The following conditions influenced this month's forecasts:

Precipitation:
Seasonal, October through April precipitation was 97 percent of average in the Sevier River Basin. April precipitation was 125 percent of average.

Streamflow:
April streamflow estimates were 85 percent of average for the Sevier River at Hatch.

Snowpack:
May 1st, indexed snow water equivalent was 104 percent of average. In the headwater region, indexed snow totals were 144 percent of average. In the lower Sevier Basin, indexed snow totals were 68 percent of average.

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

Soil Moisture:
Modeled Soil Moisture, states were below average heading into this winter and tend to lower seasonal volume forecasts.

Climate Forecasts:
The 30 day outlook suggest a shift in probabilities to above average temperature and above average precipitation for the Sevier River Basin. El Nino conditions weakened during April however anomolies remain near +0.5 degrees C across most of the Pacific Ocean. The El Nino influence was not directly accounted for in the generation of these forecasts.

Forecast Summary:
The Sevier Basin seasonal precipitation remains much above average. The southern headwaters have much above average snow and precipitation numbers, where as the northern portions of the basin are much below average. This situation presents a challenge when describing the overall condition of the basin, hence the wide range of forecasts. April through July Seasonal volume forecast for the Sevier River Basin for May 1st range between 60 and 155 percent of average. The median April through July forecast this month was 107 percent of average compared to 105 percent last month. Residual May through July forecasts range from 55 to 163 percent of average with a median forecast volume of 112 percent of average.

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