November 18, 2014
Highlights
- October was much drier than average over most of the region. The first half of November has been more mixed for precipitation, with the Colorado and Wyoming mountains faring the best, and much of Utah remaining on the dry side.
- After a very slow start to the season, the region’s snowpack has improved markedly since November 1, but is still below normal in most basins.
- An El Niño event is still likely to occur by spring, but the latest forecasts have again backed off slightly on the odds.
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October and early November Precipitation and Temperatures, and Current Drought
Water year 2015 began with a whimper: October was very dry over most of the region, with nearly all of Utah, most of Wyoming, and far western and northeastern Colorado receiving less than 50% of average precipitation. Only Colorado had substantial areas with above-average precipitation, mainly in the southeastern quarter. Southern and northwestern Utah saw virtually no precipitation for the month. The first half of November has been a mixed bag for the region. Many areas have seen above-average precipitation for this recent period, especially in the mountains of Colorado and Wyoming. Most of Utah has experienced much-drier-than-average conditions.
Consistent with the low precipitation, October brought above-average temperatures across the entire region, with most locations running 2–6°F warmer than the monthly normals. So far in November, the regional temperature anomalies have been strongly influenced by intrusions of unusually cold Arctic air which began on the 9th, with warmer-than-average temperatures south and west of the intrusions in southern Utah and Colorado, and colder-than-average temperatures elsewhere.
Despite the generally below-average precipitation, the latest US Drought Monitor, based on conditions as of November 11, shows no change in drought conditions compared to mid-October. The proportion of the region in D2 or worse drought remains: Utah at 13%; Colorado at 12%, and Wyoming at zero.
Current Streamflows
As of November 14, the maps of current streamflows for Colorado, Utah, and Wyoming show that flows are normal to above normal in Colorado and Wyoming, and mainly normal or below normal in Utah. Ice is affecting an increasing number of gages, as is typical for this time of year. On November 14, the Colorado River near Cameo, CO was in the 69th percentile, 109% of median flow; the Green River near Greendale, UT was in the 37th percentile, 80% of median flow; and the San Juan River near Bluff, UT, was in the 25th percentile, 81% of median flow.
Current Snowpack
After a very slow start to the snow-accumulation season, conditions have sharply improved since November 1. But even with the most recent storms, the Current Basin Snowpack map from NRCS as of November 17 still shows below-normal (i.e., below-median) SWE across much of the region. Colorado, northwestern Wyoming, and southern Utah have the best conditions. Mid-November is still relatively early in the season, with most of the normal seasonal accumulation yet to come.
Seasonal Climate and Drought Forecasts, including ENSO
The monthly Climate Outlook for November released on October 31 by NOAA CPC shows equal chances (EC) of above-average or below-average precipitation for the month of November. The seasonal outlooks released October 16 were described in the previous briefing. They 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.
As described in the previous briefing, 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') for the January–March period shows a tilt towards above-average precipitation over northeastern Colorado and a smaller wet tilt over far southern Utah. There is a slight dry tilt forecasted for northern Utah. (The statistical SWcast model targets different indicators of climate system behavior than the dynamical model that drives the CPC outlooks.)
The second season of experimental SWE forecasts for Colorado from NOAA PSD begins with the January 1 SWE forecast, issued in late October. The median (50th percentile) forecast calls for above-average January 1 snowpack (101–120% of average) for 5 of the 7 major basins in Colorado, and below-average snowpack in the Yampa (94%) and Colorado Headwaters (87%).
In the tropical Pacific, ENSO indicators are giving mixed signals, and the likelihood of a full-blown El Niño event may be declining. The Multivariate ENSO Index (MEI), dropped again September–October period, back into neutral territory. The benchmark Niño 3.4 sea-surface temperature index has finally climbed well above the 0.5°C threshold (currently 0.8°C), though it needs to stay above that for several more months to officially signal an El Niño.
According to the latest ENSO forecasts, it is still more likely than not that an El Niño event will officially emerge in the next several months. The early-November ENSO Probabilistic Forecast from NOAA CPC and IRI puts the odds of El Niño onset at 58%, a drop from the mid-October Forecast (65%). If an El Niño does develop, it is increasingly likely to be a weak event. The forecasts indicate virtually no chance of a La Niña event developing before spring 2015.