The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 90 percent of average in the Upper Green basin. May precipitation was 120 percent of average.

Snow:
On June 1st, the basinwide snowpack was 95 percent of average. However, the upper part of the basin, above 10,000 feet in elevation, is not gaged. The water year snowpack peaked on April 15th at much below the average peak.
--- Upper Green basin snow water equivalent plot.

Streamflow:
May streamflow was near 20 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 primarily to the much below average water year peak snowpack, the April through July streamflow volume forecasts are much below average. These forecasts range between 47 and 73 percent of average, with a median value of 55 percent. The June through July streamflow volume forecasts range between 54 and 82 percent of average, with a median value of 60 percent.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 95 percent of average in the Yampa/White basin. May precipitation was 110 percent of average.

Snow:
June 1st snow water equivalent was 80 percent of average in the basin as a whole. The seasonal snowpack peaked on April 10th at 80 percent of the average peak. This year cooler temperatures retained some of the snowpack later into the season.
--- Yampa basin snow water equivalent plot.

Streamflow:
May streamflow was near 85 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 the near average precipitation to date, and because some of the snowpack was retained later into the season because of cooler temperatures, the April through July streamflow volume forecasts are near average at this time. These forecasts range between 54 and 128 percent of average, with a median value of 105 percent. The June through July streamflow volume forecasts range between 51 and 115 percent of average, with a median value of 80 percent.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 80 percent of average in the Duchesne basin. May precipitation was 90 percent of average.

Snow:
June 1st snow water equivalent was 85 percent of average in the basin as a whole.
--- Duchesne basin snow water equivalent plot.

Streamflow:
May streamflow was near 40 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 water year precipitation, below average seasonal snowpack, and much below average soil moisture heading into winter, the April through July streamflow volume forecasts continue to be much below average. These forecasts range between 34 and 76 percent of average, with a median value of 55 percent. The June through July streamflow volume forecasts range between 25 and 73 percent of average, with a median value of 65 percent.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 95 percent of average in the headwaters of the Lower Green basin. May precipitation was 90 percent of average.

Snow:
June 1st snow water equivalent was 50 percent of average in the Price and San Rafael basin. Elsewhere within the Lower Green the snow was essentially depleted.
--- Price basin snow water equivalent plot.

Streamflow:
May streamflow was near 50 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 seasonal snowpack, and the much below average soil moisture heading into winter, the April through July streamflow volume forecasts continue to be much below average. These forecasts range between 32 and 61 percent of average, with a median value of 45 percent. The June through July streamflow volume forecasts range between 45 and 65 percent of average, with a median value of 50 percent.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was near 90 percent of average in the Upper Colorado mainstem basin. May precipitation was also near 90 percent of average.

Snow:
June 1st snow water equivalent was near 55 percent of average in the basin as a whole. The majority of the remaining snow is above 10,000 feet. The snow peaked in May at 85% of the average seasonal peak.
--- Upper Colorado basin snow water equivalent plot

Streamflow:
May streamflow was near 80 percent of average.

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

Forecast Summary:
Basin wide precipitation was near average for the month of May. However, some headwater areas received greater amounts of precipitation. June through July streamflow volume forecasts range from 43 to 85 percent of average,with a median value of 69%. April through July streamflow volume forecasts remained the same or increased slightly in certain headwater areas. The Upper Colorado basin April through July streamflow volume forecasts now range from 58 to 85 percent of average, with a median value of 72%.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 95 percent of average in the Gunnison Basin. May precipitation was 60 percent of average.

Snow:
June 1st snow water equivalent was 35 percent of average in the Gunnison Basin. There is still some snow above 11,500 feet with minimal areal extent. The snow peaked above Blue Mesa in early April around 95 percent of the seasonal average.
--- Gunnison basin snow water equivalent plot

Streamflow:
May streamflow was 70 percent of average.

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

Forecast Summary:
May precipitation was much below average in the Gunnison Basin, as were the monthly streamflow volumes. Also, June 1st snow water equivalent for the Gunnison dropped to 35 percent below average. June through July volume forecasts range from 56 to 69 percent of average, with a median value of 62%. This resulted in a 5 percent decrease to the April through July volume forecasts. The Gunnison Basin April through July forecasts now range between 66 and 86 percent of average, with a median value of 74%.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 100 percent of average in the entire Dolores Basin. May precipitation was 45 percent of average.

Snow:
June 1st snow water equivalent was 0 percent of average in the Dolores Basin. However, there is still enough snow above 11,500 feet with minimal areal extent for some runoff into June. The snow peaked in early April at about 110 percent of the seasonal average.
--- Dolores basin snow water equivalent plot

Streamflow:
May streamflow was 70 percent of average.

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

Forecast Summary:
May precipitation was much below average in the Dolores Basin. In addition, the May 1st snow water equivalent was below average at 0%, but still some snow was left above 11,500 feet for runoff into June. June through July volume forecasts range from 30 to 53 percent of average, with a median value of 34%. This resulted in about a 5% decrease in the April through July forecast volumes. The Dolores Basin April through July forecasts now range between 67 and 72 percent of average, with a median value of 69%.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 95 percent of average in the San Juan Basin.
May precipitation was 30 percent of average.

Snow:
As of June 1st, most snow at SNOTEL locations in the San Juan Basin were at zero. However, there is still enough snow above 11,500 feet for some runoff for the remainder June. The snow peaked in early April at around 110% of the average seasonal peak.
--- Animas River Basin Snow Plot.
--- Above Navajo Basin Snow Plot.

Streamflow:
Streamflow for the entire San Juan Basin was 90 percent of average in May. Seasonal streamflow since October was 65% due to the much below average summer through fall precipitation.

Soil Moisture:
Modeled soil moisture as of November 1st, 2009 was below to much below average.

Forecast Summary:
May precipitation was much below average in the San Juan Basin and the June 1st snow water equivalent was much below average as well. June through July volume forecasts range from 37 percent to 67 percent of average with a median value of 55%. The much below average precipitation in May resulted in 5% to 10% decreases at most points for the April through July forecast volumes. The San Juan Basin April through July forecasts now range between 70 and 91 percent of average, with a median value of 75%.

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