April 13, 2016

Highlights

• March was wetter than normal for the northern half of our region, including Wyoming, northern Utah, and northern Colorado, with drier conditions to the south.
• Snowpack conditions held steady or improved in the northern basins in the past month, but declined in most southern basins, with rapid melt since April 1. Most of region's basins have 80–110% of median SWE for early April, with an increasing number now below 80%.
• The April 1 spring-summer runoff forecasts improved from the March 1 forecasts in the northern basins with high recent precipitation, but worsened in southern Colorado and southern Utah. 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 wind down and is barely hanging onto “strong” status. The likely persistence of El Niño conditions through the spring and early summer is reflected in a wet tilt for most of the region in the seasonal precipitation forecasts.

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

March Precipitation and Temperatures, and Current Drought

The pattern of March precipitation in our region reflected an active northerly storm track that split the region into “haves” and “have-nots," with wetter-than-normal conditions across Wyoming and in northern Utah and northern Colorado, while the southern half of the region ended up much drier than normal, especially southern and central Utah, and southeastern and southwestern Colorado. On a statewide basis, February's precipitation was in the 90th percentile for Wyoming, the 48th percentile for Utah, and the 40th percentile for Colorado.

Despite the above-normal precipitation for the northern half of the region, March’s temperatures were warmer than normal in those areas, and elsewhere in the region. On a statewide basis, February's temperatures were in the 91st percentile for Wyoming, the 91st percentile for Utah, and the 84th percentile for Colorado.

The latest US Drought Monitor, based on conditions as of April 5, shows that after above-average precipitation in March, several large areas in Wyoming improved by one category in the past month, as did smaller areas in northwestern and northeastern Colorado. Conversely, the dry March in southern Colorado led to degradation by one category, to D0 or D1, over a broad area. The proportion of of the region in D2 or worse drought did not change over the past month and remains at: Colorado, 0%; Utah, 13%; Wyoming, 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 April 8, the 2-week and 4-week EDDI maps show that conditions have shifted to wet (low-thirst) since early March for much of the region, especially since the 3rd week of March. At the 8-week and 12-week timescales, dry conditions are more prevalent in the regional picture, especially in eastern Wyoming and eastern Colorado. The 6-month EDDI map shows moderate dryness over eastern Wyoming and eastern Colorado, while the 12-month EDDI map shows no dry patches except in far northeastern Wyoming, with large areas of low-thirst conditions in eastern Utah and western Colorado.

Snowpack
The Current Basin Snowpack map from NRCS as of April 13 shows that since early March, SWE conditions have held steady for most of Wyoming, northern Utah, and northern Colorado, but have worsened for central and southern Utah, southern Colorado, and northeastern Wyoming. As in February, the northern basins picked up more snow than the southern basins in March and early April. Currently, basins north of I-70 in Colorado have near-normal SWE, while those to the south are below normal. In Utah, the southern basins have near-normal to well-below-normal SWE, while central and northern basins are mostly below normal. In Wyoming, the southern and western basins are mainly near normal or above normal, while the central and northeastern basins are below normal or well below normal.

In the southern parts of the region where there has been below-average snowfall, there has been early and rapid melt of the snowpack, especially on southern aspects and at lower elevations (below 9000’). Cascade SNOTEL site (8880’) north of Durango has nearly melted out, about 3 weeks ahead of the median melt-out date, after reaching a near-normal peak in early February.

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

Spring-Summer Streamflow Forecasts

The April 1 spring-summer streamflow forecasts, released by NOAA CBRFC and the NRCS in early April, are improved over the March 1 forecasts in much of Wyoming, and parts of northern Utah and northern Colorado, but have slipped elsewhere, especially in southwestern Colorado. 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, but more points are now in the below-average category.

In southern Utah, most forecast points are expected to have near-average or below-average runoff, while further north, the Wasatch Range-fed forecast points are expected to have mostly below-average runoff, or near-average runoff, and the Uinta Range-fed forecast points, below-average or much-below average (50–69%) runoff.

In Wyoming, the picture is much improved over month ago, with near-average runoff expected at the majority of across the state, with a few points in central Wyoming now expecting above-average runoff, and the other points split between below-average and much-below-average runoff. The north-central and northeastern basins are expected to have the worst outcomes.

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. For example, the NRCS April 1 forecast for Lake Powell April–July inflows is 5400 KAF, 75% of average, down from 77% of average last month. The NOAA CBRFC April 1 forecast for the same point is 5300 KAF, 74% of average, down from 80% of average one month ago.

Seasonal Climate and Drought Forecasts, including ENSO

The monthly Climate Outlook for April released on March 31 by NOAA CPC shows a moderate wet tilt for most of Colorado, and a slight wet tilt for the rest of Colorado, all of Utah, and far southern Wyoming. The seasonal outlook for April–June released March 17 shows a very similar pattern, with the area of wet tilt expanded northwards, so all of Wyoming has a slight wet tilt. The outlook for May–July shows the area of wet tilt having retreated back towards the south. The overall forecasted pattern for the western US, including the wet tilt for our region, reflects the expected influence of El Niño conditions through spring.

The skill-masked experimental PSD precipitation outlook (“SWcast”) issued March 14 for the April–June period shows a moderate-to-strong wet tilt (>+10%) for central and eastern Colorado, which is very similar to the wet tilt shown in the CPC seasonal outlook for that period.

The latest CPC Seasonal Drought Outlook released on March 17 projects that areas of drought (D1 or worse) in Utah will improve or be removed by the end of June. The drought conditions in north-central Wyoming are also expected to improve or be removed. Again, these changes are expected mainly due to the forecasted impacts of the El Niño event.

Sea-surface temperatures in the tropical Pacific and other ENSO indicators tell us that the current El Niño event, while fading, remains in the "strong" category. In the past month, the benchmark Niño 3.4 index has trended downward, to +2.0°C for the latest weekly value. The Multivariate ENSO Index (MEI) fell slightly to +2.0 for the February–March period, still the 3rd-highest value for this time of year since 1950.

The IRI/CPC probabilistic ENSO forecast based on the mid-March ENSO Prediction Plume calls for the El Niño event to weaken rapidly through spring 2016, with a 60% likelihood of returning to neutral ENSO conditions in the summer, and a 45% chance of transitioning to La Niña conditions by fall, compared to the baseline climatological likelihood of La Niña conditions of 30%.