June 11, 2015

Highlights

• An extraordinarily wet May has dramatically improved drought conditions and the outlook for spring-summer runoff across the region. But runoff is still expected to be below average or much below average at most forecast points outside of central and eastern Colorado.
• May precipitation was greater than 200% of average over most of the region, with record-wet conditions at many locations.
• With the cool and wet weather, significant snow persisted at high-elevation sites in Colorado and Wyoming through May, delaying and enhancing the expected runoff.
• The June 1 spring-summer runoff forecasts from NOAA CBRFC are much higher than the May 1 forecasts for the Upper Colorado River Basin and eastern Great Basin.
• El Niño conditions have continued to strengthen and are expected to continue through the winter. Seasonal precipitation outlooks for our region show a wet tilt through the summer, partly due to El Niño’s expected influence.

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May Precipitation and Temperatures, and Current Drought

In late April the weather pattern shifted to one bringing frequent moist systems to the region. Unusually, the pattern persisted for several weeks, and so the rain and snow kept coming. And coming. And not just in a few places. Nearly the entire region received more than 200% of average precipitation for the month. The wettest areas, with over 400% of average, were southwestern Utah and portions of west-central and northwestern Utah. The driest areas—relatively—were in northern and eastern Wyoming, where “only” 125–200% of average precipitation fell. The moisture was particularly welcome in the context of the overall dry winter conditions; Salt Lake City saw as much precipitation in May (4.19”, 215% of average) as in January, February, March, and April combined.

On a statewide basis, it was the record-wettest May in Colorado (record begins in 1895), with an area–averaged 5.27” of precipitation, 261% of average. May was just shy of the wettest on record in Utah (99th percentile) and Wyoming (97th percentile). For the 3-state region as a whole, it appears that this was the wettest May on record, edging out May 1995 and May 1957. In those two years, the very heavy May precipitation occurred after a wet winter, unlike this year.

As one would expect, the very wet weather in May was accompanied by below-average temperatures, with nearly all locations ending up 2–6°F below the monthly normal. For statewide average temperatures for May, Utah was in the 36th percentile; Wyoming, the 32nd percentile; and Colorado, the 19th percentile.

The latest US Drought Monitor, based on conditions as of June 2, shows widespread improvement since early May, especially in Colorado. Southeastern Colorado is nearly drought-free after improving by 1 to 3 categories. Improvements of 1 or 2 categories occurred in western Colorado, southwestern Wyoming, and northeastern Utah. The proportion of Colorado in D2 or worse drought plummeted from 32% last month to zero; in Utah it decreased from 46% to 34%; and Wyoming lost its smidgen of D2 drought and is also at zero.

Current Streamflows

As of June 8, 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 South Platte Basin at record high flow for the date. In Utah the flows are either normal or in the below-normal classes. For the most part, flows on larger streams and rivers in Colorado and Wyoming are still ramping up towards their seasonal peaks. On June 8, the Colorado River near the CO-UT State Line was in the 71st percentile, 154% of median flow; the Green River at Green River, WY was in the 63rd percentile, 144% of median flow; and the San Juan River near Bluff, UT, was in the 52nd percentile, 103% of median flow.

Current Snowpack

The Current Basin Snowpack map from NRCS as of June 8 needs to be taken with a grain of salt this late in the season, since this a time when most SNOTEL sites have typically melted out or are well on their way. But the very high % of median values for most basins in Colorado, and several in Wyoming, do reflect the unusual late-season trajectory of the high-elevation snowpack in those two states: as snow kept falling in the mountains through May, it offset the melt that occurred, and high-elevation sites either declined slowly, held steady, or showed increases in SWE from May 1 to June 1. Since June 1, with warmer temperatures, rapid melt has occurred at all sites.

According to the Colorado Dust-on-Snow program, this season’s three dust events—an unusually low number—have produced a merged dust layer that is exposed in many areas of western Colorado but is “comparatively weak.” Accordingly, the dust is expected to have only a modest influence on the remaining snowmelt, and runoff.

Spring-Summer Streamflow Forecasts

Normally we don't see dramatic changes in forecasted spring-summer streamflows after May 1—but this is not a normal year. The June 1 forecasts from NOAA CBRFC for the Upper Colorado River Basin and the eastern Great Basin show large increases in expected April–July runoff compared to the May 1 forecasts, generally by 15–40 percentage points. For example, the June 1 forecast for spring-summer Lake Powell inflows is 5000 KAF, 70% of average, well above the May 1 forecast of 3000 KAF, 42% of average. Overall, the May precipitation has boosted the 2015 runoff picture from “grim” to “not so bad, considering.” (Note: NRCS does not produce official forecasts after May 1 so the Westwide NRCS Streamflow Forecast map has not been updated; see the CBRFC's special June water supply webinar for more information on their June 1 forecasts.)

Seasonal Climate and Drought Forecasts, including ENSO

The monthly Climate Outlook for June released on May 31 by NOAA CPC shows a significant wet tilt for precipitation over most of Colorado and southeastern Wyoming, and a slight wet tilt for the rest of Wyoming and Utah. The seasonal outlooks released May 21 show similar patterns, with moderate wet tilts for the June–August and July–September periods for much of the region. This overall forecasted wet-tilted pattern for the region is partly due to the expected moderate to strong El Niño conditions through the summer.

The latest CPC Seasonal Drought Outlook released on May 21 projects that areas of drought conditions (D1 or worse) will improve or be removed by the end of August in northern and eastern Utah, western Colorado, southeastern Colorado, and southwestern Wyoming. Drought areas in western Utah are expected to persist or intensify. No additional areas of drought development in the region are expected.

Sea-surface temperatures anomalies have continued to strengthen in the central and eastern tropical Pacific, shifting from moderate to strong El Niño conditions. The Multivariate ENSO Index (MEI) likewise increased again in the April–May period, also into moderate-to-strong El Niño territory. Nearly all ENSO model forecasts, compiled by IRI in the mid-May ENSO Prediction Plume, project that El Niño conditions will continue at least through the summer, with 80% calling for persistence through the winter.