The following conditions influenced this month's forecasts:

Precipitation:
Seasonal October through April precipitation was 85 percent of average in the San Juan Basin.

April was similar to March with much below average precipitation. At individual sites precipitation ranged mostly between 30 to 75 percent of average. Basin wide April preciptation was 55 percent of average.

Snow:
May 1st snow water equivalent was 25 percent of average for the San Juan Basin. However, at many measuring sites snow has melted out and where it remains values ranged from less than 5 to near 45 percent of average on May 1st.

Due to dry conditions and above averages temperatuers in March and April, the snow melted off more than a month earlier than average at several locations.

Animas River Basin Snow Plot.

San Juan Basin (above Navajo Reservoir) Snow Plot.

Streamflow:
Streamflow volumes were generally above average in April due to the early snow melt. Streamflow volumes in April ranged from 85 to over 200 percent of average.

Soil Moisture:
Modeled soil moisture was near to above average at highest elevations entering the winter season. Below average soil moisture conditions existed over the southern half of the basin and in tributaries further downstream.

Climate Forecasts:
La Nina climate conditions existed through the winter months. The La Nina climate condition suggests drier than average conditions possible over the San Juan Basin. The La Nina climate condition was considered when developing the forecasts.

Forecast Summary:
Despite a wet October and near or above average soil moisture conditions in some headwater areas, the dry and warm spring, low snowpack, and early snow melt had a detrimental impact on anticpated April-July streamflow runoff volumes. Forecast volumes have been further reduced from those issued April 1st. Forecast April-July runoff volumes range from 35 to 65 percent of the 1981-2010 average. May-July runoff volumes volumes are expected to range from near 20 to 50 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 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