Issued April 20, 2009, Vol. 5, Issue 3

Brad Udall – WWA Director
Jessica Lowrey, Kristen Averyt, Andrea Ray – Editors/Writers
Julie Malmberg – Writer
Lucia Harrop – Graphic Designer
Klaus Wolter, Gary Bates – Asst. Editors


April 2009 Mini Summary
Announcements & News

This month we introduce a mini IWCS that focuses on drought, snowpack conditions, and streamflow forecasts. If you are interested in updated version of other maps and products we feature in the full length IWCS, please follow the links on the Notes and Weblinks page.

We will release the next full length IWCS next month on May 26, 2009.

We appreciate your input, and please let us know what you think of these changes. Email: wwasummary@wwa.colorado.edu


Drought Conditions

In March, precipitation was below average for Colorado and Utah and above average in Wyoming.  As a result, drought expanded in western Colorado and eastern Utah (to D0-abnormally dry) and decreased in southern and northwestern Wyoming (from D0 to no drought status) (Figure RC-1).  Below average temperatures and a few big snow storms at the end of March/beginning of April in the mountains of central and western Colorado, western Wyoming and extreme southeastern Wyoming kept the drought status away from those areas.



Figure RC-1. Drought Monitor from April 14, 2009 (full size) and March 17, 2009 (inset, lower left) for comparison. (Source: National Drought Mitigation Center)


According to the U.S. Seasonal Drought Outlook, southeastern Colorado should see some improvement in drought status in the next two months, but drought will persist in southwestern Wyoming and northwestern Utah (Figure DO-1).  The projected improvements in Colorado (and other areas in the central Plains) are largely due to moderate to heavy rain forecasted from April 16-20.  On the other hand, climate outlooks show increased changes for below average precipitation in May–July across the drought areas of the Interior West, including southwestern Wyoming.  As a result, drought should persist everywhere it exists from the central Rockies westward. 

Forecasters report that forecast confidence across the across the southern Plains is moderate to high and confidence across the central Interior West is high.



Figure DO-1. Seasonal Drought Outlook for April 14, 2009–July 2009. (Source: NOAA Climate Prediction Center)


Intermountain West Snowpack

Precipitation in March was above average in Wyoming, and below average in Colorado and Utah. As a result, snowpack as a percent of average increased in parts of Wyoming, and decreased throughout most of Colorado and Wyoming (Figure SP-1). 

Between March 1 and April 1, increases of 6–15 percentage points occurred in parts of the Yellowstone, Clarks Fork, Snake, and Wind-Big Horn River Basins in the northeast, as well as the North Platte River Basin in central Wyoming. As of April 1, snow water equivalent (SWE) across Wyoming is slightly below average (98%) for this time of year. SWE in the northwest portion of Wyoming is now about 97% of average, northeast Wyoming SWE is currently about 111% of average, the southeast Wyoming SWE is currently about 100% of average, and the southwest Wyoming SWE is about 94% of average. 


Figure SP-1. Snow water equivalent (SWE) as a percent of average for available SNOTEL and snow course sites in the Intermountain West as of April 1, 2009 (Source: NRCS).


The first three weeks of March were very dry across Colorado. As a result, the month ended with lower snowpack percentages than in the beginning of the month for all the major river basins of the state. March was the third consecutive month where snowpack percentages declined in every basin in Colorado. The greatest declines in basinwide percents of average were measured in the San Juan, Animas, Dolores and San Miguel (-20 percentage points), Rio Grande (-17 percentage points), and the Arkansas (-13 percentage points). Only two basins remain with above average snowpack totals: the Colorado and combined Yampa and White River Basins, both at 104% of average. The lowest snowpack percentages were measured in the South Platte and the combined San Juan, Animas, Dolores, and San Miguel River Basins, at only 86% of average.

In Utah, March started with a few productive storms, but the middle of the month was very dry. Warm temperatures brought low and mid elevation snowpacks to isothermal conditions and melting began. Snowpacks across the state began a steep decline with many areas showing declines of 5 to 15%. Then in the final week March, a series of storms hit the state and brought snow levels in northern Utah back to near average conditions. Southern and eastern Utah improved, but not as much as the north. Snowpacks across the state now range from 77% in southeastern Utah to 100% in the Weber. March precipitation was below to above average (77–118%) in northern Utah and much below to below average (35–83%) in the south.

[The majority of the text on this page comes from the NRCS State Basin Outlook Reports.]

Notes & Weblinks
(provides explanations of graphics and additional information sources)

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Spring and Summer Streamflow Forecasts for the 2009 Runoff Season

Streamflow projections are below average to much below average for the Intermountain West Region (Figure STRM-1), due to below average precipitation across most of the region in the last few months. However, most reservoirs are near average now, and they should be able to fill this year.

Below average precipitation in March across Colorado caused runoff forecasts to decrease since those made on March 1. Southern basins could face a year of below average streamflows, especially the San Juan, Animas, Dolores, San Miguel, and the South Platte River Basins. Volumes in these basins currently range between 75 and 90% of average. Other basins with predominantly below average forecasts include the Rio Grande and the southern tributaries to the Arkansas River. The state's highest outlook for the coming runoff season is confined to the northwestern basins, where the Yampa and Colorado Rivers are forecast to produce slightly above average volumes. For the remainder of the state, near average volumes are expected. It would require a wet spring to raise those lowest forecasts back to near average in the drier basins. Water users can expect a better chance for recovery in the South Platte Basin, which is entering its wettest period of the year; however this is less likely in the southwestern basins given their spring climatologies.


Figure STRM-1. NRCS outlook for natural streamflows for spring and summer in the Intermountain West region as a percent of average streamflows (data through April 1, 2009). (Source: NRCS)


Reservoir storage continues to be the bright spot for water supplies in Colorado, especially in basins where runoff forecasts have diminished recently. Storage volumes remain near to slightly above average across most of the state. Most likely, the state's reservoir storage will help supplement water supplies across most of the state this summer. However, with inflows now forecast to be below average in most basins we can now expect to enter water year 2010 with below average volumes.

Streamflows are expected to be below average across Wyoming. Most probable yield for the entire state of Wyoming varies from 44 to 131% of average. The highest expected streamflows (about 230% of average) are in the Belle Fourche & Cheyenne River Basins in northeast Wyoming. The lowest expected streamflows are in the Little Bear River Basin (68% of average) in southwest Wyoming and in the Green River Basin (83% of average) in western Wyoming. Other river basins across the state are expected to see near average streamflows. 

Reservoirs on the North Platte River are well below average at 78% of average, while the reservoirs in the rest of Wyoming are near or slightly above average (range from 101–111% of average). 

General water supply conditions are near average in northern Utah and the Virgin River Basin and near to much below average in central and southeastern Utah. Streamflow forecasts range from 53% for the Bear River at Stewart Dam to 115% of average on the Beaver River near Beaver. Surface Water Supply Indices range from 12% on the Bear River to 57% for the Virgin. The extremely low value for the Bear River is a reflection of Bear Lake storage, which continues to be well below average. Snowmelt streamflows are expected to have a wide range from much below average to above average across Utah this year. Forecast streamflows range from 32% on South Creek near Monticello to 115% on the Beaver River near Beaver. Most flows are forecast to be in the 80–105% range. Most reservoirs in Utah should easily fill this runoff season. A notable exception is Bear Lake. All reservoirs with previous fill restrictions are now able to store.

[The majority of the text on this page comes from the NRCS State Basin Outlook Reports.]


The Intermountain West Climate Summary is published periodically by Western Water Assessment (WWA), a joint project of the UNiversity of Colorado Cooperative Institute for Research in Environmental Sciences (CIRES) and the National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory (ESRL) researching water, climate, and societal interaction.

Disclaimer - This product is designed for the provision of experimental climate services. While we attempt to verify this information, we do not warrant the accuracy of any of these materials. The user assumes the entire risk related to the use of this data. WWA disclaims any and all warranties, whether expressed or implied, including (without limitation) any implied warranties of merchantability or fitness for a particular purpose. This publication was prepared by CIRES/WWA with support in part from the U.S. Department of Commerce/NOAA, under cooperative agreement NA17RJ1229 and other grants. The statements, findings, conclusions, and recommendations are those of the author(s) and do not necessarily reflect the views of NOAA.