October 21, 2014

Highlights

• Water year 2014 closed with a wetter-than-average September for most of the region, but October has been much drier than average so far, except in southeastern Colorado.
• At the end of September, nearly all of the region's reservoirs were in better shape than at the same time last year, but most reservoirs in Utah and southern Colorado were still lagging the long-term average.
• The NOAA CPC monthly and seasonal outlooks show mostly equal chances for above-average or below-average precipitation for late fall and early winter, while the 'SWcast' shows more of a wet tilt for Utah and Colorado.
• ENSO indicators are more consistently pointing towards El Niño onset, which is likely to officially occur by spring, according to the latest forecasts.

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

September and early October Precipitation and Temperatures, and Current Drought

Water year 2014 concluded with a wet September for most of the region, with nearly all of Utah and most of Wyoming and Colorado seeing above-average precipitation for the month. Through October 19, water year 2015 has gotten off to a dry start across the region, with the exception of southeastern Colorado.

For the 12 months of water year 2014, most of the region saw above-average precipitation, with nearly all of Wyoming, northern and central Colorado, and eastern, northern, and southeastern Utah ending up on the wet side of the ledger. Southeastern and southwestern Colorado, southeastern Utah, and central Utah were on the dry side. Very few areas saw less than 70% of average water year precipitation. Because the best months for precipitation across Utah were July, August, and September—a time of year that produces less efficient runoff—water-year streamflows were generally lower than would be expected given the water-year precipitation.

Despite the generally above-average precipitation in September, the temperatures were 0-4°F warmer than average over nearly the entire region. Thus far, October has also been running warmer than average.

With the ample precipitation in September, the latest US Drought Monitor, based on conditions as of October 14, shows overall less drought in the region compared to early September. Drought conditions improved in southeastern and southwestern Colorado, southern Wyoming, and multiple areas in Utah. For the first time since June 2012, there are no areas of D3 or D4 drought in the region. The proportion of the region in D2 or worse drought is likewise on the decline: Utah down to 13% from 19%, Colorado down to 12% from 16%, and Wyoming unchanged at zero.

Current Streamflows

As of October 19, 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 normal in Utah. On October 17, the Colorado River near Cameo, CO was in the 97th percentile, 150% of median flow; the Green River near Greendale, UT was in the 46th percentile, 94% of median flow; and the San Juan River near Bluff, UT, was in the 34th percentile, 76% of median flow.

Current Reservoir Storage

The majority of Utah's major reservoirs ended the 2014 water year with less storage than the long-term average for the end of September. In Colorado, most of the major reservoirs had more storage than the long-term average for the end of September, while in Wyoming, nearly all major reservoirs had more storage than the long-term average.

As of October 19, Lake Powell held 12.34 MAF (51% full).

Current Snowpack
Across the region, October's mostly dry, sunny weather has not helped to build on or retain the initial snows left mainly by storms in September. The Current Basin Snowpack map from NRCSindicates below-normal (i.e., below-median) SWE across nearly all of the region. We are still very early in the snowpack season, and the % of normal SWE values are much more volatile than later in the season. This is demonstrated nicely by the very high % of normal values in several Utah basins; this is due mainly to the median SWE being zero for this time of year at most sites in these basins, and no site in Utah currently has more than 0.6" of SWE. The forecasted precipitation over the next several days does not look like it will add much to the snowpack, but the increased cloudiness may at least stem the losses.

Seasonal Climate and Drought Forecasts, including ENSO

The monthly Climate Outlook for October released on September 30 by NOAA CPC shows equal chances (EC) of above-average or below-average precipitation for the month. The seasonal outlooks released October 16 show mostly equal chances for the region, with a slight wet tilt in November–January period for extreme southern Colorado Utah, and a dry tilt in northwestern Wyoming. For the December–February period, the area of wet tilt is the same, but the area of dry tilt shifts out of Wyoming.

The latest CPC Seasonal Drought Outlook released October 16 projects that the areas of drought conditions (D1 or worse) in Colorado and southern Utah will improve, or be removed, by the end of January. However, the areas of drought in central and northern Utah 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 September 15 for the October–December period shows a slight tilt towards above-average precipitation over southern and eastern Utah and southeastern Colorado. (The statistical SWcast model targets different indicators of climate system behavior than the dynamical model that drives the CPC outlooks.)

In the tropical Pacific, ENSO indicators are slowly converging on El Niño status. The Multivariate ENSO Index (MEI), which has actually been signaling El Niño conditions since April, dropped sharply in August–September, but is expected to rebound in the coming month. Indicators based only on sea-surface temperatures, such as Niño 3.4, have generally moved towards the warm edge of neutral territory. The climate system has already exhibited some of the typical "side effects" of El Niño in the last several months, including a weak Indian Monsoon, a suppressed North Atlantic hurricane season, and drought in Central America.

According to the latest ENSO forecasts, it is likely that an El Niño event will officially emerge in the next several months. In the IRI ENSO Prediction Plume from mid-October, about 65% of the models call for El Niño onset in the late fall period (October–December), with that proportion remaining steady through the February–April period before declining. The forecasts indicate virtually no chance of a La Niña event developing before spring 2015.