The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was 85 percent of average.

April precipitation was 80 percent of average in the Sevier River basin.

Streamflow:
Streamflow for the Sevier River at Hatch was 130 percent of average for April. The above average April flows in the Sevier River basin are a result of snowmelt occurring earlier than usual.

Snowpack:
May 1st snow water equivalent was 25 percent of average for the overall Sevier River Basin. For the Sevier River headwaters, the May 1st snow water equivalent was 45 percent of average.

Sevier River Drainage Snow Plot.
Sevier River Basin Headwaters Snow Plot.
Lower Sevier River Basin Below Piute Snow Plot.

Soil Moisture:
Modeled soil moisture content was above average in the Sevier River headwater areas going into the winter.

Climate Forecasts:
Generally, climate scenarios like El Nino and La Nina do not strongly influence conditions in the Sevier River Basin. The southern part of the basin sees more of an influence from El Nino and La Nina than the northern part of the basin does. However, even in the southern part of the basin, the statistical correlations between climate indices like the Southern Oscillation Index and streamflow volumes are minor. As such, climate conditions are given only low weight, if any, in our water supply forecasts for the Sevier River basin.

Forecast Summary:
Above average temperatures in April continued to melt the snowpack in the Sevier River Basin, especially at the lower and middle elevations. The snowpack continues to be much below average. May 1st snow water equivalent was 25 percent of average for the overall Sevier River basin. The snowpack that remains in the high elevations and in the headwater areas is at 45 percent of average.

Volumes during the May-July period are expected to range from 24 to 47 percent of average. Current April through July Seasonal volume forecasts range between 40 and 60 percent of average. The median April through July forecast volume is 50 percent of average.

The forecasts for the Sevier River at Hatch, the Sevier River near Kingston, the East Fork of the Sevier River, Piute Reservoir, and the Sevier River near Gunnison are 'Regulated or Observed' flow forecasts rather than 'Natural or Unregulated' forecasts, which are corrected for all upstream diversions.

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