Forecasts for Hams Fork, Blacks Fork, and Flaming Gorge inflow are in the top five historical observed volumes.
The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was 130 percent of average in the Upper Green basin. April precipitation was 150 percent of average in the basin as a whole.

Snow:
May 1st snow water equivalent was 160 percent of average in the basin.
--- Upper Green basin snow water equivalent plot.

Streamflow:
April streamflow was near 100 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 La Nina conditions and winter precipitation in the Upper Green basin.

Forecast Summary:
With the addition of much above average precipitation in April, the seasonal precipitation and snow water equivalent have increased since April 1 and are now much above average for the Upper Green. As a result of the current conditions, the April through July streamflow volume forecasts increased by approximately 10 to 20 percent from the April 1 forecasts. Current forecasts range between 108 and 179 percent of average with a median value of 139 percent. Forecasts for Hams Fork, Blacks Fork, and Flaming Gorge inflow are in the top five historical observed volumes.

The majority of these forecast volumes are in the top three or record of historical observed volumes.
The following conditions influenced this month's forecasts:
Precipitation:
Seasonal October through April precipitation was 135 percent of average in the Yampa/White basin. April precipitation was 160 percent of average in the basin as a whole.

Snow:
May 1st snow water equivalent was 165 percent of average in the basin.
--- Yampa basin snow water equivalent plot.

Streamflow:
April streamflow was near 120 percent of average.

Soil Moisture:
Modeled soil moisture states were near average heading into the winter for the Yampa, Little Snake,and White River basins.

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:
With the addition of much above average precipitation in April, the seasonal precipitation and current snow water equivalent have increased since April 1 and are now much above average for the Yampa basin. It is also important to note that the majority of the snotel locations have record snow water equivlent for May 1. As a result of the current conditions, the April through July streamflow volume forecasts have increased by approximately 15 to 25 percent. Current forecasts range between 148 and 216 percent of average with a median value of 193 percent. The majority of these forecast volumes are in the top three or record of historical observed volumes.

Many of these forecast volumes are in the top five historical observed volumes.
The following conditions influenced this month's forecasts:
Precipitation:
Seasonal October through April precipitation was 160 percent of average in the Duchesne basin. April precipitation was 180 percent of average the basin as a whole; however, portions of the basin only received near average precipitation such as the headwaters of Ashley Creek, Big Brush Creek, and Whiterocks.

Snow:
May 1st snow water equivalent was 190 percent of average in the basin.
--- Duchesne basin snow water equivalent plot.

Streamflow:
April streamflow was 120 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 in the forecasts because there is not a strong correlation between La Nina conditions and winter precipitation in the Duchesne basin.

Forecast Summary:
Seasonal precipitation and current snow water equivalent have increased as a result of much above average April precipitation and still remain much above average. As a result of the current conditions, the majority of the April through July streamflow volume forecasts increased by approximately 15 to 20 percent. However, forecasts for Ashley Creek, Big Brush, and Whiterocks remained similiar to the April 1 forecasts due to near average precipitation. Current forecasts range between 108 and 227 percent of average with a median value of 181 percent. Many of these forecast volumes are in the top five historical observed volumes.

Many of these forecast volumes are in the top five historical observed volumes.
The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was 160 percent of average in the Lower Green basin. April precipitation was 180 percent of average.

Snow:
May 1st snow water equivalent was 160 percent of average in the basin as a whole.
--- Lower Green basin snow water equivalent plot.

Streamflow:
April streamflow was near 100 percent of average.

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

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:
Seasonal precipitation and current snow water equivalent have increased as a result of much above average April precipitation and still remain much above average. As a result of the current conditions, the April through July streamflow volume forecasts have increased by approximately 15 to 20 percent from the April 1 forecasts. Current forecasts range between 151 and 190 percent of average with a median value of 167 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