May 6, 2013 (updated May 7)

Highlights

  • Recent snows have improved the runoff picture, but most of the Intermountain West is still facing low or very low 2013 spring-summer runoff with reservoirs already at low levels
  • April precipitation was mixed for the region, with northern and central Colorado, portions of Wyoming, and eastern Utah being wetter than average, and northern Utah, southwestern Wyoming, and southern Colorado being mainly drier than average.
  • Snowpacks in eastern Utah, much of Wyoming, and northern and central Colorado saw large gains in April, reaching near-normal peak levels and delaying snowmelt. Southern Colorado and southern Utah did not see these gains and meltout began early, from well-below-normal peaks.
  • May 1 forecasts for spring-summer streamflow for the region are still below-average or well-below-average except in far northwestern Wyoming, but are generally improved from the April 1 forecasted flows across Wyoming, northern and eastern Utah, and northern and central Colorado.
  • The NOAA CPC seasonal climate outlooks show a dry "tilt" for spring and summer precipitation for most of our region. In contrast, the “SWcast” for April–June shows a wet tilt over much of Colorado.

April Precipitation and Temperatures, and Current Drought

April brought a mixed bag of precipitation for the region, with storms consistently favoring northern and central Colorado, portions of Wyoming, and eastern Utah, with some areas seeing over 200% of normal precipitation. Conversely, northern Utah, southwestern Wyoming, and southern Colorado were mostly skipped by these storms and saw below-average precipitation. The HPRCC Water Year Precipitation map shows that the wetter areas in April have emerged as the largest "green islands" of above-average water-year precipitation, in a sea of drier-than-normal conditions since October 1. Southeastern Colorado remains by far the driest part of the region over that period, with isolated pockets in southern Colorado and in Utah also seeing less than 50% of average October–April precipitation.

Several severe cold-air outbreaks led to April temperatures being colder than average across the region, except for parts of southeastern Utah and southwestern Colorado. Dozens of daily record-low minimum temperatures were set during April throughout the region, including 2°F in Boulder, CO on April 10, and -7°F in Cody, WY on April 9, which was also a monthly record for April.

The latest US Drought Monitor, representing conditions as of April 30, shows further improvement in regional drought conditions since late March; extreme (D3) drought in central-western Colorado and central Wyoming has been upgraded to D2 and D1. While the majority of the region is still classified in severe (D2) or worse drought, relatively little is now in D3 and D4, with the largest and most persistent area of D4 occupying most of southeastern Colorado.

Current Snowpack and Streamflows

As reported in the previous briefing, a series of storms after April 8 significantly boosted snowpacks in many mountain areas, with the last storm sweeping in on April 30. This late in the season, the % of normal SWE values, as seen in the Current Basin Snowpack map from NRCS, can be misleading since nearly all SNOTEL sites have typically begun to melt out by now. But it is fair to say that the basins in northern and central Colorado, northeastern Utah, and much of Wyoming clawed their way back to near-normal (90–100%) snowpack conditions at the higher elevations by May 1. Snowpacks in southern Utah and southern Colorado did not seem the same boost in April as the basins to the north, and the snow there has been melting out steadily since mid-April, leaving May 1 values at 70% of normal or less, even at the highest elevations. Overall, the snowpack picture for the region is generally improved over April 1.

In Colorado, the new snow in April was accompanied by two substantial dust-on-snow events, with the storm on April 15–16 depositing dust from the San Juans to the Front Range. The dust loading in the San Juans is now similar to the high-dust springs of 2009 and 2010, and has been speeding up the snowmelt. In the northern and central mountains of Colorado, the dust layers are still buried for the most part, but once exposed will likewise cause earlier meltout than in the absence of dust. As WWA researcher Jeff Deems was quoted in the Denver Post, the April snow was "a mixed blessing" because of the dust deposition.

In the maps of current streamflows across the three states, the “normal” category is most frequently reported, but with more gages in the below-normal categories than above-normal, particularly in Wyoming. The Green River near Greendale, UT gage, was in the 12th percentile, at only 55% of the median flow for May 2, probably reflecting delayed snowmelt with the recent storms. Similarly, the Colorado River near the CO-UT State Line was in the 24th percentile, at 63% of the median flow for May 2. These flows should improve relative to normal as snowmelt picks up over the next few weeks.

Spring-Summer Streamflow Forecasts

Like the snowpacks, the May 1 spring-summer streamflow forecasts from NOAA and the NRCS are generally improved across the region compared with April 1, except in southern Colorado and southern Utah. The NRCS Westwide Streamflow Forecast Map shows a strong north-south gradient, with basins in northern Wyoming forecasted to have near-average (90–109%) or below-average (70–89%) runoff; in southern Wyoming, northern and central Colorado, and northern and eastern Wyoming, mainly below-average (70–89%) or well-below-average (50–69%) runoff; and in southern Utah and southern Colorado, far-below-average (25–49%) runoff. Only a few small basins remain in the very lowest category (<25% of average). For Lake Powell inflows, the official CBRFC forecast for May 1 is at 3.0 MAF (42% of average), slightly higher than the April 1 forecast of 2.7 MAF.

The runoff outlook for the region is lower than the final snowpack numbers (and their late-season improvement) would suggest, which reflects the unusually dry soil moisture last fall that is expected to significantly reduce runoff efficiency. (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.)

Seasonal Climate and Drought Forecasts

The latest monthly Climate Outlooks released on April 30 by NOAA CPC show a slight dry tilt for precipitation for May in northern and western Wyoming. The seasonal outlooks released April 18 show a tilt towards drier-than-average conditions for the May–July period for all of the region, with the strongest tilt over western Colorado and eastern Utah. For the June–August period, the area with dry tilt shrinks and shifts to southeastern Colorado. The CPC Climate Outlooks once again show enhanced odds for above-normal temperatures for the upcoming seasons, consistent with the long-term warming trend. The latest CPC Seasonal Drought Outlook released May 2 projects that drought conditions will persist for most of our region, but some improvement is expected in far eastern Wyoming and the eastern half of Colorado over the next three months.

The latest PSD Precipitation Forecast Guidance ("SWcast")released April 12 for April–June 2013 conditions, continues to show a wet tilt for spring and early summer for Colorado, though shifted more to eastern Colorado compared with the SWcast released in March. A modest tilt towards dry conditions is forecasted for northwestern Utah. The SWcast released April 15 for late summer (July–September) continues the slight dry tilt for northwestern Utah, and also for extreme north-central Colorado and southern Wyoming, with a slight wet tilt for eastern Colorado.

ENSO indicators continue to show ENSO-neutral conditions, on the cool (La Niña) side of neutral. Since last fall, ENSO conditions have not exerted much influence on the climate system, nor on the seasonal forecasts. The models in IRI's mid-April ENSO Prediction Plume indicate a strong consensus towards ENSO-neutral conditions continuing through next summer and fall, with the dynamical models tending towards the warm side of neutral, and the statistical models tending to stay on the cool side.

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