 F over the 1981- 2010 period, depending on location. The warmest areas are in the San Luis Valley and the coolest areas are the high elevation peaks. Large temperature swings are observed from summer to winter across the Forest. Mean winter temperatures (DJF) range from 14-24 F. Mean spring temperatures (MAM) range from 25-40 F. Mean summer temperatures (JJA) range from 46-60 F. Mean fall temperatures (SON) range from 30-39 F. 2 The contour plot above depicts annual average temperature for south-central Colorado on a latitude-longitude plane. The Rio Grande National Forest is outlined in black. Data are provided at 4km resolution by the PRISM climate group at Oregon State for the 19810-2010 period of record. Large temperature swings are also observed from day to night (maximum temperature to minimum temperature). Diurnal variability is largest in the mountain valleys and smallest over mountain peaks. Using spatially gridded climate data, annual average precipitation in the Forest varies from about 12” to 55”. This large spatial variability in precipitation exists as a function of elevation, and whether or not a location is favored for orographic lift by prevailing winds. Meteorological summer (June, July, August) brings more total rainfall to the Forest than any other season as monsoonal moisture allows all locations of the Forest to average five or more inches of rainfall. For the lowest and middle elevations of the Forest, the monsoon season of July through September is most important for the production of precipitation. The highest elevation areas within the Forest average consistently higher precipitation totals from July through April with a short dry season in May and June. Precipitation and snowpack were found to be highest over the southwest end of the Forest. Temperature and Precipitation in Mountain Valleys and Passes. The longest climate records come from National Weather Service Cooperative Observing Network in the Rio Grande National Forest (Del Norte, Hermit, Wolf Creek). These stations are not only important to use in an historic climate assessment of the Forest because of their length of record, but also because these data are ground-truth weather 3 observations. Maximum temperatures at the high mountain valley station of Hermit are similar to low valley station at Del Norte year-round, but July is one of two months where the average maximum temperature is slightly higher at the Hermit station (Table 1). The average minimum temperature for high mountain valley station, Hermit, is cooler for Hermit than Del Norte every month of the year. The average maximum temperature at the mountain pass station at Wolf Creek is cooler than for Hermit and Del Norte in every month of the year. The average January minimum at Wolf Creek, however, is warmer than both the average January low for Del Norte and Hermit, but still cold at 6.1 F. Temperature variations are greater in the winter than in the summer and are greater for minimum temperatures than maximum temperatures. The observed range of 15-day running average minimum daily temperatures for Del Norte is from 38.1 to 56.1 F in July and from -19.2 to 27.2 F in January. This range goes from 26.3 to 50.6 F in July and from -35.9 to 22.2 F in January for Hermit. Wolf Creek pass has an observed range from 28.5 to 47.9 F in July and from -17.8 to 21.0 F in January. Cool nighttime air makes for a frost-free season of roughly 20-120 days depending on location, which is short relative to the majority of the United States, and even Colorado. Table 1. Average maximum and minimum temperatures (1981-2010) for the Del Norte, Hermit, and Wolf Creek stations. January July Average Average Average Average Maximum minimum Maximum Minimum Low Mountain 32.4 F 2.6 F 77 45.8 Valley (Del Norte) High Mountain 30.3 F -3.5 77.2 41.1 Valley (Hermit) Mountain Pass 39.5 6.1 65.6 40.6 (Wolfe Creek) Because not all stations have the same period of record, precipitation variability was measured over the 1980-2015 period of record. The amount of precipitation received in any given year is highly variable as it depends on the storm tracks taken by a highly chaotic and dynamic atmosphere. Total variability in annual precipitation is greatest in the areas of the Forest that receive the largest amounts of precipitation climatologically. 4 The contour plot shown above illustrates the amount of precipitation (inches) expected annually for south-central Colorado. The Rio Grande National Forest is outlined in black. Data are provided at 4km resolution by the PRISM climate group at Oregon State for the 1981-2010 period of record. For the lowest and middle elevations of the Forest, the monsoon season of July through September is most important for the production of precipitation. The highest elevation areas within the Forest average consistently higher precipitation totals from July through April with a short dry season in May and June. Precipitation and snowpack were found to be highest over the southwest end of the Forest. Precipitation variation from year-to-year is large relative to annual averages. Differences between precipitation in wet years and dry years can be as much as a factor of two. The low mountain valley of Del Norte, averages 9.37 of precipitation, yet the range between the 1980-2015 period encompassed only 4.78 inches of accumulation in 2002 and 15.67 inches in 1985. For Hermit, which is representative of high mountain valleys, average annual precipitation is 14.74”. Yearly totals that have been observed range from 8.36” in 2002 to 22.10” in 1990. Wolf Creek Pass, which represents mountain pass conditions in this report, averages 47.95” of precipitation, but keep in mind that mountain passes in the north and east parts of the forest will receive less. The range of annual precipitation received at Wolf Creek pass extends from 25.80” in 2002 to 59.47” in 1986. The Rio Grande National Forest air is typically quite dry, particularly with respect to other more populated regions around the country. Surface vapor pressure averages between two and six 5 millimeters of mercury. There is a diurnal cycle to airflow in which cold air pools in the valleys at night and moves back upslope in midday and afternoon hours. This circulation is driven by radiative heating and cooling, and often produces afternoon thunderstorms, especially during the monsoon season. Large 24-hr precipitation totals can occur even in the driest parts of the RGNF. The highest 24-hr precipitation measurements at Del Norte, Hermit, and Wolf Creek Pass are 2.55”, 3.05”, and 4.10” respectively. Heavy precipitation is most likely to occur in the late summer and early fall for low and high mountain valleys. Up on the mountain passes heavy daily totals are more likely in winter.