October 13, 2015

Highlights

• September was dry overall for our region, with many areas receiving less than 25% of average precipitation for the month. Temperatures were extremely warm, with both Colorado and Utah recording their record-warmest September.
• Water-year 2015 ended up with near-average (Utah) and above-average (Colorado and Wyoming) statewide precipitation for the 12-month period. The water-year average temperatures were unusually warm in all three states.
• Nearly all reservoirs in Colorado and Wyoming had above-average storage as of the end of September; Utah's reservoirs had mostly below-average storage.
• El Niño conditions have strengthened further and are virtually certain to continue through the winter. El Niño’s expected influence is seen in seasonal precipitation outlooks showing a wet tilt for most or all of our region through the winter.

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September Precipitation and Temperatures, and Current Drought

September was overall a dry month for our region, with nearly all areas seeing below-average precipitation. Much of Wyoming, northeastern Colorado, and west-central Utah received less than 25% of average precipitation for the month. The only areas with above-average precipitation were in northern Utah, far western Wyoming, and parts of southern Colorado. On a statewide basis, September's precipitation was in the 19th percentile for Colorado, the 48th percentile for Utah, and the 24th percentile for Wyoming.

After a mostly cool July and average August, September was considerably warmer than average, with widespread temperatures of 3–9°F above the monthly normal. Denver (Stapleton) set a new record for warmest September, 5.9°F above normal. Salt Lake City had its 3rd-warmest September, 4.7°F above normal, despite above-average precipitation for the month. On a statewide basis, this was the warmest-ever September for both Colorado and Utah, and the 2nd-warmest September for Wyoming, since the start of the record in 1895.

The latest US Drought Monitor, based on conditions as of October 6, shows that with the very dry conditions since August, several areas in eastern Colorado and eastern Wyoming have degraded to D0 (abnormally dry) in the past month. In northwestern Utah, there has been a one-category improvement due to wetter conditions there. The proportion of Colorado and Wyoming in D2 or worse drought remains at zero; in Utah it is holding steady at 27%.

Water Year 2015 recap

Very dry months in October, January, and March put the regional snowpacks well behind normal conditions in nearly all basins. The "Miracle May" bailed us out of a 2012-type drought scenario, and improved the water supply picture dramatically, though spring runoff was still low in most of Utah and southwestern Colorado. Above-average precipitation in June and July damped down water demand and generated some additional runoff. And then September ended the water year on a very dry note. The final water-year precipitation %-of-average map, showing most of the region ending up with above-average precipitation, presents a benign picture of the roller-coaster ride that actually occurred over the course of the year.

Water year 2015 was also unusually warm across the region. Utah ended up with its 2nd-warmest water year on record, 2.7°F above the 1981–2010 normal, while both Colorado (+1.7°F) and Wyoming (+2.3°F) had their 4th-warmest water years on record. It is notable that these warm anomalies occurred with near-average (Utah) and above-average (Colorado and Wyoming) statewide precipitation; historically, the warmest years in our region have been those with well-below-average precipitation, such as 1934, 1954, 1981, 2000, and 2012.

Current Streamflows

The maps of current streamflows for Colorado, Utah, and Wyoming show that flows are tending towards normal or above normal in Colorado and Wyoming, while in Utah the flows are more spread out among the below-normal, normal, and above-normal classes. As of October 11, the Colorado River near the CO-UT State Line was in the 62nd percentile, 108% of median flow; the Green River at Green River, UT was in the 55th percentile, 108% of median flow; and the San Juan River near Bluff, UT, was in the 27th percentile, 71% of median flow.

Snowpack
The first week of October brought a healthy dollop of moisture to many mountain areas, especially in Colorado and northern Utah. However, it mainly fell as rain, and higher-elevation SNOTEL sites accumulated only a few tenths of an inch of SWE at most. Above 11,000’ more snow fell, but even that may melt out in the next week or two of expected warmer, drier weather. The region’s snowpacks may not begin building in earnest until the first significant snowstorms, perhaps towards the end of October.

Reservoir Storage

In Colorado and Wyoming, nearly all reservoirs were above average for the end of September, with most reservoirs’ storage being ahead of last year at this time, and the remainder being comparable to last year.  

In Utah, reservoirs are mainly below average for the end of September, and at similar levels as last year at this time. On September 30, Lake Powell held 12.33 MAF (51% full), almost identical to the storage of 12.29 MAF on the same date last year.

Seasonal Climate and Drought Forecasts, including ENSO

The monthly Climate Outlook for October released on September 30 by NOAA CPC shows a slight wet tilt for precipitation for the southern parts of Utah and Colorado. The seasonal outlooks released September 17 show this area of wet tilt expanding northward, so that for the October–December and November–January periods there is a slight wet tilt for southern Utah, most of Colorado, and southeastern Wyoming, and a moderate wet tilt for southeastern Colorado. This overall forecasted wet-tilted pattern for the region is at least partly due to the expected strong El Niño conditions through the fall and winter.

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 September 17 projects that areas of drought (D1 or worse) in southern Utah will improve or be removed by the end of December. However, in northern Utah, drought is projected to persist or intensify, with additional drought development expected in the far northwestern corner of Utah, and northwestern 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 September, while holding steady in the central tropical Pacific. The benchmark Niño 3.4 index is currently at +2.4°C, well above the threshold for strong El Niño conditions, and within 0.5°C of the all-time record set in December 1997 and January 1983. The Multivariate ENSO Index (MEI) climbed again in the August–September period to +2.5, the 2nd-highest value for this time of year since 1950, indicating very strong El Niño conditions.

The IRI/CPC consensus ENSO forecast, base on the mid-September ENSO Prediction Plume, calls for a >95% chance that El Niño conditions will continue through the winter, and a 70% chance for the El Niño to persist for spring 2016.