September 8, 2014

Highlights

  • After mixed outcomes for precipitation in July, the summer ended on a wet note in August, with much of the region receiving over 200% of average precipitation for the month.
  • The vast majority of reservoirs across the region now have more storage than at the end of August last year. Reservoirs in Colorado and Wyoming are also ahead of the long-term average, while most reservoirs in Utah are below the long-term average.
  • The NOAA CPC monthly and seasonal outlooks are tilted towards wetter-than-average conditions for our region for the fall months.
  • While official ENSO indicators remain in neutral territory, an El Niño event is still expected to emerge this fall or winter.

 

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July and August Precipitation and Temperatures, and Current Drought

After a very mixed precipitation picture in July, the summer of 2014 finished on an overall wet note in August. July was drier than average across Wyoming except for the southeastern and far southwestern parts of the state. An active start to the summer monsoon season saw wet anomalies for July in most of eastern Colorado and portions of Utah, but some areas in both states saw below-average precipitation. In August, the monsoonal activity favored western Colorado and southern Utah, while a very potent Pacific storm tracked across the northern part of the region towards the end of the month. Only south-central Colorado ended the month with below-average precipitation, while much of the region saw over 200% of the average precipitation for August.

For July, temperatures were 0-4°F warmer than average over most of the region, while the wetter conditions in August led to nearly all of the region at 0-4°F cooler than average for the month.

The latest US Drought Monitor, based on conditions as of September 2, shows slightly less drought across the region compared to early July. Drought conditions improved in northern Utah and in eastern Colorado, but worsened in parts of southern and central Utah. The proportion of the region in D2 or worse drought: Utah down to 19% from 22%, Colorado slightly down to 16% from 17%, and Wyoming unchanged at zero.

Current Streamflows

As of September 7, the maps of current streamflows for Colorado, Utah, and Wyoming show that flows are generally normal to much above normal in Colorado and Wyoming, except in far southern Colorado, and mainly normal or below normal or in Utah. On September 7, the Colorado River near Cameo, CO was in the 89th percentile, 133% of median flow; the Green River near Greendale, UT was in the 69th percentile, 127% of median flow; and the San Juan River near Bluff, UT, was in the 33rd percentile, 67% of median flow.

Reservoir Storage

In Utah, even after a drier-than-average runoff season, most of the major reservoirs tracked by NRCS had more storage at the end of August than on the same date last year, though more than half had less than the long-term average for the end of August. In Wyoming and Colorado, where runoff was above-average, nearly all major reservoirs had both more storage than last year and more than the long-term average for the end of August.

As of September 4, Lake Powell held 12.31 MAF (51% full), up by more than 1.5 MAF from storage on the same date last year (10.79 MAF; 44% full).

Seasonal Climate and Drought Forecasts, including ENSO

The monthly Climate Outlook for September released on August 31 by NOAA CPC shows a slight wet tilt for precipitation for all of Colorado, the southern half of Utah, and far southeastern Wyoming. The seasonal outlooks released August 21 also show a wet tilt in precipitation for most of the region for the September–November period, with the strongest tilt in Colorado and southern Utah. For the October–December period, the area of wet tilt moves south, so that only the far southern portions of both Utah and Colorado show a slight wet tilt. The relatively wet CPC outlooks reflect the expectation that an El Nino will develop.

The latest CPC Seasonal Drought Outlook released August 21 projects that the areas of drought conditions (D1 or worse) in Colorado and southern Utah will improve, or be removed, by the end of November. However, the areas of drought in central and northern Utah, and south-central Wyoming, are projected to persist or intensify during that period. No additional areas of drought development in the region are expected.

The experimental PSD precipitation forecast guidance ('SWcast'), released July 16 for the July–September period has a drier outlook than the CPC forecasts over the past few months, calling for a dry tilt over most of Utah and Colorado, with the strongest dry tilt in eastern Colorado. The difference from the CPC outlooks stems from the statistical SWcast model targeting different indicators of climate system behavior than the dynamical model that is driving the CPC outlooks.

Since early July, ENSO indicators have generally moved away from signaling impending El Niño conditions, though there has been some rebound in the past few weeks. The Multivariate ENSO Index (MEI) dropped slightly in the June–July time frame and then came back up in July–August to nearly the level of May–June, remaining in El Niño territory. The Oceanic Niño Index (ONI), used by NOAA for determining official ENSO status and based on the adjusted 3-month Niño 3.4 sea-surface temperature anomaly, is at +0.1°C, well short of the +0.5°C threshold. ENSO forecasts are still calling for an El Niño event to emerge in the next several months, though with less certainty than three months ago. In the IRI ENSO Prediction Plume from mid-August, about 55% of the models call for El Niño onset by the fall period (September–November), rising to over 70% for the December–February period. If and when the El Niño does develop, the forecasts indicate that it is more likely to be a weak or moderate event than a strong event. The forecasts indicate virtually no chance of a La Niña event developing before spring 2015.

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