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Water Supply Forecast Summary:
The majority of the Upper Colorado River and Great Basin April-July water supply forecasts increased between April and May. The forecasts at locations that did not increase had minimal changes from early April.
Widespread significant precipitation occurred over the Green River Basin in Wyoming, the Great Basin and the Sevier and San Rafael River basins during the first half of April. The remainder of the Upper Colorado River Basin was mostly dry and received minimal precipitation the first half of the month. However, river basins in Colorado benefited from a significant precipitation event the last four days of the month. Specifically, the Gunnison River basin and Upper Colorado River basin headwaters received up to or more than the average monthly total precipitation during this time period.
The largest increases in water supply forecasts between April 1st and May 1st occurred in the Green River basin in Wyoming, and the San Juan, Gunnison, and Dolores River Basins. Significant increases also occurred throughout the Great Basin, Duchesne, San Rafael and Sevier River Basins in Utah. Forecasts in the Upper Colorado River headwaters and Yampa River basins had slight increases or remained similar to the April 1st forecasts. April-July runoff volume forecasts now range from near 115 to 200 percent of average. Currently only a few northern headwater basins of the Green River Basin in Wyoming and the Great Basin (Bear River Basin) have forecasts below average for the 2019 season.
Very dry soil moisture conditions were widespread entering the winter season. These may have some impact on the overall yield of runoff that ends up in the streams depending on how the snow melt plays out. In areas with significant snowpack or where snowmelt is delayed the impacts of dry soils may be lessened.
April-July unregulated inflow forecasts for some of the major reservoirs in the Upper Colorado River Basin include Fontenelle Reservoir 740 KAF (102% average), Flaming Gorge 1050 KAF (108% of average), Blue Mesa Reservoir 970 KAF (144% of average), McPhee Reservoir 420 KAF (142% of average), and Navajo Reservoir 930 KAF (127% of average). The Lake Powell inflow forecast is 9.20 MAF (128% of average).
April Weather Synopsis-Precipitation-Temperature:
Storm systems favored central/northern Utah and southwest Wyoming for the first half of April. Areas including the Great Basin, Sevier River Basin and the Green River Basin received above average precipitation for the first two weeks of April. River basins in Colorado did not benefit from the storm track early in the month. However, areas in Colorado benefited from a significant precipitation event the last four days of the month which continued into the first few days of May. Specifically, the Gunnison River basin and Upper Colorado River basin headwaters received up to or more than the average monthly total precipitation during this time period.
By the end of the month, the highest wet anomalies (in percent of normal terms) were across the Green River basin in Wyoming, the Duchesne River Basin, parts of central Utah and the Great Basin including the Bear, Weber, Six Creeks, Provo, and Sevier River basins where precipitation was 120-140% of average. Other basins including the Upper Colorado Headwaters, Gunnison River Basin and the Yampa River Basin ended the month with precipitation near 100-105% of average. These areas would have ended the month with below average precipitation and a resulting decrease in water supply forecasts had it not been for the storm at the end of the month. The San Juan River Basin, Dolores River Basin and the Lower Colorado River Basin in Arizona all received below average precipitation for April…
Soil moisture conditions in the higher elevation headwater areas are important entering the winter, prior to snowfall, as it can influence the efficiency of the snowmelt runoff the following spring. The effects are most pronounced when soil moisture conditions and snowpack conditions are both much above or much below average. In areas where the soil moisture was below average entering the winter and the current snowpack is also much below median, spring runoff may be further reduced.
Modeled soil moisture conditions as of November 15th were below average over most of the Upper Colorado River Basin and Great Basin. In the Upper Colorado River Mainstem River Basin, soil moisture conditions were below average in headwater basins along the Continental Divide, and closer to average downstream. Soil moisture conditions in the Gunnison, Dolores, and San Juan basins were much below average.