COLORADO BASIN RIVER FORECAST CENTER
  NATIONAL WEATHER SERVICE / NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION

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New Averages for 2012 Water Year

Beginning January 1, 2012, we are using the 1981-2010 period for averages, forecast equations, and model calibration.

Frequently Asked Questions
1. Why Do we update averages every 10 years using the most recent 30 year period for average computation?
Answer: We follow the NOAA and World Meteorological Organization (WMO) convention. NOAA/NCDC developed an explanation of this here

2. How does the 1981-2010 period compare with the 1971-2000 period and previous 30 year periods?
Answer: Streamflow volumes in the new 30 year period is generally between 5% and 20% lower than in the previous period. The largest decreases are observed in the upper Green River and the Bear River. For the entire upper Colorado above Lake Powell, the difference is 11%. The new 30 year period has the lowest average volume of any of the 30 year periods in the instrumental record. More details are available here

3. How does using the new period effect CBRFC forecasts and data on this webpage?
Answer: Long-lead water supply and peak flow forecasts generally use a combination of current conditions for snowpack and streamflow, a weather prediction, and the climatological distribution of precipitation and temperature. Thus long lead forecasts using the new average period will generally be lower than forecasts that used the previous 30 year period. The amount of the difference depends on the difference between the means and the duration of climatology assumed in the forecast period. For example, January 1 water supply forecasts require a climatological assumption for the months of January through July whereas April 1 water supply forecasts require only April through July. Thus the January 1 forecasts are influenced more greatly by the change in the 30 year period. An example illustrating this effect is available here
The snow time series plots on the CBRFC pages are now using the 1981-2010 period of record for calculated statistics including the daily mean, median, maximum, and minimum snow water equivalents. CBRFC requires at least 20 years of record to calculate any of these statistics.

4. How is this change being coordinated with other forecast groups?
Answer: NOAA/NCDC has published new means for the meteorological 30 year means. The NRCS and other RFCs plan to begin using the 1981-2010 average period in 2013.

5. Where can I get more information?
Answer: CBRFC staff is here to help. Please feel free to contact us.