June 10, 2013

Highlights

• The region is heading into summer with long-term drought conditions comparable to or worse than early June 2012 in nearly all areas; significant drought impacts (water supply, agricultural, wildfire) can be expected
• May precipitation was above average in central Colorado, far northern Wyoming, and portions of eastern Utah, and below-average in northern Utah, southeastern Wyoming, and southern and eastern Colorado.
• Snowpacks have melted out except at the highest elevations; snowmelt occurred 1-3 weeks earlier than the median in most basins.
• The latest (June 1) runoff forecasts continue to call for below-average or well-below-average spring-summer streamflow over nearly all of the region; observed flows in April and May were generally well below average
• The NOAA CPC seasonal climate outlooks show a dry "tilt" for summer precipitation for southeastern Colorado, but no tilt elsewhere. The “SWcast” for July–September shows a slight wet tilt over eastern Colorado, and a dry tilt over north-central Colorado and northern Utah.

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May Precipitation and Temperatures, and Current Drought

As in April, May brought variable precipitation for the region, with central Colorado, far northern Wyoming, and portions of eastern Utah seeing well above-average precipitation. Northern Utah, southeastern Wyoming, and southern and eastern Colorado saw below-average precipitation. The HPRCC Water Year Precipitation map shows a handful of areas with above-average water-year precipitation, but most of the region has experienced drier-than-normal conditions since October 1.

May temperatures were very close to average across the region, with nearly all areas departing less than 2°F from monthly average temperatures. Wyoming overall was slightly warmer than average, and Utah and Colorado were split between warmer- and cooler-than-average conditions.

The latest US Drought Monitor, representing conditions as of June 4, shows some amelioration in drought conditions since late April in north-central Colorado, northern Utah, and northern Wyoming. An area of severe (D2) drought in southwestern Colorado and extreme southeastern Utah has deteriorated to D3 (extreme). Overall, the percentage of each state classified in severe (D2) or worse drought has decreased, with Colorado still highest at 71%, and Utah and Wyoming both at 47%. Colorado remains the only state in the region with D4 (exceptional) drought, occupying much of the southeastern quadrant of the state. Despite the recent improvement, however, the region on the whole is experiencing worse drought conditions than in early June 2012.

Current Snowpack and Streamflows

By early June, the vast majority of SNOTEL sites across the region have melted out, and this year is no exception. Thus the % of normal SWE values, as seen in the Current Basin Snowpack map from NRCS, need to be treated with extreme caution. That said, the map broadly conveys the state of the remaining high-elevation snowpack: below-normal to near-normal conditions in northern and central Colorado, and south-central and north-central Wyoming, and much below normal or prematurely melted out elsewhere.

In the maps of current streamflows across the three states, Colorado is seeing an increased number of gages in the above-normal categories compared to a month ago, reflecting the seasonal peak runoff in those basins with near-normal snowpacks. But across all three states, there are more gages in the below-normal categories than the above-normal categories. The Colorado River near the CO-UT State Line has slipped from the 24th percentile on May 2 to the 15th percentile (52% of median flow) on June 7, reflecting overall below-normal snowpacks in the Colorado headwaters and the Gunnison basins.

Spring-Summer Streamflow Forecasts

The June 1 spring-summer streamflow forecasts from NOAA and the NRCS have deteriorated compared with May 1 for most basins across the region, except for the Colorado headwaters and upper Gunnison basin, which saw above-normal May precipitation. The NRCS Westwide Streamflow Forecast Map shows that the vast majority of basins are forecasted to have well-below-average (50–69%) or lower runoff. For Lake Powell inflows, the official CBRFC forecast for June 1 is at 3.0 MAF (42% of average), unchanged from the May 1 forecast.

Note that these April–July runoff forecasts issued on June 1 include two months of observed streamflow (April and May). The observed volumes this spring have been consistent with the dry outlooks of the previous months' forecasts; across the Upper Colorado River Basin, for example, observed streamflows for May ranged from 40–85% of average.

(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 May 31 by NOAA CPC show a dry tilt for precipitation for June in northern Utah, western Wyoming, and southeastern Colorado. The seasonal outlooks released May 16 show a tilt towards drier-than-average conditions for the June–August period for southeastern Colorado, but no tilt for the rest of the region. 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 June 6 projects that drought conditions will persist for most of our region, but some improvement is expected in eastern Wyoming and northeastern Colorado over the next three months.

As reported in the previous briefing, the SWcast released April 15 for late summer (July–September) has a 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. 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-May ENSO Prediction Plume indicate a 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, with a few models calling for development of weak La Niña conditions by the end of summer.