The April 1, 2019 Water Supply Forecast Discussion is hot off the presses from the #ColoradoRiver Basin Forecast Center #COriver

Click here to read the discussion. Here’s an excerpt:

The ​Colorado Basin River Forecast Center (CBRFC)​ geographic forecast area includes the Upper Colorado River Basin, Lower Colorado River Basin, and Eastern Great Basin.

Water Supply Forecast Summary:
With the exception of the Green River Basin of Wyoming all of the Upper Colorado River Basin experienced an increase in the April-July water supply forecasts between early March and early April. Similarly, much of the Great Basin noted increases with the exception of some of the northernmost sections.

Widespread significant precipitation occurred over most of the area during the first half of March. Storm systems with a sub-tropical moisture source, similar to those that occurred in February, resulted in large precipitation amounts that extended from southwest and central Utah into parts of southwest and central Colorado. In the areas that experienced the heaviest precipitation, snowpack conditions now range in the top three of the historical records dating back 35-40 years.

The largest increases in water supply forecasts between March 1st and April 1st occurred in the San Juan, Gunnison, and Dolores River Basins. Significant increases also occurred throughout the San Rafael and Sevier River Basins in central and southwest Utah. Due to record February-March precipitation amounts in these areas, April-July runoff volume forecasts range from near 115 to 200 percent of average. Currently only parts of the Green River Basin in Wyoming and the northern 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 630 KAF (87% average), Flaming Gorge 830 KAF (85% of average), Blue Mesa Reservoir 925 KAF (137% of average), McPhee Reservoir 430 KAF (146% of average), and Navajo Reservoir 920 KAF (125% of average). The Lake Powell inflow forecast is 9.20 MAF (128% of average). [ed. emphasis mine]

The Lower Colorado River Basin also started out March very wet particularly in the Gila, Salt, Little Colorado and Virgin River Basins. These areas were also very wet in February. While this area typically experiences drier weather over the next couple of months, many sites in Arizona and New Mexico have already reached their historical seasonal median Jan-May volumes due to rainfall and snowmelt over the past couple of months. April-July runoff volumes in the Virgin River Basin are expected to range from 115-120 percent of average (175-205 percent of median).

Upper Colorado, Great, Virgin River Basins: 2019 April-July forecast volumes as a percent of 1981-2010 average (50% exceedance probability forecast)
Upper Colorado, Great, Virgin River Basins: 2019 April-July forecast volumes as a percent of 1981-2010 average (50% exceedance probability forecast)

Water Supply Discussion
March Weather Synopsis-Precipitation-Temperature:
Following a wet February across much of the Colorado River Basin, the first two weeks of March were a game changer with significantly above normal precipitation across much of Utah and Colorado. An anomalous trough across the Western U.S. during the first half of March was responsible for bringing multiple, moisture-laden storm systems through the Colorado River Basin. By the end of the month, the highest wet anomalies (in percent of normal terms) were across the San Juan, Dolores, and Gunnison Basins where 200-300% of average precipitation fell. Over

much of Utah and the Upper Colorado Headwaters, 130-200% of average was observed. In general, the only area that saw below normal monthly precipitation was the Green River Basin of Wyoming. Over Arizona, precipitation was mostly near to slightly above normal but increased to much above normal in the Gila River Basin and eastern Salt River Basin. The combination of two successive very wet months across much of the Colorado River Basin (particularly Utah/Colorado) has dramatically improved seasonal snowpack and resulting water supply forecasts…

March was cooler than normal over most of the CBRFC forecast area. This acted to preserve snowpack, even at lower elevations, that will contribute to the overall April-July runoff volumes…

Much above normal (median) snow conditions exist across much of Utah and western Colorado and are generally higher in locations farther south. Currently, the only basin that does not have above normal snow is the Upper Green River in Wyoming which is near normal overall.

The following maps show the SNOTEL sites as a percent of normal (1981-2010 median) and also as a historical ranking for their period of record. The snow as represented in the CBRFC hydrologic model is also displayed.

The image below displays the SNOTEL sites as a percent of their historical median as of April 2nd 2019. Those sites in the dark blue currently exceed 150 percent of median (or normal) for this time of year while those in the dark purple are at 200 percent or more of normal.

The snow percentile image displayed below indicates where the current snow measurement ranks in the historical record (typically 35-40 years) for each site. Those sites in black are the highest on record. Those in the dark blue are in the top 3 of their historical record, while those in the brighter blue are in the top ten. This map helps highlight the areas with unusually high snowpack at this time, such as the San Juan and Dolores basins in southwest Colorado.

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