June 13, 2014

Highlights

• May was wetter than average across most of Colorado, adding to the above-average snowpacks in many basins. Drier than average conditions were experienced in most of Wyoming and northern Utah.
• The spring runoff, in line with previous forecasts, has been above average to much above average in nearly all of Wyoming, and in northern, central, and southeastern Colorado. Below to much-below-average runoff has been observed in southwestern and south-central Colorado, southwestern Utah, and most of the eastern Great Basin.
• Many gages in Colorado and Wyoming have seen record-high or near-record-high daily flows in the past three weeks, with minor to moderate flooding in some areas.
• With the above-average spring runoff in the northern two-thirds of the region, reservoirs are regaining storage depleted by the last two years of dry conditions.
• Atmospheric and oceanic conditions are still pointing to the very likely emergence of an El Niño event by fall. Sea-surface temperatures have continued to warm in the equatorial Pacific.

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

May Precipitation and Temperatures, and Current Drought

May was wetter than average across Colorado (except the southeastern quarter), southern Utah, and far southeastern Wyoming. The month was drier than average in northern Utah and nearly all of Wyoming, though only relatively small areas saw less than 50% of average precipitation.

May temperatures were up 3°F to cooler than average in most of Colorado, eastern Utah, and eastern Wyoming, and up to 4°F warmer than the monthly average, in the remainder of Wyoming and Utah.

The latest US Drought Monitor, representing conditions as of June 10, shows slightly less drought for the region compared to early May. Drought conditions improved in eastern and southwestern Colorado, while worsening in a smaller area of south-central Colorado. The proportion of the region in D2 or worse drought: Utah unchanged at 22%, Colorado at 17%, down from 19%, and Wyoming unchanged at zero.

Current Snowpack

Early June is typically well past the peak accumulation at even the highest-elevation SNOTEL sites, and many sites have normally melted out by this date. Thus, the numerical percent-of-median-SWE values on the Current Basin Snowpack Map should be treated with caution. That said, the prevalence of dark blue colors (>150% of median) on the map in Colorado, Wyoming, and far northern Utah does indicate an unusually large and persistent late-season snowpack in those basins. The basins in southwestern Colorado and southern Utah that were lagging behind seasonal normals throughout the spring have completely melted out and are not shown on the map.

According to the Colorado Dust-on-Snow Program (CODOS), no more dust events have occurred since May 13, leaving the seasonal total at eight events. The latest CODOS update noted that consolidated dust layers from events D3–D8 were exposed at the snow surface during field surveys done from June 2–4 at CODOS sites at Grand Mesa, Hoosier Pass, Loveland Pass (Grizzly Peak), Berthoud Pass, Willow Creek Pass, Rabbit Ears Pass, and McClure Pass, as well as Independence Pass. With the exposed dust, snow-surface albedo (reflectivity) is unusually low, and melt rates will be above-average through the remainder of the season, leading to a steeper-than-average declining limb of the annual hydrograph. 

Spring Runoff and Current Streamflows

Overall, observed runoff in April and May has been consistent with the May 1 water supply forecasts made by the NOAA Colorado Basin River Forecast Center and by the NRCS. Runoff has been above-average to much-above average in most of Wyoming, and northern and central Colorado. Below to much-below-average runoff has been observed in southwestern and south-central Colorado, southwestern Utah, and most of the eastern Great Basin.

As is typical for early June, mountain snowpacks have been melting rapidly, and rivers in the basins that accumulated above-normal snowpacks have produced high daily flows. Many gages in northern and central Colorado saw record-high or near-record daily flows in late May or early June. The Colorado River near Dotsero peaked slightly above flood stage, while on the other side of the Continental Divide, the Cache la Poudre River peaked well above flood stage west of Fort Collins and in Greeley, causing damage to several dozen homes and businesses. Flows at these gages, and elsewhere in northern and central Colorado, are now probably past the seasonal peak but remain much above normal. Most parts of the Front Range impacted by flooding last September have been spared further flooding so far.

As of June 12, the maps of current streamflows for Colorado, Utah, and Wyoming show that flows are normal to much above normal in Colorado and Wyoming, and mainly below normal or normal in Utah.

Reservoir Storage

After two years of below-average runoff depleted the region’s water storage, this spring's runoff thus far has helped most of the region's reservoirs regain lost ground. At the end of May, nearly all reservoirs in Colorado and Wyoming had more storage than last year, while Utah's reservoirs are more split between gains and losses compared to last year. As of June 11, Lake Powell held 11.76 MAF (48% full), down very slightly from 11.79 MAF (48% full) on the same date last year. However, last year at this time Powell's storage had nearly peaked for the season, while now Powell is still gaining about 80,000 AF per day, and should continue to gain storage through June.

Spring–Summer Streamflow Forecasts

The June 1 forecasts issued by NCRS and NOAA CBRFC for spring–summer streamflow across the region are generally similar to the May 1 forecasts, except in the Colorado Headwaters, Yampa-White, and South Platte basins in Colorado, where above-average May precipitation led to considerable increases in the runoff forecasted for June and July. Note that NRCS does not update their Westwide Streamflow Forecast Map, shown above in the Dashboard, past May 1.

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. The NRCS June 1 forecast for Lake Powell April–July inflows is 7300 KAF, 102% of average, up from 7160 KAF in the May 1 forecast. The NOAA CBRFC June 1 forecast for Lake Powell inflows is 7550 KAF, 105% of average, which is unchanged from the May 1 forecast.

Seasonal Climate and Drought Forecasts, including ENSO

The monthly Climate Outlook for June released on May 31 by NOAA CPC shows a wet tilt for precipitation for nearly all of Wyoming, and northern and eastern Colorado, with the strongest tilt in far eastern Wyoming. The seasonal outlooks released May 15 show a wet tilt in precipitation for our entire region for the June–August and July–September periods, with the strongest tilt in eastern Utah, western Colorado, and southwestern Wyoming.

The latest CPC Seasonal Drought Outlook released May 15 projects that the areas of drought conditions (D1 or worse) in Colorado and southeastern Utah will improve or be removed by the end of August, while drought areas in western Utah will persist through that time frame. No additional areas of drought development are expected.

As reported in the previous briefing, according to the experimental PSD precipitation forecast guidance ('SWcast'), for the April–June period there is a dry tilt for southern Utah, extending into northwestern Colorado, and a slight wet tilt for southwestern Colorado. For the July–September period there is a dry tilt for all of Utah, and northern and eastern Colorado. Note that forecast skill is relatively poor for the latter period and lead time.

Atmospheric and oceanic conditions continue to indicate that an El Niño event is emerging. The Multivariate ENSO Index (MEI) experienced its 2nd largest increase ever for the April–May timeframe, moving sharply into El Niño territory. In the IRI ENSO Prediction Plume from mid-May, nearly all of the models call for continued near-term warming in the Niño 3.4 region. By the late-summer period (July–August), a little under two-thirds of the models are calling for El Niño onset, reaching over 70% by fall. The consensus of the forecasters is that El Niño onset is even more likely than indicated by the models. Assuming the El Niño develops, it is still very uncertain how strong it may become.