-Snow Water Equivalent & Precipitation ... The entire Great Basin received much above average precipitation during April 2009. Monthly totals were 144% and seasonal values were 110% of average. April departure from maximum temperatures were 1 to 5 degrees below normal while minimum temperatures were slightly above normal. Overall we built snowpack during the month of April and then began melting it during the mid and latter half of the month. Indexed snow totals were at their seasonal peak during the first week of May and on May 1 were 107% of average in the Weber and Ogden basins, 108% in the Provo, 100% in the Bear and 120% in the Six Creeks drainages. Overall, April precipitation improved water supply conditions.

-Soil Moisture & Streamflow... Modeled soil moisture has been dry this season due to dry conditions during the late summer and fall of 2008. Model guidance all year has suggested it will lower volumes in 2009. Stream flow on the Weber, Provo and Bear rivers were 134%, 112% and 110% of average.

-Short Term Precipitation Forecasts... Weather guidance from National Weather Service Meteorological Models show mostly average temperature and precipitation conditions during the first 6-10 days of May. Flow is currently zonal which will bring seasonal temperatures, which moderate conditions helping to slowly, instead of quickly, melt the snowpack. Short term CPC guidance shows a slight shift to warmer and dryer conditions for the second half of May which will accelerate the melt of snowpacks.

-General Discussion... Longer range predictions from NOAA's Climate Prediction Center are still factoring in a weak La Nina episode, predicted to last through the spring of 2009. Please reference the web link above for further details and effects within the Great Basin.

-Volumetric forecasts... Currently May through July forecast are average to above average reflecting the above average monthly conditions during April. April through July volume were dervived by summing observed April flows to the May through July forecasts. Median Forecast have now increased between 5-20% by volume.

-Great Basin Elevation... Current ESP estimates for the 2009 GSL maximum summer elevation are 4194.97 ft and we expect the Salt Lake Marina's maximum elevation to reach 1 to 1.5 feet higher than the Saline value in late June. The Saline gage as of May 1st was reading 4194.51 ft. and the harbor gage was reading 4195.81 ft. above mean sea level. The minimum elevation at Saline between May 1 and October 1 is forecast to be 4192.7 ft.

-Snow Water Equivalent & Precipitation... Monthly precipitation was 135% of average and snow water equivalent increased to 100% of average. Seasonal precipitation rose last month to 110% of average.

-Streamflow... Observed streamflows for Bear at the UT/WY State line were 110% of average. These flows contributed to Woodruff Narrows Reservoir completely filling and spilling began on April 21. Observed flows on the Logan River nr Logan above State Dam were also near normal at 97% of average.

-Short Term Precipitation Forecasts... Weather guidance from National Weather Service Meteorological Models show mostly average temperature and precipitation conditions during the first 6-10 days of May. Flow is currently zonal which will bring seasonal temperatures, which moderate conditions helping to slowly, instead of quickly, melt the snowpack. Short term CPC guidance shows a slight shift to warmer and dryer conditions for the second half of May which will accelerate the melt of snowpacks.

-General Discussion... May through July forecast ranged from 89% of average on the Bear at Monpelier to 105% on the Logan Nr State Dam. Volumetric forecast for April through July range from 78% at Bear River nr Montpelier to 115% of average at Little Bear River near Paradise. The median April through July forecast is 98% of average, with a range from 78% t0 115% of average.

-Snow Water Equivalent and Precipitation... Very wet condtions prevailed during April across the Weber basin. Monthly precipiation for the Weber and Ogden basins totalled 148% of average and improved seasonal numbers to 106% of average. Snow water equivalent numbers peaked during April and as of May 1st were 107% of average.

-Streamflow... Streamflows were 112% of average for the Weber at Oakley, 114% for the Weber at Rockport, 132% for the Weber at Echo Reservoir. Melt began during May but several storms brought additional snows and slowed or shut down the melt.Temperature departures from the daily maximums were 1-5 degrees below normal last month and held back the melt.

-Short Term Precipitation Forecasts ... Weather guidance from National Weather Service Meteorological Models show mostly average temperature and precipitation conditions during the first 6-10 days of May. Flow is currently zonal which will bring seasonal temperatures, which moderate conditions helping to slowly, instead of quickly, melt the snowpack. Short term CPC guidance shows a slight shift to warmer and dryer conditions for the second half of May which will accelerate the melt of snowpacks.

-General Discussion ... May through July residual forecast ranged from 100% to 122% of average, with Rockport Reservoir coming in at 122% of average. Monthly streamflow observation were added to these May through July forecast to come up with new April thorugh July volumes and for example, using East Canyon Reservoir is now 116% of average. The median April through July forecast is now 106% of average and the range is 91% to 118% of average.

-Snow Water Equivalent & Precipitation... The Six Creeks drainages received an amazing 169% of average monthly precipitation during April. Many of our snow stations reached their peak season snow during mid April. Snow water equivalent as of May 1 was indexed at 108% of average. Seasonal precipitation totals climbed 7% percent to 120% of average.

-Streamflow... Steam flows for both Big and Little Cottonwood canyons were near average during April. Big Cottonwoods monthly flow was 114% of average and Little Cottonwood ended the month at 96% of average. Temperature departures from the daily maximums were 1-5 degrees below normal last month and held back the melt. Currently the upper elevations on both canyons has greater than average peak seasonal snow.

-Short Term Precipitation Forecasts... Weather guidance from National Weather Service Meteorological Models show mostly average temperature and precipitation conditions during the first 6-10 days of May. Flow is currently zonal which will bring seasonal temperatures, which moderate conditions helping to slowly, instead of quickly, melt the snowpack. Short term CPC guidance shows a slight shift to warmer and dryer conditions for the second half of May which will accelerate the melt of snowpacks.

-General Discussion... May through July volume forecasts ranged from 97% to 154% of average with a median of 112% of average . Observations and or estimates from April were added to the May through July forecast to then calculate the April through July volumes. Currently the range of April through July forecasts range from 100%-125% of average with a median for Six Creeks of 108% of average. City Creek near Salt Lake City is currently forecast at 118% of average for the April through July period.

-Snow Water Equivalent & Precipitation... The drainages that contribute to Utah Lake saw monthly precipitation numbers at 185% of average. Snow water equivalent reached their seasonal peak in mid April at and finished the month at 108% of average. Seasonal precipitation increase last month to 111% of average.

-Streamflow... Stream flow in the upper reaches of the Provo River was much above average this month. The Provo at Woodland came in at 134% of the 30 year average. Temperature departures from the daily maximums were 1-5 degrees below normal last month and held back the melt.

-Short Term Precipitation Forecasts... Weather guidance from National Weather Service Meteorological Models show mostly average temperature and precipitation conditions during the first 6-10 days of May. Flow is currently zonal which will bring seasonal temperatures, which moderate conditions helping to slowly, instead of quickly, melt the snowpack. Short term CPC guidance shows a slight shift to warmer and dryer conditions for the second half of May which will accelerate the melt of snowpacks.

-General Discussion... May through July forecasts averaged from 100% to 107% with a median of 100% of average. The April through July forecasts were calculated by adding April observed or estimated values and ranged from 100% to 108% of average with a median of 102% of average. Inflow for Utah Lake is now forecast to be 108% 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