October - December 2004

Latest Seasonal Assessment - The water supply situation in drought-affected areas of the West is not expected to change significantly into early winter, but the seasonal outlook does show limited improvement over large parts of the Rockies and the Intermountain West. Most of the Southwest will see persisting drought into December, as progress toward drought improvement tends to be difficult during the fall or early winter period before heavy snows accumulate in the mountains. However, the recent development of an El Niño makes forecasters cautiously optimistic that the Southwest and Colorado River Basin will see a normal or better snow season this winter. Not every El Niño is accompanied by above-normal precipitation in the West, but it does tilt the odds toward drought improvement in the Southwest. The official Climate Prediction Center long-lead precipitation outlook for the winter months shows a weak tendency toward above-normal precipitation in the Southwest. Elsewhere, limited drought improvement is expected from southwestern North Dakota southward into central South Dakota and central Nebraska into Kansas, while drought persists from western South Dakota into the Panhandle of Nebraska and into northeastern Wyoming. Recent hot, dry weather has reduced soil moisture from Kansas southward into Oklahoma, while short-term dryness has also become a concern in an area extending from Arkansas into parts of Louisiana. With indications that below-normal rainfall could persist well into fall from Arkansas northeastward into the Midwest, this area bears watching. Hurricane Ivan will ensure that there is scant chance for dryness developing east of the Mississippi River into much of the Southeast or Mid-Atlantic, but soil moisture could be a concern elsewhere in the middle and lower Mississippi Valley. Drought development is not forecast at this time, as the historical odds are low that a full-fledged drought can develop this time of the year in the Arkansas-Missouri region. Drought is not forecast to expand southward from northern Kansas due to the forecast indicating a tilt of the odds toward above-normal rainfall during October-December over the southern Plains. In Alaska, the shift to much cooler and damper weather has ended drought there, although large precipitation deficits continue. However, a lack of immediate impacts will preclude drought from returning to Alaska before next spring. In Hawaii, short-term dryness has developed on the Big Island. An El Niño often brings dry weather to the state, but it is unlikely that drought will develop by December.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for October-December, the drought termination and amelioration probabilities for December, 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 West mainly considers recent trends, the medium-range forecasts, and the seasonal outlook for precipitation during October-December. For the short-term, models develop an impressive upper level trough over the West around September 19-24, and this should bring wet weather, with some mountain snows, to northern parts of the western drought region. Associated with the trough, a fairly strong frontal system should bring rain to the Great Plains around September 20-21, raising hope for a break in the dry pattern for the Kansas area. The week 2 forecast issued on September 15 suggests a return to a flatter trough pattern beyond the 6-10 day period, so somewhat drier weather may prevail across the West by late September.

With an El Niño expected to last into next year, the odds start to tilt toward normal to above-normal precipitation in the Southwest as we go into winter. However, the wet signal does not show up for the October-December period in past occurrences of weak to moderate El Niños, so the drought outlook still shows persisting drought across the Southwest. The odds for wetness increase for December-February and January-March, as shown in the current long-lead precipitation outlooks, so the seasonal drought outlook may provide a more optimistic picture for the Southwest in future months. It is important to note that a look at past occurrences of weak to moderate El Niños shows that drought relief is by no means a sure thing even later in winter. The last warm event, just two years ago, failed to bring much improvement to most areas, and the 1976-77 and 1963-64 events resulted in drier-than-normal weather during November-March. Nevertheless, the overall Pacific SST pattern, with plentiful warm water in the east and a lack of warm water in the tropical western Pacific and eastern Indian Ocean, provides a more hospitable environment for drought improvement.

Persisting drought is called for in portions of the High Plains, including western South Dakota and the Nebraska Panhandle, due mainly to climatological considerations. October-December precipitation tends to be too light to provide drought improvement. The odds, however, improve farther east in South Dakota and Nebraska, due both to climatology and the current 2-week outlook.

Recent dryness from Kansas southward and also in the Arkansas-Louisiana area bears watching. Both short and long-range forecasts suggest dryness from Arkansas into southern Illinois, with a sharp cutoff east of the Mississippi because of Ivan's potential rains. Drought development was not forecast because the Palmer drought probabilities show less than a 10 percent chance for the index to drop below -2.0 by December. Nevertheless, with some El Niño analogues showing dryness for October-December, the area needs to be monitored.

The southern Plains area is also vulnerable to increased dryness over the short-term, but drought expansion is not forecast because of increased rainfall expected during the October-December period, a pattern that is consistent with El Niño. The analogue soil moisture forecasts for December also suggest wet conditions in this region.

In Alaska, the shift to much colder weather and increased moisture has alleviated drought, but sizeable precipitation deficits remain. Impacts from those deficits should not be very important as winter progresses, with the wildfire threat already diminished.

Hawaii is a candidate for drought development, as the weather has turned decidedly drier in recent weeks, and the U.S. Drought Monitor has recently introduced an area of D0 dryness on the Big Island. With the long-range forecast calling for below-normal rainfall during December-February, consistent with an El Niño, drought may develop during the winter. For the October-December drought outlook period, however, development is not forecast because the official precipitation forecast sees equal chances for wet or dry.