March 10, 2016

Highlights

• February was overall dry and very warm for our region, with one major storm very early in the month providing most of the moisture.
• Snowpack conditions worsened in most basins in the past month, and the vast majority of the region's basins now have 80–100% of median SWE for early March.
• The March 1 spring-summer runoff forecasts are sharply reduced from the February 1 forecasts, with the region's forecast points roughly split between near-average (90–109% of average), below-average (70–89%), and well-below-average (50–69%) runoff.
• The current El Niño event is on the wane but remains strong. The persistence of El Niño conditions through the spring is reflected in a wet tilt for most of the region in the seasonal precipitation forecasts.

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February Precipitation and Temperatures, and Current Drought

February's precipitation was mostly below-average for the region, with one significant storm system in the first few days of February, followed by unusually dry conditions for the remainder of the month. That one storm, however, was enough so that the month ended up wetter-than-normal in eastern Colorado, southeastern and northeastern Wyoming, and parts of southern Utah. Conversely, northern and central Utah, northwestern Wyoming, and southwestern Colorado ended up much drier than normal. On a statewide basis, February's precipitation was in the 18th percentile for Utah, and the 25th percentile for Wyoming, and the 30th percentile for Colorado.

With high pressure and dry and warm conditions prevailing for most of the month, February’s temperatures were well above normal for nearly all of the region, especially in Wyoming. On a statewide basis, February's temperatures were in the 98th percentile for Wyoming, the 93rd percentile for Colorado, and the 87th percentile for Utah.

The latest US Drought Monitor, based on conditions as of March 8, shows that after the mostly below-average precipitation in February, several areas in northern Wyoming, south-central Utah, and southeastern Colorado degraded by one category in the past month, with D2 emerging in the Bighorn Mountains in north-central Wyoming. Conversely, one area in northeastern Wyoming and two in eastern Colorado improved by one category. The proportion of Colorado and D2 or worse drought remains at zero; in Utah, it remains at 13%; in Wyoming, it increased from zero to 2%.

Introducing EDDI

For the past two 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 March 2, the 2-week and 4-week EDDI maps show that drought (high-thirst) conditions have emerged since early February in most of Colorado and Wyoming, especially in northeastern Colorado and eastern and northern Wyoming. At the 8-week and 12-week timescales, dry conditions are more restricted to Colorado, while Utah is mainly wet. The 6-month EDDI map shows moderate dryness over most of Wyoming and central and eastern Colorado, while in the 12-month EDDI map those dry patches are much more limited.

Snowpack
The Current Basin Snowpack map from NRCS as of March 10 shows that SWE conditions worsened for much of the region compared to early February, with the strong southwest–northeast gradient typical of El Niño winters weakening as the northern basins picked up more snow than the southern basins in the past month. Basins across Colorado have near-normal SWE, with all being between 88–98% of median. In Utah, the southern basins are near-normal while central and northern basins are mostly below average with 75–90% of median. In Wyoming, the southern and western basins are mainly near-normal, while the central and northeastern basins are lower, with 55–80% of median SWE.

Spring-Summer Streamflow Forecasts

The March 1 spring-summer streamflow forecasts, released by NOAA CBRFC and the NRCS in early March, show widespread and significant reductions in the expected runoff for Utah and Colorado compared to the February 1 forecasts. The NRCS Westwide Streamflow Forecast Map shows that in Colorado, nearly all forecast points are now expected to have near-average (90–109%) or below-average (70–89%) spring and summer runoff. 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 similar to a month ago, with near-average runoff expected at only a handful of points in the northwestern and southeastern basins, with most points across the state expected to have have below-average to much-below-average spring and summer runoff. Note that the March 1 streamflow forecasts do not reflect the increased chances for above-normal precipitation for most of the region this spring (March–May), as described below.

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 March 1 forecast for Lake Powell April–July inflows is 5500 KAF, 77% of average, down from 98% of average last month. The NOAA CBRFC March 1 forecast for the same point is 5700 KAF, 80% of average, down from 94% of average one month ago.

Seasonal Climate and Drought Forecasts, including ENSO

The monthly Climate Outlook for March released on February 29 by NOAA CPC shows a slight to strong wet tilt for precipitation for all of Colorado and most of Utah, strongest in southeastern Colorado, and a slight wet tilt for southeastern Wyoming. The seasonal outlook for March–May released February 18 shows a very similar pattern, with the area of wet tilt expanded northwards, so that nearly of the region has wet tilt, including a strong wet tilt for most of Colorado (“Strong” = the probability for a season in the wettest third is 50% or greater, compared to the baseline probability of 33%.) The outlook for April–June shows the wet tilt being much reduced in strength, though not coverage. The overall forecasted pattern for the western US, including the wet tilt for our region, reflects the expected influence of strong El Niño conditions through spring.

The skill-masked experimental PSD precipitation outlook (“SWcast”) issued January 19 for the January–March period shows a moderate wet tilt (>+10%) for western and northern Utah and north-central Colorado, and a slight wet tilt for most of the rest of Utah and Colorado. The unusually extensive area of wet tilt has persisted through three iterations of the January–March SWcast, and follows on a high skill score (i.e., validation) for the October–December 2015 SWcast.

The latest CPC Seasonal Drought Outlook released on February 18 projects that areas of drought (D1 or worse) in Utah will improve or be removed by the end of May. The drought conditions in north-central Wyoming are also expected to improve. 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 on the wane, 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 +2.1 for the January–February period, still the 3rd-highest value for this time of year since 1950.

The IRI/CPC probabilistic ENSO forecast based on the mid-February ENSO Prediction Plume calls for the El Niño event to weaken rapidly through spring (March–May) 2016, with a >50% likelihood of returning to neutral ENSO conditions in the summer, and then in fall, a decent chance of rapidly transitioning to La Niña conditions.