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Climate Prediction Center


September - November 2004


Latest Seasonal Assessment - Although the water supply situation in drought-affected areas of the West is not expected to change significantly before the snowpacks start to build in late fall or early winter, the seasonal outlook does show limited improvement over large parts of the Rockies and the Plains, as well as relatively greater improvement in the Northwest and Alaska. Most of the Southwest will see persisting drought into November. The winding down of the monsoon during the forecast period reduces the odds for improvement as much of the Southwest and southern Great Basin enter a seasonally drier period. In contrast, cool, wet weather expected from late August into early September farther north and east should boost soil moisture, benefit grasslands, reduce the wildfire threat, and limit the need for irrigation water. Prospects for improvement are relatively greater in the Northwest, where both short-term rainfall and the typical increase in storm activity expected by the end of November contribute to the likelihood for increased moisture. In Alaska, the trend toward cooler weather and the prospects for increased storm activity as winter approaches should reduce the fire danger, which has been the main drought impact this summer. Although a weak El Niño is likely this fall and winter, impacts should be minor going into late autumn. El Niño's do not guarantee drought improvement but, if an El Niño persists into winter, there should be a slight increase in the chances for above-normal precipitation and increased streamflows over the Southwest this winter and early spring.

Discussion for the Seasonal Drought Outlook

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for September-November, the drought termination and amelioration probabilities for November, 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 drought forecast for the interior West considers the wet and cool conditions forecast by the medium-range products as the broad upper air trough retrogrades westward during the next week. The most recent runs from the GFS model have been consistent in showing a tendency for an upper level trough to be located near the West Coast or the interior West into early September. The 2-week soil moisture forecasts based on the GFS model show significant increases in moisture over the Northwest into western Montana and southward into northeastern Nevada, northern Utah, and western Wyoming, and northern Colorado. The forecast only shows limited improvement for much of this area because rainfall during late summer and early fall is not very critical to the long-term water supply situation, since significant snowpack rarely builds up before late November. Nevertheless, the forecast moisture and unseasonably low temperatures should benefit the wildfire situation and enhance growth of grasslands while reducing the need for irrigation, resulting in less use of reservoir storage. The areas with the greatest chance for improvement, as shown in Washington, western Montana and central Idaho, tend to begin to experience seasonal precipitation from winter-like storms by the end of November, so there are prospects for improvement from both a short and long-term perspective. However, little improvement to the reservoir situation by late November can be anticipated in the areas shown as improving in Montana and Idaho.

In the Southwest, the latter stages of the monsoon will provide some scattered heavy rains in this area during the forecast period, but the odds for overall drought improvement tend to diminish during autumn. Climatologically, southern California usually does not see much drought improvement before December. Drought is largely expected to persist for the southern Great Basin and over the Southwest from Southern California into Arizona.

In Alaska, the seasonal drop in temperatures along with increased storm activity should ease drought in the eastern part of the state, where several large fires were still burning as of mid-August.

Elsewhere, the first half of August was abnormally dry in several areas in the central and interior eastern parts of the country, including east Texas and much of Illinois, Indiana, and Nebraska, but there were no strong indications from the long-range or medium-range forecasts that drought would develop in any of these areas.

A weak El Niño is expected to develop shortly, but El Niño impacts are expected to be either too weak or too ambiguous to be considered a major part of the outlook through November. If an El Niño does develop, its potential impacts should be increasingly important for future drought outlooks as winter approaches. Traditionally, an El Niño tends to improve drought conditions in the Southwest, but improvement can be limited, as seen during the last event in 2002-2003. See the Climate Prediction Center's latest long-lead seasonal outlooks for the current thinking on the precipitation outlook through winter and beyond.

NOAA/ National Weather Service
NOAA Center for Weather and Climate Prediction
Climate Prediction Center
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Page Author: Climate Prediction Center Internet Team
Page last modified: August 19, 2004
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