November 12, 2015

Highlights

  • October saw wetter-than-average conditions over most of Utah and Colorado, while dry conditions prevailed for most of Wyoming. Temperatures were extremely warm for the month across all three states.
  • The seasonal snowpack is off to a slow start in mountain areas in northern Utah and most of Wyoming. Snowpack conditions in Colorado, southern and eastern Utah, and southeastern Wyoming are near normal to well above normal.
  • El Niño conditions have strengthened further by some indicators, but this event is likely near its peak. The El Niño event is very likely to continue through the spring, with varying influences on weather across the region.

 

October Precipitation and Temperatures, and Current Drought

Across the region, October saw mainly above-normal precipitation, but there were distinct winners and losers. Surges of subtropical moisture from the southwest led to well-above-average precipitation in southern and northwestern Utah, southern and eastern Colorado, and southeastern Wyoming. Conversely, much of Wyoming, northwestern Colorado, and the rest of northern Utah received below-average precipitation for the month. Unlike in September, however, there were few areas with less than 50% of average precipitation. On a statewide basis, October's precipitation was in the 74th percentile for Colorado, the 80th percentile for Utah, and the 38th percentile for Wyoming.

Like September, October was far warmer than normal, with temperatures of 4–10°F above the monthly normal. The warmth was particularly unusual given the above-average precipitation in most areas, reflecting the subtropical moisture sources for that precipitation, as well as an absence of Arctic cold air outbreaks during the month. Salt Lake City had its warmest-ever October, 7.5°F above normal. On a statewide basis, this was the 2nd-warmest October for Wyoming (99th percentile), the 3rd-warmest in Colorado (98th percentile), and the 4th warmest in Utah (97th percentile) since the start of the records in 1895.

The latest US Drought Monitor, based on conditions as of November 10, shows that after the varying precipitation outcomes in October, several areas in southern Utah, eastern Colorado, and northeastern Wyoming have improved in the past month to drought-free or abnormally dry (D0), while areas in central and northwestern Colorado and several parts of Wyoming have degraded to D0. The proportion of Colorado and Wyoming in D2 or worse drought remains at zero; in Utah, it has declined slightly to 26%.

Snowpack
The Current Basin Snowpack map from NRCSas of November 11 shows wildly varying percent-of-median SWE conditions across the region’s basins, reflecting the winners and losers in October and early November precipitation, as well as the usual volatility of early-season snowpack statistics. Basins across Colorado, in southern and eastern Utah, and in southeastern Wyoming have fared the best, with near-median to well-above-median SWE. Basins in the rest of Wyoming and northern Utah have below-median SWE, with most basins in northern and central Wyoming reporting less than 50% of median SWE.

 

Seasonal Climate and Drought Forecasts, including ENSO

The monthly Climate Outlook for November released on October 31 by NOAA CPC shows a moderate wet tilt for precipitation for all of Colorado and most of Utah and Wyoming, and a slight wet tilt for the rest of Utah and Wyoming. The seasonal outlooks released October 15 show a slight or moderate wet tilt for the November–January and December–February periods over southern Utah and southern Colorado. These outlooks also show a dry tilt over northwestern Wyoming that expands and strengthens in the December–February period. The overall forecasted pattern for the western US, including the wet and dry tilts for parts of our region, reflects the expected strong El Niño conditions through winter.

As reported in the previous briefing: The experimental PSD precipitation outlook (“SWcast”) issued September 21 also shows a wet tilt for the October–December period, but mainly for southern Utah, and also for far western Colorado and extreme northeastern Colorado. The long-lead SWcast issued October 9 for the January–March period is about as "green" as they come, with a wet tilt across all of Utah and Colorado, with the weakest tilt in western Colorado and the strongest (>+10%) in western Utah. As with the CPC forecasts, the wet tilt is mainly from the El Niño.

The latest CPC Seasonal Drought Outlook released on October 15 projects that areas of drought (D1 or worse) in southern Utah will improve or be removed by the end of January. However, in northern Utah, drought is projected to persist or intensify, with additional drought development expected in northern Wyoming. Again, these changes are expected mainly due to the forecasted impacts of the El Niño event.

Sea-surface temperatures have continued to warm in the eastern tropical Pacific since early October, while declining slightly in the central tropical Pacific. The latest weekly value of the benchmark Niño 3.4 index is +2.8°C, well above the threshold for strong El Niño conditions, tying the highest weekly value during the 1997-98 El Niño event. On a monthly basis, however, the October 2015 Niño 3.4 index lags the peak values during the 1982-83 and 1997-98 El Niño events. The Multivariate ENSO Index (MEI) declined in the September–October period to +2.2, though this is still the 2nd-highest value for this time of year since 1950, and indicates very strong El Niño conditions.

The IRI/CPC probabilistic ENSO forecast based on the mid-October ENSO Prediction Plume calls for a 100% chance that El Niño conditions will continue through the winter, a 90% chance for the El Niño to persist into spring (March–May) 2016, and then a >50% chance of returning to neutral ENSO conditions by late summer.

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