Most of these forecast volumes are in the top two historical volumes of the last 40 years for both the April-July and the June-July period. The following points are forecast at or above the record volume: Lake Granby, Willow Creek Reservoir, Wolford Reservoir, Colorado River near Kremmling, and Colorado River near Dotsero

The following conditions influenced this month's forecasts:

Precipitation:
May precipitation was near 130 percent of average overall in the Upper Colorado mainstem basin making it above average for seven out of the eight months of this water year so far. Seasonal precipitation (the total since October) remained at 140 percent of average.

Snow:
June 1st snow water equivalent was near 290 percent of average in the basin as a whole; please note that this time of year the percent of average may not provide a valid measure of conditions. Another month of cooler than average temperatures has delayed the bulk of the snow melt and as of June 1st there was still 100% of the average seasonal peak, which usually occurs in mid April. Conditions are even more extreme in the headwaters of the basin where the June 1st snow water equivalent was near 130 percent of the average seasonal peak. 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 110 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:
Most forecast volumes were raised from last month due to the much above average precipitation during May as well as cool temperatures which inhibited 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 235 percent of average with a median value of 165 percent. Current June through July streamflow volume forecasts range between 165 and 315 percent of average with a median value of 190 percent. Most of these forecast volumes are in the top two historical volumes of the last 40 years for both the April-July and the June-July period. The following points are forecast at or above the record volume: Lake Granby, Willow Creek Reservoir, Wolford Reservoir, Colorado River near Kremmling, and Colorado River near Dotsero

The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through May precipitation was 120 percent of average in the Gunnison basin. May precipitation was 115 percent of average.

Snow:
June 1st snow water equivalent was near 230 percent of average in the Gunnison basin; please note that this time of year the percent of average may not provide a valid measure of conditions.
--- Gunnison basin snow water equivalent plot

Streamflow:
May streamflow was 90 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:
May precipitation throughout the Gunnison basin was above average at 115%. Seasonal precipitation for the Gunnison did not change from last month's 120% of average. The current April through July streamflow volume forecasts dropped 5% from last month's forecast. Current April through July streamflow volume forecasts range between 100 and 133 percent of average with a median value of 125 percent.

The following conditions influenced this month's forecasts:

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

Snow:
June 1st snow water equivalent was 140 percent of average in the Dolores basin; please note that this time of year the percent of average may not provide a valid measure of conditions.
--- Dolores basin snow water equivalent plot

Streamflow:
May streamflow was 70 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:
May precipitation throughout the Dolores basin was much above average at 150%. Seasonal precipitation for the Dolores basin remained the same from last month at 105% of average. Due to the cool temperatures in May and above average monthly precipitation, this month's April-July forecast volumes rose slightly from last month. Current April through July streamflow volume forecasts range between 76 and 93 percent of average, with a median value of 80 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