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.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was near 90 percent of average in the Upper Colorado mainstem basin. April precipitation was near 140 percent of average.

Snow:
May 1st snow water equivalent was near 75 percent of average in the basin as a whole.
--- Upper Colorado basin snow water equivalent plot

Streamflow:
April streamflow was near 110 percent of average.

Soil Moisture:
Modeled soil moisture states were 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 Colorado mainstem basin.

Forecast Summary:
Precipitation and streamflows were much above average and above average for the month of April in the Upper Colorado Basin. However, May 1st snow water equivalent remained below average at 75%. May through July streamflow volume forecasts range from 58 to 85 percent of average, with a median value of 72%. April precipitation and streamflow conditions resulted in a 5-15% increase to the April through July volume forecasts for most locations. The Upper Colorado basin April through July volume forecasts now range between 60 and 87 percent of average, with a median value of 75%.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was 95 percent of average in the Gunnison Basin. April precipitation was 115 percent of average.

Snow:
May 1st snow water equivalent was 80 percent of average in the Gunnison Basin.
--- Gunnison basin snow water equivalent plot

Streamflow:
April streamflow was 125 percent of average.

Soil Moisture:
Modeled soil moisture states were 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 Gunnison Basin.

Forecast Summary:
April precipitation was above average in the Gunnison Basin as a whole, as were the monthly streamflow volumes. However, May 1st snow water equivalent for the Gunnison dropped to below average at 80%. May through July volume forecasts range from 65 to 84 percent of average, with a median value of 75%. This resulted in very little change overall to the April through July volume forecasts, with some points going up a little and others down a little. The Gunnison Basin April through July forecasts now range between 73 and 91 percent of average, with a median value of 80%.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was 105 percent of average in the entire Dolores Basin. April precipitation was 105 percent of average.

Snow:
May 1st snow water equivalent was 70 percent of average in the Dolores Basin.
--- Dolores basin snow water equivalent plot

Streamflow:
April streamflow was 120 percent of average.

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

Climate Forecasts:
The correlation of El Nino and May through July water volumes in the Dolores Basin is so small that it did not influence the forecast process.

Forecast Summary:
Although April precipitation was near average in the Dolores Basin as a whole, the Dolores River headwaters were once again below average at 85%. In addition, the May 1st snow water equivalent was below average at 70%. May through July volume forecasts range from 62 to 75 percent of average, with a median value of 67%. Even with the above average April streamflow volumes, this resulted in about a 5% decrease in the April through July forecast volumes. The Dolores Basin April through July forecasts now range between 75 and 80 percent of average, with a median value of 75%.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was 100 percent of average in the San Juan Basin.
April precipitation was 70 percent of average.

Snow:
May 1st snow water equivalent was 75 percent of average for the San Juan Basin as a whole; it was 65% in the Animas Basin and 85% in the area above Navajo Reservoir.
--- Animas River Basin Snow Plot.
--- Above Navajo Basin Snow Plot.

Streamflow:
Streamflow for the entire San Juan Basin was 120 percent of average in April. 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.

Climate Forecasts:
The correlation of El Nino and May through July water volumes in the San Juan Basin is so small that it did not influence the forecast process.

Forecast Summary:
April precipitation was below average in the San Juan Basin and the May 1st snow water equivalent was below average as well, having decreased quite a bit from last month. May through July volume forecasts range from 66 percent to 93 percent of average with a median value of 75%. Even with the above average April streamflow volumes, this resulted in up to 10% decreases at some points in the April through July forecast volumes. The San Juan Basin April through July forecasts now range between 45 and 98 percent of average, with a median value of 80%.

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