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December precipitation was close to twice of average across much of the intermountain West.

La Niña to persist into spring, potentially leading to drier than average conditions

By Summit Voice

SUMMIT COUNTY — Based on persistent La Niña conditions, much of the intermountain West could face an enhanced risk of above-average temperatures and below normal precipitation during the upcoming late winter and spring, according to the latest forecast from NOAA’s Climate Prediction Center.

The current La Niña has weakened slightly but is still the strongest on record for this time of year since 1988. Under La Niña, cooler-than-average sea surface temperatures prevail across much of the central and eastern equatorial Pacific. In the fall and winter, the pattern tends to drive storms into the Pacific Northwest and then down across the northwestern corner of Colorado. But that pattern can shift dramatically in the spring, and the latest long-term forecast maps show most of Colorado in an area with higher-than-usual chances of dry weather.

A map from NOAA's Climate Prediction Center shows most of Colorado in an area of below average precipitation for the spring season.

But December was unusually warm and wet across the entire intermountain West, resulting in nearly all river basins in the region reporting above-average snowpack. Much of western Colorado saw December temperatures as much as 6 to 15 degrees above average, with temperatures between 3 to 6 degrees above average across the rest of Colorado, Wyoming and Utah.

Nearly all the intermountain West recorded well above-average temperatures in December 2010.

Utah was even wetter than Colorado, with some sites setting precipitation and snowpack records.Utah’s statewide snowpack on Jan. 1 was 193 percent of average, with the Virgin River Basin at 349 percent of average.

Here’s an excerpt from the Western Water Assessment summary for December:

New records for January 1 Snow-water-equivalent were set at 18 SNOTEL sites with periods of record in excess of 20 years. Precipitation in Utah was above average in all basins for October, November and December. Seasonal (October–December) precipitation statewide was 197% on January 1, ranging from a low of 154% for the Upper Green River in Utah, to 349% for the Virgin River. Southwestern Utah experienced significant flooding in late December as a result of an intense “Pineapple Express” storm. January 1 statewide average snowpack was 193%, ranging from 131% in the Escalante to 315% in the Virgin (Figure SP-1). The Uinta Mountains had the largest January 1 SWE since SNOTEL Measurements began in the late 1970s.

Based on Jan. 1 readings from around the region, inflow into Lake Powell is expected to be about 120 percent of normal, though the forecasters cautioned that those figures can change between now and April 1.
Here’s the summary for the Colorado streamflow forecast:

In Colorado, January 1 streamflow forecasts are the highest since 1997. Forecasts are for near-average to slightly above-average runoff in most of the state.  Forecasts in the range of 120% to 140% of average have been made for the Grand Mesa, the Upper Yampa, the North Platte, and headwater tributaries of the Colorado. The Sangre de Cristo Mountains saw little benefit from the December storms, and the streams in this area reflect this with runoff forecasted at only 51 to 79% of average. October precipitation was above average in all drainages except the Upper Rio Grande, and this should lead to high soil moisture, and high runoff efficiency this spring.

La Niña

Many strong La Niña episodes last up to three years, but most models are showing a trend toward neutral conditions, and two models predict a swing back to El Niño.

Here is the latest La Niña outlook:

Model forecasts of SST anomalies as compiled by the International Research Institute for Climate and Society (IRI) show a robust consensus that La Niña conditions will persist through Spring 2011 (March–May), albeit while progressively weakening and moving towards neutral conditions (Figure EN-2). There is little difference between the average trajectories of the dynamical models and of the statistical models at all time frames.

As was the case last October, a majority of the models project that a shift towards neutral conditions will occur by Summer 2011 (June–August), about 12 months after the emergence of the current La Niña. By Fall 2011 (September–November) only 4 of 16 models are forecasting that La Niña conditions will persist, with two models forecasting the emergence of El Niño conditions. Again, this model consensus is not entirely consistent with the observed history of strong La Niña events in the past century, which suggests that such La Niñas tend to persist for 1–3 years.

And the precipitation outlook for February:

The CPC precipitation outlook for February 2010 (Figure PPT-1) shows an enhanced risk of below-average precipitation across the southwestern U.S., extending into far southern Utah and much of Colorado. For the February–April and March–May seasons, this region of enhanced drying risk expands slightly to the north, covering southern Utah and all of Colorado.  In the April–June season, the risk of drier-than-average conditions shifts eastward and northward, covering all of Utah and Colorado, and southern Wyoming, with southern Colorado having greater drying risk (Figure PPT-4). Overall, the outlook for February–June is that drier-than-average conditions are likely across Colorado and Utah.

Get the full report at the Western Water Assessment website.

Filed under: climate and weather, Colorado, El Niño, La Niña, seasons, Summit County snow and weather, Weatherblog Tagged: | La Niña spring impacts Colorado, NOAA Climate Prediction Center, Summit County News, Western Water Assessment