May 12, 2015

Highlights

• While April was overall wetter and cooler than the previous three months, and May has gotten off to a wet start, very poor runoff is still expected in most basins across the region.
• April precipitation was well-above-average in some areas, and below-average in others. The first part of May has been very wet so far over most of the region.
• Snowpacks in most basins peaked 4–6 weeks early and are well along towards meltout, with many individual SNOTEL sites at lower and middle elevations having already melted out.
• The May 1 spring-summer runoff forecasts are even lower than the April 1 forecasts in most basins. Most points are expected to see much-below-average (50–69%), far-below-average (25–49%) or extremely low (<25%) runoff, especially in Utah.
• El Niño conditions have strengthened further and are expected to continue into the winter. A wet tilt in seasonal precipitation through the summer is forecasted for our region, largely due to El Niño’s influence.

Click on the thumbnail images in the text below to bring up the full-sized graphic

April Precipitation and Temperatures, and Current Drought

April precipitation was quite variable around the region. Many of the areas that have had consistently dry conditions this water year were dry again in April, including southwestern Colorado, much of southern Utah, northeastern Utah, and northeastern Wyoming. Central and southeastern Wyoming, central and northeastern Colorado, and east-central Utah saw well-above-normal precipitation. Overall, the region’s mountain areas were split between wet and dry. On a statewide basis, April precipitation came out as near average in all three states: Utah was in the 40th percentile, Colorado was in the 66th percentile, and Wyoming in the 60th percentile.

A very wet weather pattern that began the last week of April has continued into May, bringing well-above-normal precipitation to nearly all of Utah and Colorado, and southern Wyoming. While this weather has been beneficial in replenishing soil moisture, slowing snowmelt, and reducing early-season irrigation demand, it is “too little, too late” with respect to the overall regional water supply picture.

The temperatures in April were generally above average, but the warm anomalies were not nearly as unusual as in the previous three months. Most of the region saw temperatures within 2°F of the monthly normal. The statewide average temperatures for April were similar among the three states relative to the respective long-term records: Utah was in the 68th percentile; Wyoming, the 73rd percentile; and Colorado, the 77th percentile.

The latest US Drought Monitor, based on conditions as of May 5, shows that since early April, there has been some degradation in southwestern Wyoming, from D0 (abnormally dry) to moderate drought (D1) and in northeastern Wyoming, from drought-free to D0.  But other areas saw improvement, including an area in west-central Utah in far western Colorado that went from D2 to D1, and in northeastern Colorado from D0 to drought-free. The proportion of Utah in D2 or worse drought decreased from 52% last month to 46%; likewise in Colorado, which dropped from 40% to 32%. Wyoming remained at 1%.

Current Streamflows

As of May 11, the maps of current streamflows for Colorado, Utah, and Wyoming show that flows are generally above normal or much above normal in Colorado and Wyoming, with several gages in the Arkansas and South Platte Basins at record high flow for the date after a very rainy week. In Utah the flows are more distributed across the below-normal, normal, and above-normal classes, with normal flows predominating. For the most part, flows on larger streams and rivers across the region are still rising towards their seasonal peak flows, but many streams in Utah have already seen the pulse of snowmelt runoff. On May 11, the Colorado River near the CO-UT State Line was in the 66th percentile, 131% of median flow; the Green River at Green River, UT was in the 40th percentile, 88% of median flow; and the San Juan River near Bluff, UT, was in the 12th percentile, 34% of median flow.

Current Snowpack

The Current Basin Snowpack map from NRCS as of May 11 continues to paint a bleak picture, with only three basins at more than 80% of median SWE for the date, and many basins below 50%. The worst conditions are in Utah, where the May 1 statewide snowpack was record-low at 23% of normal. Across the region, the snowpack peaked early at well-below-normal levels in most locations, and melt has been proceeding for 4–6 weeks. Many low- and mid-elevation sites have nearly or completely melted out, well ahead of normal meltout. Some sites in northern Utah for which median SWE is over 20” in early May are down to their last dregs. The exceptions to the regional picture are the Arkansas and South Platte basins, which have been favored by upslope storms in the late winter and spring, including a very wet start to May, and the snowpack is just above 100% of median SWE.

According to the Colorado Dust-on-Snow program, even after a third and more substantial event in mid-April, the total seasonal dust-on-snow deposition in Colorado has been lighter than in recent seasons. In northern Utah, that same mid-April storm system deposited substantial dust on what snow remained in the Wasatch, per Jim Steenburgh's Wasatch weather blog.

Spring-Summer Streamflow Forecasts

In the May 1 spring-summer streamflow forecasts, released by NOAA CBRFC and the NRCS in early May, the expected runoff declined further at many forecast points relative to the April 1 forecasts. With the very wet conditions in early May, the daily ESP forecasts from NOAA CBRFC as of May 12 show an uptick in many basins, but only back to the April 1 forecasted level at best. Overall, the outlook for regional runoff is still grim—one of the worst five seasons since 1980 in most basins.

The NRCS Westwide Streamflow Forecast Map shows that nearly all forecast points in Utah are still expected to have far-below-average (25–49%) or extremely low (<25%) spring and summer runoff. Most Wyoming and Colorado forecast points are expected to have much-below-average (50–69%), and far-below-average (25–49%) runoff, with a lesser number seeing below-average (70–89%) runoff. Lower flows are expected in southwestern and northwestern Colorado, and central Wyoming, than elsewhere. Only in the South Platte River Basin and the adjacent Colorado River headwaters is near-average (90–109%) spring and summer runoff expected. The Lake Powell seasonal inflow forecast has declined again since April 1, to around 40% of average inflows; see Note below.

IMPORTANT NOTE: While NRCS and NOAA continue to collaborate on streamflow forecasts, the forecast values are no longer being strictly coordinated between NRCS and some of the RFCs, including the CBRFC. For example, the NRCS April 1 forecast for Lake Powell April–July inflows is 2700 KAF, 38% of average. The NOAA CBRFC April 1 forecast for the same point is 3000 KAF, 42% of average.

Seasonal Climate and Drought Forecasts, including ENSO

The monthly Climate Outlook for May released on April 30 by NOAA CPC shows a moderate wet tilt for precipitation for all of Colorado, and a slight wet tilt for most of Wyoming and Utah. The seasonal outlooks released April 16 show a moderate wet tilt in both the May–July and June–August periods for nearly all of the region. Much of this overall forecasted pattern of wet tilt in the region US is associated with the expected persistence of El Niño conditions.

The latest CPC Seasonal Drought Outlook  released on April 16 projects that areas of drought conditions (D1 or worse) will improve or be removed by the end of July in northern Utah, western Colorado, and southeastern Colorado. Drought areas in southern Utah and the San Luis Valley of Colorado are expected to persist or intensify. No additional areas of drought development in the region are expected.

The updated Experimental PSD Seasonal Precipitation Forecast Guidance ('SWcast') for the April–June period issued on April 14 is very similar to the initial forecast released in mid-March. It shows a tilt towards above-average precipitation over most of eastern, central, and southwestern Colorado, and northern Utah.  The wet tilt is strongest in east-central Colorado. There is a slight dry tilt forecasted for the far northeastern corner of Colorado. (Note: "Skill-masked" means that the forecast only shows the tilts for those areas where there has been demonstrated forecast skill for this season in the past.)

Sea-surface temperatures anomalies have recently strengthened in across the tropical Pacific, and are now indicative of moderate El Niño conditions. The Multivariate ENSO Index (MEI) likewise jumped in the March–April period into moderate El Niño territory. There is a stronger consensus from the ENSO model forecasts, compiled by IRI in the mid-April ENSO Prediction Plume, that El Niño conditions will continue at least through the summer, with 70% calling for persistence into the winter.