May 10, 2016

Highlights

• April was wetter than normal for most of our region, with wide swaths of much-above-average precipitation in Wyoming, Utah, and Colorado.
• With the melt season well underway, snowpack conditions are above normal in Colorado, southern Utah, and southern and central Wyoming, but below normal in northern and central Utah and northern Wyoming.
• The May 1 spring-summer runoff forecasts improved from the April 1 forecasts in some basins, but worsened in northern and central Utah, and far western Wyoming. Most of the region's forecast points are expected to have near-average (90–109% of average) or below-average (70–89%) runoff.
• The current El Niño event continues to decline, but is still expected to exert some influence on our region through spring, tilting the odds towards wetter-than-average conditions for most of the region.

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

April Precipitation and Temperatures, and Current Drought

April saw overall wetter-than-normal conditions for our region, with greater than 150% of normal precipitation over broad swaths of Wyoming, Utah, and Colorado, and only relatively small areas seeing below-normal precipitation. On a statewide basis, April's precipitation was in the 82nd percentile for Wyoming, the 77th percentile for Utah, and the 89th percentile for Colorado.

Despite the above-normal precipitation, April’s temperatures were warmer than normal for most of the region. On a statewide basis, April temperatures were in the 81st percentile for Wyoming, the 80th percentile for Utah, and the 66th percentile for Colorado.

The latest US Drought Monitor, based on conditions as of May 3, shows that after above-average precipitation in April, drought conditions improved in southeastern Colorado, northern Utah, and several areas in Wyoming. A swath of central and southern Utah, however, degraded from drought-free to D0. The proportion of Utah in D2 or worse drought decreased from 13% to 0%; Colorado remained at 0%; and Wyoming remained at 2%.

Introducing EDDI

For the past three months, the Drought Monitoring portion of the Dashboard has featured a new product: EDDI, the Evaporative Demand Drought Index. EDDI is based on atmospheric data—wind speed, humidity, temperature, and sunshine—and it reflects the atmosphere’s “thirst” for water from the land surface. (EDDI does not explicitly incorporate precipitation, but many of the component variables are strongly related to precipitation anomalies.)

EDDI has been shown to capture fast-emerging (“flash”) drought earlier than other drought indices, while also providing useful information at longer time scales. Like SPI, it is calculated over a specific time period in the context of normal conditions over that time period. EDDI is expressed in discrete categories based on the current conditions' percentile ranking relative to past conditions. The EDDI drought categories (ED0-ED4) have the same percentile “bins” as as the corresponding US Drought Monitor categories (D0-D4). EDDI also has wetness categories (EW0-EW4), indicating wetter-than-normal surface conditions, occupying the 70th percentile and above, mirroring the drought categories.

As of May 4, the 2-week, 4-week, and 8-week EDDI maps show widespread wet (low-thirst) anomalies, consistent with more humid conditions associated with the above-normal precipitation. At the 12-week timescale, dry (high-thirst) conditions are seen in eastern Wyoming. The 6-month EDDI map shows wet conditions predominating for the region, while the 12-month EDDI map shows no dry patches except in far northern Wyoming, with large areas of wet (low-thirst) conditions Utah, Colorado, and western Wyoming

Snowpack

The Westwide Current Basin Snowpack map from NRCS as of May 9, while reflecting the usual volatility in basin snowpack statistics after the peak of the accumulation season, does indicate that above-normal snowpack persists in northern and central Colorado, southern and central Wyoming, and southern and northeastern Utah, while snowpacks are below normal in northern and central Utah, and northwestern Wyoming. With wet weather over most mountain areas in March and April, the SWE time-series for many sites showed a bumpy "plateau" rather than a distinct peak. Since May 1st, nearly all sites have seen rapid melt.

According to the Colorado Dust-on-Snow program, this season has seen low to moderate dust loading, with six dust events through early May recorded at the Senator Beck Basin and elsewhere in western Colorado.

Current Streamflows

As of May 9, the maps of current streamflows for Colorado, Utah, and Wyoming show that flows are generally normal or above normal in Colorado, and above normal to much above normal in Wyoming. In Utah, the flows are more distributed across the below-normal, normal, and above-normal classes, with normal flows predominating. For the most part, flows on larger streams and rivers across the region are still rising towards their seasonal peak flows. On May 9, the Colorado River near the CO-UT State Line was in the 61st percentile, 120% of median flow; the Green River at Jensen, UT was in the 71th percentile, 141% of median flow; and the San Juan River near Bluff, UT, was in the 27th percentile, 58% of median flow.

Spring-Summer Streamflow Forecasts

The May 1 spring-summer streamflow forecasts, released by NOAA CBRFC and the NRCS in early May, are improved over the April 1 forecasts for much of Wyoming, northern and central Colorado, and southern Utah, but have slipped in northern Utah and far western Wyoming. The NRCS Westwide Streamflow Forecast Map shows that in Colorado, nearly all forecast points are still expected to have near-average (90–109%) or below-average (70–89%) spring and summer runoff, with several now in the above-average (110–129%) category.

In southern Utah, most forecast points are expected to have near-average (90–109%) or below-average (70–89%) runoff, while in central and northern Utah, most forecast points are expected to have mostly below-average (70–89%) or much-below average (50–69%) runoff.

In Wyoming, near-average (90–109%) or above-average (110–129%) is expected at nearly all forecast points east of the Continental Divide, but mostly below-average (70–89%) runoff is expected west of the Divide.

IMPORTANT NOTE: While NRCS and NOAA continue to collaborate on streamflow forecasts, the forecast values are no longer being strictly coordinated between NRCS and some of the RFCs, including CBRFC. That said, the NRCS May 1 forecast for Lake Powell April–July inflows is 5500 KAF, 77% of average, up from 75% of average last month. The NOAA CBRFC May 1 forecast for the same point is also 5500 KAF, 77% of average, up from 74% of average one month ago.

Seasonal Climate and Drought Forecasts, including ENSO

The monthly Climate Outlook for May released on April 30 by NOAA CPC shows a strong wet tilt for most of Utah, a moderate wet tilt for the rest of Utah and for Colorado and southern Wyoming, and a slight wet tilt for the rest of Wyoming. The seasonal outlook for May–July released April 21 shows a similar pattern, though the wet tilt is weaker. The outlook for June–August shows a smaller area of wet tilt centered over our region, with a moderate wet tilt for northern Utah, northern Colorado, and southern Wyoming.

The skill-masked experimental PSD precipitation outlook (“SWcast”) issued April 14 for the April–June period shows a moderate-to-strong wet tilt (>+10%) for central and eastern Colorado, and a slight dry tilt for east-central Utah and northwestern Colorado.

The latest CPC Seasonal Drought Outlook released on April 21 projects that most areas of drought (D1 or worse) in Utah will improve or be removed by the end of July, but some drought is expected to persist. The drought conditions in north-central Wyoming are also expected to improve or be removed.

Sea-surface temperatures in the tropical Pacific and other ENSO indicators show that the current El Niño event is continuing to decline. In the past month, the benchmark Niño 3.4 index has dropped sharply, to +0.8°C for the latest weekly value, not far above the +0.5°C threshold for El Niño conditions. The Multivariate ENSO Index (MEI), however, increased slightly to +2.1 for the March–April period, the 4th-highest value for this time of year since 1950.

The IRI/CPC probabilistic ENSO forecast based on the mid-April ENSO Prediction Plume calls for the El Niño event to weaken rapidly to neutral conditions through spring 2016, with a 30% likelihood of transitioning to weak La Niña conditions by summer, increasing to 60% by fall.