December 17, 2015

Highlights

• November saw wetter-than-average conditions over the majority of the region, especially in Colorado, though some key mountain areas were on the dry side.
• Snowpack conditions continue to lag behind normal in northern and central Utah, northwestern Colorado, and most of Wyoming. Snowpack conditions in the rest of Colorado, southern and eastern Utah, and southeastern Wyoming are near normal to well above normal.
• The current El Niño event has likely peaked but remains very strong. El Niño conditions are very likely to continue through the spring, with varying influences on weather across the region.

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

November Precipitation and Temperatures, and Current Drought

In November, precipitation was overall above-normal for the region. Colorado was the wettest state in the region, with large areas of above-normal precipitation in the western, northeastern, and southeastern parts of the state. Wyoming and Utah didn’t fare quite as well, but both had above-normal precipitation over the majority of the state. However, several key mountain areas, including the Wasatch and the Wind River ranges, were drier than normal. On a statewide basis, November's precipitation was in the 86th percentile for Colorado, the 64th percentile for Utah, and the 60th percentile for Wyoming. The first half of December has been generally wet as well, with the storms particularly favoring northern Colorado, southeastern Wyoming, and western Utah, and few areas seeing less than 75% of normal precipitation for the period.

After the unusual warmth of September and October, November’s temperatures were closer to normal across the region, with Utah joining the states to its west in experiencing a relatively cool month, while Colorado and Wyoming were at the edge of the unusually warm conditions in the central and eastern U.S., and ended up slightly above the monthly normals. On a statewide basis, November's temperatures were in the 32nd percentile for Utah, the 53rd percentile for Wyoming, and the 59th percentile for Colorado.

The latest US Drought Monitor, based on conditions as of December 15, shows that after above-normal precipitation in November and early December, several small areas in central Utah; northwestern, central, and eastern Colorado; and southern Wyoming have improved by one category in the past month. Conversely, an area in north-central Wyoming has degraded to D0. The proportion of Colorado and Wyoming in D2 or worse drought remains at zero; in Utah, it declined to 23%.

Snowpack
The Current Basin Snowpack map from NRCSas of December 17 shows sharp differences in percent-of-median SWE conditions across the region. Basins in most of Colorado, in southern and eastern Utah, and in far southeastern and far northwestern Wyoming have benefited the most from early-season storms, and have near-median or above-median SWE. Basins in northwestern Colorado, northern and central Utah and most of Wyoming have below-median SWE, with most basins in these areas reporting 55–85% of median SWE. (Note that NRCS is experiencing problems with data reporting at many SNOTEL sites in our region, and several basins are "blanked out" on the map due to an insufficient number of sites reporting from those basins. Go to the NRCS Interactive SNOTEL Map to see the sites reporting from those basins.)

Seasonal Climate and Drought Forecasts, including ENSO

The monthly Climate Outlook for December released on November 30 by NOAA CPC shows a slight to moderate wet tilt for precipitation for most of Colorado and the far southeastern corner of Utah, and a slight to moderate dry tilt for western and northern Wyoming. The seasonal outlook released December 15 for January–March shows a very similar pattern: slight or moderate wet tilt over most of Colorado and southern Utah, and a slight to moderate dry tilt for most of Wyoming. The outlook for February–April shows the area of wet tilt extending north into Wyoming, while the dry-tilt area retreats to the north. The overall forecasted pattern for the western US, including the wet and dry tilts for different parts of our region, reflects the expected strong El Niño conditions through winter into early spring.

The skill-masked experimental PSD precipitation outlook (“SWcast”) issued November 16 for the January–March period has a moderate wet tilt (>+10%) for northern Utah and central Colorado. The non-masked version shows this wet tilt covering all of Utah and most of Colorado, very similar to the previous forecast for January–March, but the skill of forecasts at this time of year is lower so more of the region is masked out.

The latest CPC Seasonal Drought Outlook released on December 17 projects that areas of drought (D1 or worse) in Utah will improve or be removed by the end of March. Additional drought development is expected in far northern Wyoming. Again, these changes are expected mainly due to the forecasted impacts of the El Niño event.

Sea-surface temperatures appear to have peaked in tropical Pacific in the past few weeks, after rising slightly from early November values. The latest weekly value of the benchmark Niño 3.4 index is +2.8°C, well above the threshold for strong El Niño conditions. The Multivariate ENSO Index (MEI) recovered slightly in the September–October period to +2.3, the 3rd-highest value for this time of year since 1950, indicating very strong El Niño conditions.

The IRI/CPC probabilistic ENSO forecast based on the mid-November ENSO Prediction Plume calls for a >90% chance for the El Niño to persist into spring (March–May) 2016, while weakening through the spring, with a ~50% chance of returning to neutral ENSO conditions by summer.