The following conditions influenced this month's forecasts:

Precipitation:
Seasonal, October through May precipitation was 122 percent of average in the Sevier River Basin. May precipitation was 74 percent of average.

Streamflow:
April streamflow estimates were 99 percent of average for the Sevier River at Hatch.

Snowpack:
Peak snow in the upper Sevier River Basin occurred this season during the first week of April at approximately 145 percent of average. However further north in the lower basin peak snow values were only 85 percent of average. This divergence was the result of a classic El Nino pattern that affected the entire state. Currently the lower Sevier has very little snow remaining while the upper basin still has significant snow at several stations above 9000 feet.

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 El Nino that affected the Sevier River Basin this winter has now ended and neutral conditions persist in the tropical Pacific Ocean.

Forecast Summary:
The Sevier Basin seasonal precipitation decreased slightly during May due to the below average monthly precipitation. There was a noticable decrease in the median April through July volume forecast this month as streamflows in the upper basin were below average in April and average for May but in the lower basin flows were well below average. The soil moisture models were prediting lower flows this year but not to the extent that it would lower the forecast by 20 percent given the above average snow conditions that persisted in the headwaters. This months April through July Seasonal volume forecast for the Sevier River Basin range between 59 and 129 percent of average. The median April through July forecast this month was 84 percent of average compared to 107 percent last month. Residual June through July forecasts range from 55 to 176 percent of average with a median forecast volume of 97 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