December 2004 - February 2005

Latest Seasonal Assessment - Powerful Pacific storms striking the Pacific Coast dumped heavy mountain snows over California and the Great Basin during late October, resulting in an excellent start to the West's water year. As of mid-November, snowpack in the Upper Colorado Basin above Utah's Lake Powell was 139 percent of average, while snow water content ranged up to four times normal in southwestern Utah. Below-normal snowpack was mainly confined to Montana and central and eastern Wyoming, as well as the Pacific Northwest. The early snows represent a good start to alleviating long-term water shortages, although the larger reservoirs have not yet responded to the increased moisture. Lake Powell, for example, was only 38 percent full in mid-November, and 129 feet below its full elevation. Larger reservoirs like Powell will take a long time to respond to the unseasonable snows, and currently remain near multi-decade lows. The overall drought outlook through February 2004 calls for limited improvement to water supplies for the Great Basin and Southwest, where there is generally an equal chance for above or below-normal precipitation. A tendency for above-normal precipitation through February over large parts of New Mexico should result in general drought improvement in that region, although large reservoirs should show little or no increase in water volume during this time period. Drought is expected to persist in southeast Oregon and southern Idaho due to a tendency for below-normal precipitation, while the odds favor persisting drought from Montana and eastern Wyoming eastward into the Dakotas and Nebraska Panhandle because of the drier time of the year and recent below-normal snowpack. Short-term precipitation should result in limited improvement in northwestern Kansas and southern Nebraska. The long-lead outlook for December-February favors below-normal precipitation in the Great Lakes area, mid-Mississippi Valley, and the Ohio Valley, but widespread drought is not expected to develop by February. In Hawaii, the odds favor below normal rainfall during December-February, and drought is expected to develop in those areas that currently remain abnormally dry, including western Maui and eastern and northern areas of the Big Island. Recent heavy rains have reduced the odds for drought development on the other islands.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for December-February, the drought termination and amelioration probabilities for February, 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. For this outlook, a number of analogues based on historical instances of weak to moderate El Niño events were consulted, as well as updated runs of the Coupled Forecast System (CFS).

The current outlook is largely based on the December-February precipitation outlook, medium-range outlooks and, to a lesser extent, current mountain snowpack. Well-above normal snowpack in the Great Basin and California got the water year off to an excellent start, with unprecedented October snows in Utah. The early-season boost to soil moisture bodes well for runoff later in the season, boosting reservoir and streamflow prospects. The Seasonal Drought Outlook generally shows limited improvement in the Great Basin and Southwest due to equal chances of above or below normal precipitation through February and the expectation that the reservoirs will likely show limited response to the unseasonable snows until melt occurs later in the season. Where the snowpack is greatest, the eventual improvement to water supplies could be substantial unless conditions turn drier. With weak to borderline moderate El Niño conditions expected this winter, the odds for wetness in the Southwest and Colorado Basin are not as great as could be expected for a stronger event, but historical analogues suggest a slight tilt toward improvement by spring. Both dynamic models, especially the NCEP Coupled Forecast System, and analogues depict the odds for wetness in the Southwest as being better in spring than winter. As a result, more improvement may result after the end of the current outlook period. Above-normal precipitation is likely from Texas westward into New Mexico through the period, and the latter drought area is highlighted as showing relatively greater improvement. Nevertheless, larger reservoirs in New Mexico, such as Elephant Butte, are not expected to see significant increases this winter. Farther north, the odds tilt toward drier weather in the Northwest, and this should result in persisting drought in southeast Oregon and southern Idaho. The area of persisting drought in Montana, eastern Wyoming, and the western Dakotas and Nebraska Panhandle is based on current below-normal mountain snowpack and the relatively dry time of the year, when drought alleviation is difficult. Forecast precipitation, however, in Kansas and southern Nebraska during late November suggests better odds for minor improvement in this area.

The precipitation outlook for December-February tilts toward dryness in parts of the Midwest, and there is a chance that the existing dryness could expand or intensify. Both the dynamic models and El Niño analogues show a dry signal for much of the Midwest. Given that this is the cold time of the year, it is difficult to develop a drought with meaningful impacts by February, so widespread development is not anticipated. Streamflows have declined in northern New England due to recent dryness, so this is another area that bears watching.

In Hawaii, heavy, flood-producing rains, mainly on Oahu, have increased moisture supplies. Current areas of lingering dryness, specifically northern and eastern Big Island and western Maui, are seen as the most vulnerable to drought development, and these areas are highlighted in this month's Outlook. The area shown for development is reduced from last month because of the recent heavy rains, making it more difficult for widespread drought to gain a foothold.