Many of these forecast volumes are in the top five historical observed volumes.

The following conditions influenced this month's forecasts:

Precipitation:
April precipitation was 176 percent of average. Seasonal October through April precipitation was 169 percent of average. In the upper basin the monthly precipitation was 183 percent and the seasonal precipitation was 198 percent of average.

Streamflow:
Streamflow for the Sevier River at Hatch was 315 percent of average.

Snowpack:
May 1st snow water equivalent was 160 percent of average.Many SNOTEL sites in the basin have snow water equivalent values in the top 5% of their historical records.

---Upper Sevier River Basin Snow Plot.
---Sevier River Basin Snow Plot.
---Lower Sevier River Basin Snow Plot.

Soil Moisture:
Modeled Soil Moisture states were much above average heading into the winter.

Climate Forecasts:
Generally climate scenarios like El Nino and La Nina have a weak to moderate effect in the Sevier River Basin. However because we a lack of good statistical correlation, these events are not specifically factored into our water supply forecasts.

Forecast Summary:
April precipitation throughout the Sevier Basin was 176 percent of average. Seasonal October through April precipitation in the Sevier Basin remained much above average at 169 percent. Current April through July seasonal volume forecasts range between 150 to 319 percent of average. The median forecast was 204 percent of average. Many of these forecast volumes are in the top five historical observed volumes.

As a reminder, forecasts for the EF Sevier, Sevier at Kingston, Piute Reservoir and Sevier near San Pitch Below Gunnison are all 'Regulated or Observed' flow forecasts rather than 'Natural or Unregulated' forecasts which are corrected for all upstream diversions. Also because of this during large runoff years the regulated averages are exceeded in a significant way and show fairly large numbers. The Sevier at Kingston is a case in point for 2011.

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