May 13, 2014

Highlights

• April was generally drier than average across the region, with few mountain areas seeing above-average precipitation, in contrast with previous months. The second week of May brought heavy precipitation to many parts of the region.
• As of May 13, snowpack conditions are similar to one month ago, with near-normal to above-normal SWE in Wyoming, northern and central Colorado, and northern Utah, and below-normal SWE in central and southern Utah and southern Colorado.
• The May 1 spring-summer streamflow forecasts slipped a little from the April 1 forecasts, though still calling for most forecast points in Wyoming and northern and central Colorado to have above-average or much-above-average spring runoff. Most forecast points in south-central and southwestern Colorado, and in Utah south of the Wasatch Front, are expected to have below-average or much-below-average runoff.
• Watersheds in the northern Front Range of Colorado most affected by the floods last September still have well-above-normal SWE. There is an elevated potential for snowmelt flooding given the large snowpacks and sediment-impacted stream channels.
• The emergence of an El Niño event in the next several months looks increasingly likely, with atmospheric and oceanic conditions consistently moving in that direction.

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April Precipitation and Temperatures, and Current Drought

After a wet late winter and early spring for the region’s mountains, April brought below-average precipitation to most mountain areas, with the exception of northwestern Wyoming. Utah was on the dry side except for the far northern and the far eastern portions of the state. In Colorado, there were wet pockets in the south-central and northeast, with the rest of the state seeing below-average to average precipitation. Wyoming was mostly drier than average, especially the southwestern quarter of the state. The second week of May brought a couple of potent systems that dropped 0.5"–3” of precipitation across the region, providing a big boost to the snowpack, with north-central Colorado and southeast Wyoming seeing the highest totals.

April temperatures were close to normal across the region, with few areas more than 2°F above or below the monthly average.

The latest US Drought Monitor, representing conditions as of May 6, shows a net increase in drought severity in our region since early April. Drought conditions worsened in southeastern Utah and adjacent southwestern Colorado, and in eastern Colorado. Improvement was seen in southeastern Wyoming. The proportion of the region in D2 or worse drought increased again, with Utah up to 22% from 16%, Colorado up to 19% from 15%, and Wyoming unchanged at zero.

Current Snowpack

May 13 is past the typical peak snow accumulation date for all basins in our region and all but the highest-elevation and northernmost sites. The percent-of -median-SWE values may be misleading in those basins that are well down the declining limb of their seasonal snowpack trajectory, such as in southern Utah.

A generally dry April was balanced out by a wet early May in most mountain areas, so the Current Basin Snowpack map from NRCS shows that conditions as of May 13 are similar to where they were one month ago. Wyoming and northern Colorado continue to have above-normal to much-above-normal SWE, with nearly all basins at 130% of median SWE or greater. Northern Utah basins are near or above normal, while the remainder of the state is below-normal and headed to early meltout, albeit delayed by the most recent storms. In Colorado, the San Juan-San Miguel-Dolores (75% of median SWE) and Rio Grande (56% of median SWE) basins are lagging well behind the rest of the state.

According to the Colorado Dust-on-Snow Program, three dust-on-snow events affected at least southwestern Colorado in late April and early May, bringing the seasonal total to eight, one more than the average number of seasonal events since 2005. Consolidation and surface exposure of the previous dust layers from the larger events D3, D4, and D5 led to earlier and faster snowmelt throughout April in many parts of the Colorado mountains, especially in the San Juans. While these dust layers have now been reburied by the most recent snows, they will re-emerge in coming days to weeks and continue to hasten snowmelt and runoff.

Current Streamflows

The maps of current streamflows for Colorado, Utah, and Wyoming show that flows are near normal to above normal in Colorado and Wyoming, with several gages in Wyoming reporting daily record high flows. Flows are mainly normal to much below normal in Utah, with a few gages recording record daily low flows.

Spring-Summer Streamflow Forecasts

The Westwide Streamflow Forecast Map based on the NRCS May 1 forecasts shows small decreases from the April 1 forecasts in several basins, including southeastern and central Wyoming and central Utah. Most forecast points in Wyoming and northern and central Colorado are expected to have near-average (90-109%), above-average (110–129%) or much-above-average (130–149%) spring and summer runoff. Most forecast points in south-central and southwestern Colorado, and most forecast points across Utah, are expected to have below-average (70–89%) to much-below-average (50-69%) spring and summer runoff, with an increasing number of points expected to have far-below-average (25–49%) runoff, and four southern Utah points in the under-25% category. Most points in northern Utah and west-central Colorado are expected to have below-average (70-89%) to near-average (90–109%) runoff.

In the watersheds in the northern Front Range of Colorado (St. Vrain/Boulder, Big Thompson, upper Cache la Poudre) that were most affected by the floods last September, SNOTEL sites are still showing much-above-normal (>130% of median) SWE. The NRCS May 1 forecasts for these basins slipped slightly, to 107–122% of average spring–summer runoff, though these forecasts were made prior to the snow accumulation from the major storm on May 10–13. According to NOAA NWS, there is still a high potential for snowmelt flooding in these basins. Since stream channels are impacted by debris and excess sediment in many locations in these basins, the risk and severity of flooding may be greater than in previous high-runoff years.

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 May 1 forecast for Lake Powell April–July inflows is 7160 KAF, 100% of average. The NOAA CBRFC May 1 forecast for Lake Powell inflows is 7550 KAF, which is 105% of average.

Seasonal Climate and Drought Forecasts

The monthly Climate Outlook for April released on April 30th by NOAA CPC shows a wet tilt for precipitation for nearly all of Wyoming, and far northeastern Colorado, with the strongest tilt in northern Wyoming. The seasonal outlooks released April 17 show a wet tilt in precipitation for most of Utah and Colorado for May–July and June–August periods, with the strongest tilt in eastern Utah and western Colorado. The latest CPC Seasonal Drought Outlook released April 17 projects that the areas of drought conditions (D1 or worse) in Utah and Colorado will persist through at least July, but no additional areas of drought development are expected.

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.

The prospect of an El Niño event emerging in the next several months looks increasingly likely, with atmospheric and oceanic conditions indicating a continued progression towards El Niño. In the IRI ENSO Prediction Plume from mid-April, all of the models call for near-term warming in the Niño 3.4 region, and by the mid-summer period (June–August), two-thirds of the models are calling for El Niño onset, exceeding 75% by the September–November period. The experimental PSD ENSO forecast has a >80% chance of El Niño development by mid-summer. Assuming the El Niño develops, there is still great uncertainty as to how strong it may become.