Nearly the entire basin received much below average precipitation with the Yampa receiving below average precipitation. Similar to March, April temperatures were below average for all basins within the Green. Both maximum and minimum daily temperatures were 3 to 5 Fahrenheit degrees below average. While additional liquid was not added to the snowpack, cooler temperatures help preserve that which was on the ground. Additionally, many stream flow observations were below average due to decrease melt during April.

Dry antecedent soil moisture conditions continue to influence a lower volumetric forecast from the River Forecast Model. While the May 1st snowpack is 95 percent of average, the basin median forecast is only 80 percent of average.

Overall the Yampa and White basins received only 80 percent of average April Precipitation. The Upper Yampa basin received almost 100 percent of its normal precipitation. This, coupled with cooler temperatures that preserved much of the snowpack led to slightly increased forecasts on portions of the Little Snake and Yampa.

April precipitation in the Duchesne Basin was only 45 percent of average. However due to much above average precipitation in February and March, the seasonal snowpack remains near average. Soil moisture conditions prior to the melt were normal. Overall forecast dropped slightly to reflect the seasonal precipitation values.

Similar to the Duchesne, precipitation in April was much below average. Most points are forecast to see a near normal April through July runoff. Outliers include both Ferron Creek and inflow to Joes Valley where volumes are expected to be below average.

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