November 2006 - January 2007

Latest Seasonal Assessment - The first 3 weeks of October saw abundant rains offering drought improvement from the Southwest into the Lower Mississippi Valley, and the Outlook calls for additional improvement across the southern United States, with the best odds for relief extending from the southern Plains into the Southeast. The intensifying El Niño should play a role in bringing plentiful moisture to the southern tier of the U.S. this winter. There is also a good chance for at least some improvement in the drought areas in the central and northern High Plains, but drought should largely persist with minimal improvement over northern Minnesota and from north-central North Dakota westward into Montana and southward into western Wyoming. The odds also favor drought to persist in Missouri, particularly in the northern and eastern areas. The November-January precipitation outlook indicates a tilt of the odds toward below-normal rain and snow in the Northwest, and the Outlook continues to indicate an enhanced risk for drought development from Washington and Oregon into Idaho. In contrast, despite an expected tendency toward below-normal precipitation, the typical heavy autumn and early winter precipitation should bring drought improvement to northwestern Washington. In Hawaii, localized drought areas should continue to see some improvement, although there is an increased chance for dryness later in the winter, a typical occurrence during El Niño episodes.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for November-January, the four-month drought termination and amelioration probabilities, 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 Analogue on Soil moisture.

The drought outlook this month is somewhat more optimistic across the southern areas of the U.S. and somewhat less optimistic over the northern tier. El Niño conditions, which are expected to persist through the winter, usually feature above-normal precipitation across most of the southern U.S. during November-January, and this is reflected in the improvement indicated from the southern Plains into the Southeast. However, as the official CPC precipitation outlook for November-January indicates, the chances for wetness in Arizona are lower than in areas farther to the east, so limited improvement is depicted there. The most recent computer model forecasts indicate the possibility of tropical moisture from the eastern Pacific making its way northward into the Arizona area in early November, but this is too uncertain to weigh heavily in the Drought Outlook at this time.

It is important to note that improvement does not necessarily mean drought elimination, and the NCDC Palmer Drought Index probabilities for January show little chance the drought will end or even be significantly ameliorated (Palmer Index rising above -2.0) in Texas and Oklahoma by that time.

Farther north, the improvement forecast in Kansas, Nebraska, and South Dakota has less confidence than the improvement shown farther south. The long-lead precipitation outlook shows equal chances for wet or dry in the central Plains, and the improvement expected in this area is primarily based on wet 2-week soil moisture forecasts, and on El Niño soil moisture composites for January, which point toward wet soils from Nebraska southward through western Kansas into Texas.

In the northern Plains, the area of persisting drought near the Canadian border is consistent with El Niño composites of seasonal precipitation and January soil moisture, while the area of some improvement just south of this region is based on near-normal soil moisture patterns in the composite for January, and on increased wetness shown in the 2-week soil moisture model based on the GFS ensemble run. The persisting drought shown in Missouri is derived from the long-lead November-January precipitation outlook and various El Niño composites. In the first 2 weeks of the forecast period, there is much uncertainty about rainfall amounts for this area. Some model runs indicated a substantial storm affecting the Plains and Mississippi Valley toward the end of October, but more recent model runs backed off on this idea. The Constructed Analogue (CAS) soil moisture map for December is consistent with dryness across the northern tier and increasing dryness east of the Cascades in the Pacific Northwest.

The official precipitation forecast continues to show the odds tilting toward dryness in the Northwest from Washington to Montana, so the Drought Outlook maintains the area of developing drought risk in the region. As with last month, the Outlook considers the ample precipitation normally received this time of the year in northwestern Washington and, as a result, shows improvement despite the odds favoring below-normal precipitation.

In Hawaii, as with last month’s Outlook, some improvement is shown for the several drought areas that have been affecting the islands. This is consistent with the wet pattern already seen in mid-October. It should be noted again, however, that El Niño tends to bring dryness to Hawaii by late winter, so the risk for drought development will increase during the winter.