Climatology of Precipitation and Precipitation Extremes in the United States Greg Johnson Applied Climatologist USDA-NRCS National Water and Climate Center Portland, Oregon

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

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

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

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

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

Standardized Precipitation Index (SPI)

A Spatial Climate Modeling System n n PRISM (Parameter-elevation Regressions on Independent Slopes Model) n n Statistical/Dynamical/Topographic approach n n Uses point data, a DEM and a coordinated set of rules, decisions and calculations, designed to mimic an “expert” climatologist n n For good reference see PRISM Guide Book under Technical Papers at the OSU PRISM web site

PRISM n n Originally developed for precipitation only, now expanded to temperature, dewpoint, solar radiation and many derived variables such as HDD’s/CDD’s, GDD’s, frost dates, snowfall, snow water equivalent, etc. n n Most commonly applied in monthly or annual time increments, but also applied to events

PRISM Model from OSU’s Spatial Climate Analysis Service n n Funded primarily by the NRCS-NWCC since 1993 for development of spatial climate products for the U.S. n n Originally developed for precipitation only, now expanded to temperature, dewpoint, solar radiation and many derived variables such as HDD’s/CDD’s, GDD’s, frost dates, snowfall, snow water equivalent, etc. n n 4 km horizontal resolution raster data, and ARC polygon coverages both available n n Most commonly applied in monthly or annual time increments, but also applied to events

PRISM n n Any given grid cell value is determined by a linear regression of station values against elevation n n Stations assigned weights n n Combined weight of a station is a function of many factors

SNOTEL n Large Automated Climate Network –Began in 1978 –Over 650 remote site –Generally in high elevation areas –Located in the 12 Western States and Alaska –Utilizes meteor burst communication technology to telemeter data

Typical SNOTEL Site

SNOTEL Coverage

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

Avg. Ann. Precip. from UMAC web site

Cartographic-quality Map of Mean Annual Precipitation for Idaho Produced at the USDA-NRCS NCGC

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

PRISM - SWE Map - Oregon

PRISM Product Dissemination n n Web Sites: OSU (Raster and polygon coverages of practically everything produced to date (Arc, GRASS); documentation; metadata; DEM’s) NRCS (U.S., Regional and State mean annual precipitation cartographic products)

 Cartographic state mean annual precipitation maps  Full repository of “official” NRCS PRISM layers

PRISM Product Dissemination n n 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:

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

Precipitation Trend Analysis

Precipitation Trends (in./decade)

Extreme Precipitation Climatology

Spatial Considerations n 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” Portland OR MAP 37.39” Max ” Max ” Washington DC MAP 39.00” Washington DC MAP 39.00” Max ” Max ”

Reynolds Creek Watershed Idaho n Valley bottom: MAP: 11.14” 50 yr 1-hour value: 1.18” 50 yr 1-hour value: 1.18” n Mountain top location: MAP: 43.58” 50 yr 1-hour value: 1.26” 50 yr 1-hour value: 1.26” Distance between stations: 10 miles Elevation difference: 3500 feet Elevation difference: 3500 feet

Example of Variations over Small Regions: Reynolds Creek, Idaho (values in mm precipitation): n n 10 Year Return n n 3500 feet Atlas feet Atlas 2 n n 30 min n n 1 hour n n 24 hours n n n 50 Year Return n n 30 min n n 1 hour n n 24 hours

Precipitation Frequency Definition n Year Event: “An event which has a 1 percent chance of occurrence IN ANY YEAR; NOT ONE event every 100 years.”

Extreme Precipitation Climatology References n Technical Paper 40, Rainfall Frequency Atlas of the United States for Durations from 30 minutes to 24 Hours and Return Periods from 1 to 100 Years (1961, $15.50). n NOAA Atlas 2, Precipitation Frequency Atlas of the Western United States, (1973).

Extreme Precipitation Climatology References n Technical Memorandum NWS Hydro 35, Five to 60-minutes Precipitation Frequency for Eastern and Central United States (1977, $9.00) n Technical Paper 49, Two-to-Ten-Day Precipitation for Return Periods of 2 to 100 Years in the Contiguous United States (1964, $7.25)

Hydrometeorological Design Studies Center (HDSC) of the NWS: html

TP Yr 24-hour pcpn. (in.) from SCS TR-55, 1986

50 Year, 6-hour NOAA Atlas 2 Values (inches) Northwest Colorado

100 Year, 24-hour NOAA Atlas 2 Values (inches) Northwest Colorado

Ft. Collins CO Rainfall (in.), 5: pm, July 28, 1997

Probable Maximum Precipitation (PMP) n “Theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographical location at a certain time of the year.”

Storm Characteristics