January - March 2005

Latest Seasonal Assessment - The latest seasonal drought outlook calls for generally improving conditions across the Southwest and much of the central Great Basin, while prospects for relief diminish farther north. Drought will tend to persist from southeastern Oregon eastward through southern Idaho, across Montana and into the western Dakotas, western Nebraska, and northeastern Wyoming. Limited improvement is expected in a transition band between the two areas. Although many large reservoirs continue to be well below normal, or even near record lows, heavy autumn rain and snow has boosted snowpack, soil moisture, and streamflows in large parts of the West. In particular, three consecutive months of above-normal precipitation from September to November benefited the Southwest, with Utah reporting record early-season snowpack by early November. In mid-December, the water content of the mountain snowpack still totaled over 150 percent of normal from California's Sierra Nevada into Nevada, Utah, and Arizona. In contrast, snowpacks have fallen below average in the Pacific Northwest and northern Rockies. The medium-range forecasts indicate a trend toward drier weather during the last half of December for the West, but the seasonal outlook for January through March shows a tilt of the odds toward a return to above-normal precipitation in the Southwest. This outlook, along with current abundant snowpack, implies general drought improvement across the Southwest and central Great Basin. Many reservoirs, however, including Lakes Mead and Powell in the Colorado River Basin, traditionally do not see increases until summer. A slower start to the snow season along with a drier outlook through March implies a tendency for persisting drought in the Northwest, northern Rockies, and northern and central Plains. Elsewhere, Hawaii has experienced reduced dryness after heavy rains soaked windward areas on the Big Island and Maui, but an expected trend toward drier weather during January-March results in the potential for drought development.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for January-March, the drought termination and amelioration probabilities for March, various medium and short-range forecasts and models such as the 6-10 day and 8-14 day forecasts, and the soil moisture tools based on the GFS model, and the Constructed Analogues for the season.

The current drought outlook focuses on the January-March precipitation outlook and considers current mountain snowpack. Ample rain and snow through early December has produced above-normal snowpack across most of the Southwest and northward into the Great Basin. Precipitation from October 1 to mid-December totaled over 200 percent of normal from southern California into southern Nevada, western Arizona, and large areas of Utah. To the north, snowpack is less, with cumulative precipitation generally below normal from the Pacific Northwest eastward into Montana and northern Wyoming.

The main change from last month's outlook is the increased area of improvement shown in the Southwest and Great Basin. Although the second half of December is shaping up to be relatively dry across the West, as high pressure dominates the western half of the country and troughing takes over in the East, the current drought outlook period extends into early spring, a period where there is somewhat higher confidence in above-normal precipitation, as indicated by various statistical and dynamical models for the January-March period as well as historical analogues for El Niño. It needs emphasizing that weak El Niños show weak signals for wetness in the Southwest, but recent improvements in soil moisture, streamflows, and snowpack increase the confidence that even without above-normal precipitation, a 1-category improvement in drought intensity is quite feasible. In fact, the Palmer Drought Index already shows mostly normal to wet conditions across the Southwest. In contrast, the numerous signals from both medium-range and long-range models for dryness in the Pacific Northwest result in a forecast for persisting drought in this region. The persisting drought forecast for the northern Rockies and High Plains is largely based on climate, this being a time of the year when precipitation is relatively light.

Many reservoirs have reacted to the wet start of the western water year, but many other reservoirs will not see improved storage until later in the year, due to the normal lag between winter snowfall and water supply impacts. This is especially true for Lakes Powell and Mead in the Colorado River Basin as well as Elephant Butte Reservoir in southern New Mexico. However, it is anticipated that other indicators of drought, such as streamflows, smaller lakes and reservoirs, groundwater, and soil moisture will see improvement by the end of March in the Southwest and parts of the Great Basin.

Much of the country outside of the Northwest and northern Rockies has been abnormally wet this autumn, resulting in little likelihood that drought will develop elsewhere. Some areas that do bear watching include the Pacific Northwest eastward into western Montana and parts of the Midwest. The odds tilt toward below-normal precipitation for both regions during January-March, so dryness could expand, but large-scale drought is unlikely to develop by March. Some dryness has also developed in southern Florida and parts of Puerto Rico, but widespread drought impacts are also not expected in these areas.

In Hawaii, a slight tendency toward dryness during January-March may result in drought development near the areas that are currently dry, mainly on the Big Island and Maui.