The Upper Colorado mainstem and the Gunnison basin both received above average precipitation for the month, but the Dolores basin only received 80% of average. Temperatures averaged over the entire month were near to slightly below average in western Colorado during May, but daily temperatures within the month fluctuated between periods of much above and much below average temperatures. As a result, May runoff volumes were much above average in the Upper Colorado and Gunnison, but there is plenty of snow left to melt as June 1st snow water equivalent is much above average. May runoff volumes in the Dolores basin were near normal and June 1st snow water equivalent is below average. The median of the forecasts for the April through July runoff volumes increased in the Upper Colorado and Gunnison, but decreased in the Dolores.

After two months of slightly below average precipitation, the Upper Colorado mainstem basin returned to wetter conditions during May with 110% of average precipitation for the month. This, combined with already high snowpack and near to slightly below average monthly temperatures, left the June 1st snow water equivalent values at much above average. The April through July streamflow forecasts were raised at most points in the basin, although some did go down because of low observed flows to date. These forecasts now range between 98% and 145% of average, with the exception of Mill Creek which is forecast at 66% of average.

Precipitation was above average in the month of May for the Gunnison Basin. Maximum temperatures were again below average resulting in a slower melting of the snow pack. However, runoff volumes in the Gunnison Basin were above average for May because of the above average snow pack.
April through July volume forecasts in the Taylor Basin and Blue Mesa Reservoir have been been increased due to the above average precipitation in May. April through July volume forecasts in the rest of the basin remained steady.

The Dolores Basin did not fair as well as the rest of the Upper Colorado only receiving 80% of the average May precipitation. However, maximum temperatures were below average in May like the rest of the Upper Colorado. Runoff volume in the basin was around average for May.
There is little snow remaining in the Dolores Basin. This, along with below average precipitation for May, has resulted in a 5% to 10% decrease in the April through July volume forecasts in the Dolores Basin.

Differences between the full period forecasts and the residual forecasts may not exactly equal the actual observed volumes due to rounding conventions (see Definitions section).

10% exceedance forecast: Given the current hydrometeorological conditions, i.e current snowpack, soil moisture and streamflow, the volume that has a 10% chance of being exceeded. Previously referred to as "Reasonable Maximum Forecast".

50% exceedance forecast: Given the current hydrometeorological conditions, i.e current snowpack, soil moisture and streamflow, the volume that has a 50% chance of being exceeded. Previously referred to as "Most Probable Forecast".

90% exceedance forecast: Given the current hydrometeorological conditions, i.e current snowpack, soil moisture and streamflow, the volume that has a 90% chance of being exceeded. Previously referred to as "Reasonable Minimum Forecast".

Acre-Foot (af): The volume equal to one acre covered one foot deep (43,560 cubic feet). See kaf below.

Average: The arithmetic mean. The sum of the values divided by the number of values. Values from 1971-2000 are used for this publication.

Categories: Much Above Average=Greater than 130%, Above Average=111-130%, Near Average=90-110%, Below Average=70-89%, Much Below Average=Less than 70%.

CBRFC: Colorado Basin River Forecast Center.

Forecast Period: The period from April 1 through July 31, unless otherwise noted.

kaf: Thousand Acre-Feet. See Acre-Foot above.

Inflow: The volume of water that flows into a reservoir or lake.

Median: The middle value of an ordered set of values. Half of the values are higher and half of the values are lower. When the set contains an even number of values the median is the average of the two middle numbers.

NOAA: National Oceanic and Atomospheric Administration.

NWS: National Weather Service.

Water supply forecasts take into consideration present hydrometeorological conditions and use average basin temperatures and precipitation for the forecast period. As the forecast season progresses, a greater portion of the future hydrologic and climatic uncertainty becomes known and monthly forecasts become more accurate. For more information on the tools we use, consult Water Supply Forecasting Tools.

Volume forecasts represent adjusted flows; that is, observed flows with upstream water use taken into account. Adjusted flows will closely approximate natural or unimpaired flows. However, not all upstream diversions or impoundments are measured or quantifiable. For specific adjustments used with each forecast point, consult the Guide to Water Supply Forecasting.

The Water Supply Outlook is issued monthly January through May by the Colorado Basin River Forecast Center. It represents a coordinated effort between the National Weather Service, Natural Resources Conservation Service, Bureau of Reclamation, U.S. Geological Survey and local water district managers.

Note: Data used in this report are provisional and are subject to revision.

For more information, or to be included on the mailing list, please contact:

Colorado Basin River Forecast Center
2242 W North Temple
Salt Lake City, UT 84116
(801) 524-5130
www.cbrfc.noaa.gov