The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was 90 percent of average in the Upper Green basin as a whole. April precipitation was 65 percent of average.

Snow:
May 1st snow water equivalent was 50 percent of average in the basin as a whole as measured at the snotel locations. It is important to note that the highest snotel location is at 9800 feet in the Upper Green; however, the basin extends to elevations up to 13,000 feet. Nearly half of the basin is not represented by the snotel network. Low and mid elevation snowmelt has continued with another month of above average temperatures and below average precipitation. Remote sensing data suggests that a considerable amount of snow remains at high elevations.
--- Upper Green basin snow water equivalent plot.

Streamflow:
April streamflow was near 110 percent of average.

Soil Moisture:
Modeled soil moisture states indicated near average to above average soil moisture conditions last fall prior to snow accumulation.

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

Forecast Summary:
Similar to March, April had much below average precipitation and above average temperatures. However, seasonal precipitation in the Upper Green remains near average as a result of above average precipitation in months other than December, March and April. The addition of another unseasonably warm and dry month in conjunction with continued snowmelt has resulted in significant decreases in the April through July forecasts. Current April through July streamflow volume forecasts in the headwaters of the Upper Green have decreased by 10 to 20 percent while forecasts for basins located in the Wyoming range (Ham's Fork) and the North Slope of the Uintas decreased by 15 to 25 percent. April through July forecasts now range between 48 and 90 percent of average with a median value of 74 percent.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was 65 percent of average in the Yampa/White basin. April precipitation was near 50 percent of average.

Snow:
April 1st snow water equivalent was 30 percent of average in the basin. Significant snowmelt has continued with another month of above average temperatures and below average precipitation. Many of the snotel locations have record low snow or have completely melted out. The majority of remaining snow is located above 10,000 feet.
--- Yampa basin snow water equivalent plot.

Streamflow:
April streamflow was 105 percent of average over the basin as a whole. However, some headwater locations had monthly volumes between 140 and 160 percent of average.

Soil Moisture:
Modeled soil moisture states indicated above average to much above average soil moisture conditions last fall prior to snow accumulation.

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

Forecast Summary:
Above average temperatures and much below average precipitation continue to prevail in the Yampa/White. As a result, snow and water supply conditions have continued to deteriorate rapidly. Unseasonably warm and dry conditions in April, have resulted in another significant decrease in the April through July streamflow volume forecasts. Forecasts have decreased by and additional 15 to 20 percent. Forecasts have decreased by as much as 50 percent from March 1 due to the conditions in March and April and many of the May through July forecasts are in the bottom five of the historical records. April through July forecasts now range between 32 and 55 percent of average with a median value of 45 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 60 percent of average.

Snow:
May 1st snow water equivalent was 30 percent of average in the basin. Significant snowmelt has continued with another month of above average temperatures and below average precipitation. Many of the snotel locations have record or near record low snow or have completely melted out. The majority of remaining snow is located above 10,000 feet.
--- Duchesne basin snow water equivalent plot.

Streamflow:
April streamflow was near 185 percent of average over the basin as a whole. However, some headwater locations had monthly volumes greater than 200 percent.

Soil Moisture:
Modeled soil moisture states indicated much above average soil moisture conditions last fall prior to snow accumulation.

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

Forecast Summary:
Similar to March, April had much below average precipitation and above average temperatures. The addition of another unseasonably warm and dry month in conjunction with continued snowmelt has resulted in another decrease in the April through July forecasts. Current April through July streamflow volume forecasts decreased by 10 to 20 percent. May through July forecasts for many of the low elevations basins such as Strawberry near Solider Springs and Currant Creek are in the bottom five of the historical records. April through July forecasts now range between 20 and 68 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 75 percent of average in the Lower Green basin. April precipitation was 55 percent of average.

Snow:
There is no snow remaining at any of the snotel sites. Any remaining snow is located at the highest elevations.
--- Lower Green basin snow water equivalent plot.

Streamflow:
April streamflow was near 105 percent of average over the basin as a whole.

Soil Moisture:
Modeled soil moisture states indicated near average to above average soil moisture conditions last fall prior to snow accumulation.

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

Forecast Summary:
Similar to March, April had much below average precipitation and above average temperatures. The addition of another unseasonably warm and dry month in conjunction with continued snowmelt has resulted in another decrease in the April through July forecasts. As a result, current April through July streamflow volume forecasts have decreased by 15 to 30 percent and now range between 16 and 48 percent of average with a median value of 39 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 1981-2010 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