December 2006 - February 2007

Latest Seasonal Assessment - Record-breaking rains and flooding in the Pacific Northwest during the first half of November eliminated short-term dryness concerns, as precipitation amounts reached 1 to 2 feet in western Washington and the adjacent coast of Oregon. Although a much drier pattern is expected during the winter, it is unlikely that drought will develop within the next several months. Farther east, however, drought persisted over parts of central and eastern Montana, southward to Wyoming, and eastward into northern Minnesota and northern Wisconsin. The odds favor drought to largely persist into February across the region, as below-normal precipitation is forecast for the Montana area, and the northern Plains usually cannot count on much relief during this time of the year. Drought may even expand across the eastern Dakotas into western Minnesota, an area where mid-September to mid-November precipitation totaled less than one-half of normal. Across the southern tier of states, the ongoing El Niño should contribute to improving drought conditions in the Southwest, the southern Plains, and the Southeast, although many locations will see persisting or worsening drought conditions before relief arrives later in the outlook period. Prospects for relief gradually diminish going from south to north in the Plains, with more limited improvement expected in Oklahoma, and persisting drought in much of Missouri and in southern Kansas and adjacent parts of Oklahoma. In Hawaii, El Niño often brings dry weather to the islands, and this winter may be no exception. As a result, there is an enhanced risk for drought development across the islands in coming months.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for December-February, 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 main change from last month’s Outlook is the removal of the area of drought development from the Northwest due to the heavy rain and snow that pounded the region during the first half of November. Although below-normal precipitation is still expected this winter, the current abundant moisture conditions will make it difficult for drought conditions to develop by the end of February. Drought development may be seriously considered in future Drought Outlooks, however. April mountain snowpack is a key drought indicator for the West because of its influence on spring and summer streamflows and reservoir storage, and much can happen between now and then. Some monthly numerical models are forecasting a change to a more typical El Niño type of pattern in December, with the latest Japanese model (JMA), for example, showing a change to a positive Pacific North American Pattern (PNA) by early December, resulting in a high pressure ridge over the western U.S. and a trough over the East, a pattern favoring dry weather for the Northwest and northern Plains. The model also shows signs of the subtropical jet developing across the extreme south, indicative of increased rainfall. This pattern is much more consistent with the canonical El Niño circulation expected during the upcoming winter.

Persisting drought is shown for the northern Rockies and northern Plains, with expanding drought across the eastern Dakotas into Minnesota. The latest CPC precipitation outlook for December-February indicates a tendency toward dryness in the Montana-Idaho region, consistent with drought continuation. This month’s drought outlook is somewhat more pessimistic in the northern Plains, where large areas of some improvement were indicated in the October release. The change is due in part to the climate, as precipitation in the plains tends to be light in winter, and also due to indications of a tendency toward below-normal precipitation from several statistical and numerical models. Both the CFS (Climate Forecast System) model and the UKMET multimodel, for example, show below-normal precipitation extending eastward into Minnesota during December-February. Recent drying trends also played a role in the forecast expansion of drought in the northern Plains. Sixty-day rainfall through November 14 was less than one-half of normal from central South Dakota into southern Minnesota, resulting in expansion of D0 dryness in the November 14 U.S. Drought Monitor. Confidence for the development area is not strong, given this is the time of year with low evaporation and reduced water demands from plants and people, and El Niño winter composites do not indicate unusual dryness east of Montana. Also, a major Plains winter storm cannot be ruled out in coming weeks.

Farther south, drought is shown as persisting in most of Missouri, although the area may see some good precipitation events in the next couple of weeks and beyond. Confidence level is also weak for this region. The persisting outlook comes mostly from the indications of below-normal precipitation in the 3-month precipitation outlook, but dryness probabilities are weak and the status of the Arctic Oscillation (AO) has much to do with whether Missouri is wet or dry in the winter during and El Niño, neutral AO’s tending to bring wet weather to the Middle Mississippi Valley and negative AO’s dry weather.

Given the ongoing El Niño, the odds for wetness increase toward the south, and a wet winter pattern should result in drought improvement in the Southwest, southern Plains, and Southeast. Transitioning from the drought in southern Kansas to the drought in Texas, it is difficult to delineate the boundaries between areas with relatively dim prospects for relief and areas where relief is more confident. This Outlook was more pessimistic than last month’s in Kansas and northern Oklahoma due to recent trends and medium-range weather forecasts indicating a good chance that conditions will worsen before they get better in the southern Plains. Considerable rain will be needed to significantly improve reservoir conditions in some locations, including the extreme drought area in northern Oklahoma. To the south, the prospects look better in Texas and southern Oklahoma, where both the monthly and seasonal CPC forecasts show a tilt toward wetness, consistent with El Niño impacts.

Confidence for improvement on the Florida peninsula is tempered by the fact that the meteorological drought and water supply situation has been recently deteriorating, and considerable rain will be needed to turn the situation around.

The official CPC precipitation outlook for December-February shows an area of dryness centered in the Ohio Valley. Current abundant moisture conditions will make it difficult for drought to develop by February in most of this region, so no development is indicated in the current Drought Outlook.

In Hawaii, heavy early November rains banished most of the dryness on the islands, but below-normal rainfall is expected during December-February, so the risk for drought is elevated. The main concerns involve the population sensitive to short-term (1-3 month) dryness. This includes residents dependent on rain water catchment (7000+ homes on the Big Island alone and others on the rest of the islands) and those dependent on surface water diversions (primarily Big Island and Maui). The north portion of the Big Island has become more vulnerable to short-term dryness because of damage to the water supply system from October’s 6.7 earthquake. There is also some concern for out-of-season brush fires if dryness persists long enough.  The drought risk extends well beyond Hawaii to Micronesia, where El Niño-related dryness often begins in November and lasts until June or July.