Climatology of Precipitation and Precipitation Extremes in the United States

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Climatology of Precipitation and Precipitation
Extremes in the United States

• Greg Johnson
• Applied Climatologist
• USDA-NRCS
• National Water and Climate Center
• Portland, Oregon

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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)

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Orographic Precipitation Enhancement Factors

• Wind Direction (relative to topography)
• Wind Speed
• Atmos. Moisture (precipitable water)
• Elevation Rise
• Slope Angle

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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

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

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Standardized PrecipitationIndex (SPI)
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A Spatial Climate Modeling System

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

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PRISM

• Originally developed for precipitation only, now
  expanded to temperature, dewpoint, solar
  radiation and many derived variables such as
  HDDs/CDDs, GDDs, frost dates, snowfall, snow
  water equivalent, etc.
• Most commonly applied in monthly or annual time
  increments, but also applied to events

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PRISM Model from OSUsSpatial Climate Analysis
Service

• Funded primarily by the NRCS-NWCC since 1993 for
  development of spatial climate products for the
  U.S.
• Originally developed for precipitation only, now
  expanded to temperature, dewpoint, solar
  radiation and many derived variables such as
  HDDs/CDDs, GDDs, frost dates, snowfall, snow
  water equivalent, etc.
• 4 km horizontal resolution raster data, and ARC
  polygon coverages both available
• Most commonly applied in monthly or annual time
  increments, but also applied to events

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PRISM

• 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

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SNOTEL

• 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

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Typical SNOTEL Site
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SNOTEL Coverage
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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

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Avg. Ann. Precip. from UMAC web site
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Cartographic-quality Map of Mean Annual
Precipitation for Idaho Produced at the USDA-NRCS
NCGC
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Annual Precipitation Map of Elmore County, Idaho
Produced by the NRCS NCGC Cut-out of State Map
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PRISM - SWE Map - Oregon
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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 DEMs)
• NRCS www.ftw.nrcs.usda.gov/prism/prism.html
• (U.S., Regional and State mean annual
  precipitation cartographic products)

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http//www.ftw.nrcs.usda.gov/prism/prism.html

• Cartographic state mean annual precipitation
  maps
• Full repository of official NRCS PRISM layers

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PRISM Product Dissemination

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

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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

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Precipitation Trend Analysis
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Precipitation Trends (in./decade)
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Extreme Precipitation Climatology
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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

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Reynolds Creek Watershed Idaho

• Valley bottom MAP 11.14
• 50 yr 1-hour value 1.18
• Mountain top location MAP 43.58
• 50 yr 1-hour value 1.26
• Distance between stations 10 miles
• Elevation difference 3500 feet

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Example of Variations over Small Regions
Reynolds Creek, Idaho(values in mm
precipitation)

• 10 Year Return
• 3500 feet Atlas 2
  7000 feet Atlas 2
• 30 min. 14 9 13 9
• 1 hour 21 20 29 20
• 24 hours 41 43 103 48
• 50 Year Return
• 30 min. 21 13 19 13
• 1 hour 30 28 32 29
• 24 hours 54 53 129 64

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Precipitation Frequency Definition

• 100 - Year Event An event which has a 1
  percent chance of occurrence IN ANY YEAR NOT ONE
  event every 100 years.

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Extreme Precipitation Climatology References

• 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).
• NOAA Atlas 2, Precipitation Frequency Atlas of
  the Western United States, (1973).

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Extreme Precipitation Climatology References

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

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Hydrometeorological Design Studies Center (HDSC)
of the NWShttp//www.nws.noaa.gov/oh/hdsc/index
.html
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TP-40 100 Yr 24-hour pcpn. (in.) from SCS TR-55,
1986
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50 Year, 6-hour NOAA Atlas 2 Values
(inches) Northwest Colorado
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100 Year, 24-hour NOAA Atlas 2 Values
(inches) Northwest Colorado
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Ft. Collins CO Rainfall (in.), 530 -11 pm, July
28, 1997
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Probable Maximum Precipitation (PMP)

• 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.

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Storm Characteristics
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