Climatology of Precipitation and Precipitation Extremes in the United States

1. Climatology of Precipitation and Precipitation Extremes in the United States Greg Johnson Applied Climatologist USDA-NRCS National Water and Climate Center Portland, Oregon RFC Hydromet 01-1 14-21 November 2000

2. Characteristics of the Mean Precipitation Climate • The principal controlling factors are the availability of atmospheric moisture and lifting mechanisms • Moisture controlled by flow from or proximity to large water bodies • Propensity for lifting influenced by topography, convergence zones (seabreeze, etc.), preferred storm tracks (jet dynamics)

4. Orographic Precipitation Enhancement Factors • Wind Direction (relative to topography) • Wind Speed • Atmos. Moisture (precipitable water) • Elevation Rise • Slope Angle

6. Issues of Scale(Spatial and Temporal) • Over long averaging times (say, the 30 year normal maps), only the most important and consistent meteorological factors are evident • Progressively shorter time spans reveal ever-increasing nuances of the atmospheric system

7. A Snapshot in Time

8. 24 Hour Time Integration

10. July normal Precipitation (top) versus July 1993 Precipitation (bottom)

12. Statistical Properties of Precipitation • Persistence, or lack thereof • Average amount of precipitation • Variability in precipitation amount, and theoretical maximum • Frequency of precipitation • Duration of precipitation

13. Annual Mean of Wet-Dry Day Prob. (x1000), Southwest Idaho

15. A Spatial Climate Modeling System • PRISM (Parameter-elevation Regressions on Independent Slopes Model) • Statistical/Dynamical/Topographic approach • Funded primarily by the NRCS-NWCC since 1993 for development of spatial climate products for the U.S. • Developed by Dr. Chris Daly of the Spatial Climate Analysis Service, Oregon State University

16. What is PRISM?

17. PRISM • Grid based model, approx. 4 km horiz. resolution • Any given grid cell value is determined by a linear regression of station values against elevation • Stations assigned weights • Combined weight of a station is a function of many factors

18. Form of Grid Cell Prediction • Y = 1X + 0 , where Y is the predicted climate element and X is the DEM elevation at the target cell. 0 and 1 are regression slope and intercept, and are determined by x,y pairs of elevation and climate observations from nearby climate stations

19. Station Weighting • Combined weight of a station is: W = f {Wd, Wz, Wc, Wl, Wf, Wp, We} , where Wd, Wz, Wc, Wl, Wf, Wp, We are the distance, elevation, cluster, vertical layer, topographic facet, coastal proximity and effective terrain height weights.

20. Vermont Annual Precipitation

24. Olympic Mtns Terrain

25. Olympic Mtns Annual Precipitation Facets On + Vertical Extrapolation On Inches

26. Olympic Mtns Annual Precipitation Facets On + Vertical Extrapolation Off Inches

27. Olympic Mtns Annual Precipitation Facets Off + Vertical Extrapolation Off Inches

28. Olympic Mtns Annual Precipitation No elevation Inches

29. Olympic Mtns Annual Precipitation Facets On + Vertical Extrapolation On Inches

31. Hawaii Annual Precipitation No Layer Weighting

32. Hawaii Annual Precipitation Two Layers

36. SNOTEL Coverage

37. PRISM-derived Products • Mean Mon. and Ann. Precipitation • Mean Mon. and Ann. Temps (mx/mn) • Frost dates and freeze-free season • Extreme winter min. temps & probs. • Growing, heating, cooling degree days • Snow-water equivalent

40. Oregon Annual Precipitation

41. Annual Precipitation Map of Elmore County, Idaho Produced by the NRCS NCGC “Cut-out” of State Map

44. PRISM - SWE Map - Oregon

45. PRISM Product Dissemination • Web Sites: OSU: www.ocs.orst.edu/prism/prism_new.html (Raster and polygon coverages of practically everything produced to date (Arc, GRASS); documentation; metadata; DEM’s) NRCS: www.ftw.nrcs.usda.gov/prism/prism.html (U.S., Regional and State mean annual precipitation cartographic products)

46. http://www.ftw.nrcs.usda.gov/prism/prism.html: • Cartographic state mean annual precipitation maps • Full repository of “official” NRCS PRISM layers

47. PRISM Product Dissemination • Compact Discs: All precipitation layers for all of the U.S. 3 CD’s (East, Central, West) of the lower 48 states. Includes Arc Explorer viewing software, and all documentation. Available from the NRCS-NCGC: 800-672-5559

48. PRISM Product Dissemination • Hardcopy maps: Cartographic-quality, walls-size maps of mean annual precipitation for each state Available from the NRCS Climate Data Liaison in each state

49. Extreme Precipitation Climatology

50. Spatial Considerations • Means and extremes are not always correlated, especially over large spatial domains, or even in small regions with significant climate variations: Portland OR MAP 37.39” Max24 2.62” Washington DC MAP 39.00” Max24 7.19”