Nearly the entire basin received much below average precipitation with the Yampa receiving below average precipitation. Similar to March, April temperatures were below average for all basins within the Green. Both maximum and minimum daily temperatures were 3 to 5 Fahrenheit degrees below average. While additional liquid was not added to the snowpack, cooler temperatures help preserve that which was on the ground. Additionally, many stream flow observations were below average due to decrease melt during April.

The Upper Colorado mainstem basin fared pretty well in terms of precipitation during April with near normal amounts, but the Gunnison and Dolores basins received below average amounts for the month. The percent of average snow water equivalent dropped 5% in the Upper Colorado basin, 10% in the Gunnison River basin, and 30% in the Dolores River basin. As a result, median forecasts for the April through July runoff volumes remained steady in the Upper Colorado basin, but dropped 10% and 15% in the Gunnison and Dolores basins, respectively.

-Observed Streamflow...Observed streamflow for the month of April was above to much above average for the San Juan.

-Snowpack/Precipitation...Snowpack above Navajo Reservoir dropped to 120 percent of average. The Animas was near average for snowpack while the entire San Juan Basin dropped to 110 percent. Precipitation over the San Juan Basin for April was much below average with 45 percent. Seasonal average precipitation for the San Juan Basin dropped to 110 percent.

-Short Term Precipitation Forecast...The forecast models through five days keep the San Juan Basin relatively dry with near average temperatures. The CPC forecast days 6 through 10 have an increased chance for below normal temperatures and below normal precipitation.

-General Discussion...The current snowpack along with both statistical forecasts and ESP numbers suggest lowering the forecast across all of the headwaters of the San Juan Basin. Very dry conditions across the San Juan Basin over the past two months also reflect current thinking of lowering volume forecasts for the San Juan. CPC guidance for May 15th through June 1st continues to show an increased chance of above normal temperatures and below normal precipitation.

Dry antecedent soil moisture conditions continue to influence a lower volumetric forecast from the River Forecast Model. While the May 1st snowpack is 95 percent of average, the basin median forecast is only 80 percent of average.

Overall the Yampa and White basins received only 80 percent of average April Precipitation. The Upper Yampa basin received almost 100 percent of its normal precipitation. This, coupled with cooler temperatures that preserved much of the snowpack led to slightly increased forecasts on portions of the Little Snake and Yampa.

April precipitation in the Duchesne Basin was only 45 percent of average. However due to much above average precipitation in February and March, the seasonal snowpack remains near average. Soil moisture conditions prior to the melt were normal. Overall forecast dropped slightly to reflect the seasonal precipitation values.

Similar to the Duchesne, precipitation in April was much below average. Most points are forecast to see a near normal April through July runoff. Outliers include both Ferron Creek and inflow to Joes Valley where volumes are expected to be below average.

In contrast to the majority of western Colorado, which was mostly dry, the Upper Colorado mainstem basin as a whole received about 90% of the average April precipitation. This near normal precipitation combined with slightly cooler than normal temperatures for most of the month kept the high elevation snowpack largely intact, so that the May 1st snow water equivalent was still 120% of average; this is just a 5% decrease from April 1st. The Roaring Fork basin snow water equivalent remains the highest in the basin at close to 150% of average, essentially unchanged from April 1st. Most changes to the forecasts were less than 5% from last months values. The April through July streamflow forecasts now range between 100% and 140% of average, with the exception of Mill Creek which is forecast at 74% of average.

In the Gunnison Basin, precipitation was below average at 75% for the month of April. However, an April with below normal temperatures allowed snow water equivalent to remain above average at 120% of the daily average as of May 1st.
Some snow melt has started in most areas of the basin which is reflected in the streamflow being 130% of average. However, in the Taylor River basin, melt has been minimal and streamflows were below average for April. April through July volume forecasts throughout the basin either remained the same as the April 1st forecast or were lowered around 5%.

Precipitation was below average at 65% for the month of April in the Dolores Basin. As of May 1st, snow water equivialent is at 100% of the daily average.
Snow began to melt rapildly in the middle of April. This shows in monthly streamflow for the Dolores Basin being 145% of average. Inflow into McPhee Reservoir was well above average for the month of April at 178%. April through July volume forecasts in the Dolores Basin decreased by 15%.

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