April precipitation was much below average while upper elevation snow pack remained mostly unchanged. Both minimum and maximum temperatures were below normal (1-7 deg. F) and consequently runoff was well below average for most of the basin. Monthly precipitation for the Great Basin basin was 45% of average and seasonal precipitation dropped to 92% of average. Snow throughout the basin remains high at middle and upper elevation at 118% of average. Low elevation snow melted during April and reduced the risk of high peak flows for the time being.

Short term weather guidance from National Weather Service models indicates a persistent zonal pattern. This pattern brings showers, clouds and mostly average temperatures but no ridge that would signal a strong change to a melting regime. Temperatures forecast are calling for an above average probability of being below normal during the 6 to 14 day out. The probabity of a shift to above average precipitation exists during the same period. Modelled streamflows have begun to show simulated rises over the next 10 days and streamflows should be much closer to average by the end of the month.

Volumetric forecasts for the Great Basin decreased slightly this month and now range from 74% at Lost Creek near Croydon to 108% percent of average for Little Cottonwood Creek near Salt Lake city. The average forecast from 30 gaging points was 97% of the 1971-2000 average.

Conditions across the Bear River Basin were dry and cool during the month of April. Snowpack remains healthy in the middle and upper elevations at 112% of average. Monthly precipitation was 53% of average while seasonal numbers dropped to 90% of average. Streamflows at Bear Utah-Wyoming State Line were very low at 36% of average. Forecasts in the Bear ranged from 47% at Stewart Dam near Montpelier (observed flow forecast) to 99% at the Utah-Wyoming State Line.

The Weber and Ogden River basins had another much below average month for precipitation. Monthly precipitation was recorded at 54% of average and seasonal precipitation dropped to 96% of average. Snow water equivalent numbers remain high at 127% of average. Streamflows were 43% of average for the Weber at Oakley and 173% of average for Pineview Reservoir. The good inflow numbers here were the result of a substantial, low elevation snow pack melting in and around Pineview Reservoir. Most snow now lies above 8500 feet. The April through July volume forecasts range from 92% of average at Pineview Reservoir near Ogden to 103% of average at East Canyon Reservoir near Morgan.

Monthly precipitation over the Six Creeks drainages was much below average at 58% of average with seasonal precipitation dropping to 101% of average. Indexed snow numbers for Six Creeks remains much above average at 141%. Snow in the Cottonwoods is currently 118% of the average peak snow. This area currently has the largest snowpack in the Great Basin basin. Steamflow in Big Cottonwood Creek near Salt Lake City was 58% of average and never really got going due to the below average temperatures during the month of April. April through July volume forecasts range from 76% percent of average at Emigration Creek near Salt Lake City to 108% percent of average for Little Cottonwood Creek near Salt Lake City.

Monthly precipitation for Utah Lake Basin was much below average at 49% and seasonal precipitation dropped from 106% to 100% of average. Currently the indexed snow for the Utah Lake Basin is 130% of average. The snow pack SWE values are much the same as last month but because we are holding it so long and the daily averages drop during this time of year, the numbers appear to be going up. Inflows to Utah Lake last month were 94% of average and like Pineview to the north were the recipients of a low elevation snow pack melting. Currently this is a much above average snow pack. April through July inflow to Utah Lake is forecast to be 100% 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