The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was 80 percent of average in the Upper Green basin. April precipitation was 140 percent of average.

Snow:
May 1st snow water equivalent was near 60 percent of average over the entire basin.
--- Upper Green basin snow water equivalent plot.

Streamflow:
April streamflow was near 70 percent of average.

Soil Moisture:
Modeled soil moisture states ranged from below average to near average heading into the winter.

Climate Forecasts:
Climate forecasts were not a factor because there is not a strong correlation between El Nino conditions and winter precipitation in the Upper Green basin.

Forecast Summary:
Due to below average seasonal precipitation to date and much below average May 1st snow water equivalent values the May through July streamflow volume forecasts are much below average at this time. These forecasts range between 41 and 79 percent of average, with a median value of 50 percent.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was 95 percent of average in the Yampa/White basin. April precipitation was 155 percent of average.

Snow:
May 1st snow water equivalent was 80 percent of average in the basin as a whole.
--- Yampa basin snow water equivalent plot.

Streamflow:
April streamflow was near 95 percent of average.

Soil Moisture:
Modeled soil moisture states were near average heading into the winter for the Little Snake and White River basins. Modeled states ranged from below average to near average heading into winter for the Yampa basin.

Climate Forecasts:
Climate forecasts were not a factor because there is not a strong correlation between El Nino conditions and winter precipitation in the Yampa/White basin.

Forecast Summary:
Due to near average seasonal precipitation to date and below average May 1st snow water equivalent values the May through July streamflow volume forecasts are below average at this time. These forecasts range between 46 and 88 percent of average, with a median value of 75 percent.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was 80 percent of average in the Duchesne basin. April precipitation was 130 percent of average.

Snow:
May 1st snow water equivalent was 65 percent of average in the basin as a whole.
--- Duchesne basin snow water equivalent plot.

Streamflow:
April streamflow was near 65 percent of average.

Soil Moisture:
Modeled soil moisture states were much below average to below average heading into the winter.

Climate Forecasts:
Climate forecasts were not a factor because there is not a strong correlation between El Nino conditions and winter precipitation in the Duchesne basin.

Forecast Summary:
Due to below average seasonal precipitation to date and much below average May 1st snow water equivalent values the May through July streamflow volume forecasts are much below average at this time. These forecasts range between 50 and 80 percent of average, with a median value of 70 percent.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was 95 percent of average in the headwaters of the Lower Green basin. April precipitation was 100 percent of average.

Snow:
May 1st snow water equivalent was 80 percent of average in the basin as a whole. The Price and San Rafael headwaters did not receive as much snow as those further to the south. The May 1st snow water equivalent was 65 percent of average in the Price and San Rafael basin.
--- Price basin snow water equivalent plot.

Streamflow:
April streamflow was near 60 percent of average.

Soil Moisture:
Modeled soil moisture states were much below average heading into the winter.

Climate Forecasts:
Climate forecasts were not a factor because there is not a strong correlation between El Nino conditions and winter precipitation in the Lower Green basin.

Forecast Summary:
Due to the below average May 1st snow water equivalent values, and the much below average soil moisture heading into winter the May through July streamflow volume forecasts are much below average at this time. These forecasts range between 48 and 64 percent of average, with a median value of 55 percent.

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