Many of these forecast volumes are in the top three of historical volumes, with the forecasts for Lake Granby, Willow Creek Reservoir and Wolford Reservoir at or above the record volumes.

The following conditions influenced this month's forecasts:

Precipitation:
April precipitation was near 175 percent of average overall in the Upper Colorado mainstem basin. However, the uppermost headwaters of the basin received near 190% of average precipitation for the month, while the Eagle River basin had close to 230% of average. Seasonal precipitation (the total since October) increased since last month and now stands at 140 percent of average.

Snow:
May 1st snow water equivalent was near 155 percent of average in the basin as a whole. This is especially significant for this time of year as the snow melt has been delayed and so it is also near 150% of the average seasonal peak. Conditions are even more extreme in the headwaters of the basin where the May 1st snow water equivalent was near 175 percent of average. Many snotel sites throughout the basin have snow water equivalent values that are at record levels when compared to their historical period (20-30 years).
--- Upper Colorado basin snow water equivalent plot

Streamflow:
April streamflow was above average at 125 percent.

Soil Moisture:
Modeled soil moisture states were near to above average heading into the winter.

Climate Forecasts:
Climate forecasts were not a factor because there is not a strong correlation between La Nina conditions and winter precipitation in the Upper Colorado mainstem basin.

Forecast Summary:
All forecast volumes were raised significantly from last month due to the much above average precipitation during April as well as cool temperatures which have delayed the start of the snow melt. This has resulted in near to above record snow pack conditions throughout much of the basin. Current April through July streamflow volume forecasts range between 140 and 190 percent of average with a median value of 155 percent. Current May through July streamflow volume forecasts range between 145 and 200 percent of average with a median value of 165 percent. Many of these forecast volumes are in the top three of historical volumes, with the forecasts for Lake Granby, Willow Creek Reservoir and Wolford Reservoir at or above the record volumes.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was 120 percent of average in the Gunnison basin. April precipitation was 150 percent of average.

Snow:
May 1st snow water equivalent was near 140 percent of average in the Gunnison basin; this is around 120% of the seasonal peak.
--- Gunnison basin snow water equivalent plot

Streamflow:
April streamflow was 100 percent of average.

Soil Moisture:
Modeled soil moisture states were below to near average heading into the winter.

Climate Forecasts:
Climate forecasts were not a factor because there is not a strong correlation between La Nina conditions and winter precipitation in the Gunnison basin.

Forecast Summary:
April precipitation throughout the Gunnison basin was much above average at 150%. Seasonal precipitation for the Gunnison basin rose from last month to 120% of average. May 1st snow water equivalent for the Gunnison was much above average with 140%; whereas the snow water equivalent is 120% of the average seasonal peak. Since there was an increase in the snow water equivalent in the Gunnison basin, the current April through July streamflow volume forecasts rose from last month's forecast. Current April through July streamflow volume forecasts range between 98 and 146 percent of average with a median value of 130 percent.

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was 105 percent of average in the entire Dolores basin. April precipitation was 150 percent of average.

Snow:
May 1st snow water equivalent was 95 percent of average in the Dolores basin as a whole; this is about 45% of the seasonal peak.
--- Dolores basin snow water equivalent plot

Streamflow:
April streamflow was 65 percent of average.

Soil Moisture:
Modeled soil moisture states were average to slightly below average heading into the winter.

Climate Forecasts:
The correlation of La Nina and April through July water volumes in the Dolores basin is small that it did not influence the forecast process.

Forecast Summary:
April precipitation throughout the Dolores basin was much above average at 150%. Seasonal precipitation for the Dolores basin rose from last month to 105% of average. May 1st snow water equivalent for the Dolores was average with 95%; whereas the snow water equivalent is 45% of the average seasonal peak. Since the percent of average for both snow water equivalent and seasonal precipitation increased from last month, the current April through July streamflow volume forecasts rose in the Dolores Basin from last month's forecast. Current April through July streamflow volume forecasts range between 72 and 92 percent of average, with a median value of 75 percent.

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