October - December 2009

Outlook Graphic: GIF   PDF 

Latest Seasonal Assessment - Heavy, locally-flooding rains brought an abrupt end to drought in the western Carolinas during the last half of September, but recent changes have been more subtle elsewhere. New areas of drought were identified in parts of the central Appalachians, northwestern Ohio, and east-central Indiana in late September while rainfall eased drought in the northern Plains. By the end of the year, conditions are forecast to improve in all drought-affected areas east of the Rockies and in the Four Corners states, though with varying degrees of confidence. More limited improvement is anticipated for the areas of drought in northwestern Montana, northwestern Washington, most of California, and Nevada. Unfortunately, drought persistence is forecast for the dry areas in northernmost California, southern and western Oregon, and Washington from the Cascades eastward. Some eastward drought expansion is expected in the latter region, where drier than normal conditions are anticipated through December and where some moisture shortages have already been observed.

Tools used in the Drought Outlook included the official CPC precipitation outlook for October 2009 and the long lead forecast for October - December 2009, the four-month Palmer drought termination and amelioration probabilities, various medium- and short-range forecasts and models such as the 6-10 day and 8-14 day forecasts, the soil moisture tools based on the GFS model and the Constructed Analogue on Soil (CAS) moisture, the Climate Forecast System (CFS) seasonal precipitation forecasts, El Niño precipitation and temperature composites for October - December, climatology, and initial conditions.

Heavy late-September rains alleviated drought in the western Carolinas, but drought persisted in central and eastern sections of the states where rainfall totals were considerably lower. After a dry start, the odds favor above-normal precipitation for the second week of October, and both the October - December and updated October forecasts show equal chances for above- and below-normal precipitation. The expected development of moderate El Niño conditions by the end of the year could favor increased precipitation late in the period. In addition, the seasonal decline in temperatures and hence evaporation and transpiration means that the precipitation that does fall, though typically somewhat less than during other seasons, should increasingly benefit soils, streamflows, and groundwater as the season progresses.
Forecast confidence for the Carolinas is high.

Two new areas of drought were identified during the last half of September, one extending through part of east-central Indiana and western Ohio, and one covering a portion of the central Appalachians (specifically, much of northern West Virginia, southwestern Pennsylvania, and the Maryland Panhandle). Light to moderate precipitation is forecast during the first few days of October, then the odds favor above-normal precipitation into mid-October. Both the October - December and updated October outlooks show equal chances for above- and below-normal precipitation. Based on historical observations, precipitation deficits observed in early autumn have only a fair chance of improving significantly by the end of December, and the existence of moderate El Niño conditions has done nothing to improve those chances. Still, the likelihood of at least moderate precipitation into mid-October, along with the typical seasonal drop in temperatures, water demand, and surface moisture losses, tilts the drought forecast for these regions toward improvement, though not with a lot of confidence.
Forecast confidence for eastern Indiana, northwestern Ohio, and the central Appalachians is low.

Drought eased in the northern Plains during the last half of September, but dry conditions continued in the upper Mississippi Valley and western Great Lakes region, with intensification observed in a few areas. The first few days of October are expected to bring moderate to heavy precipitation to southern and western Minnesota and roughly the southwestern half of Wisconsin, with at least a few tenths of an inch expected elsewhere. Thereafter, above-normal precipitation is anticipated through the first one-third of October regionwide, and the odds continue to favor enhanced precipitation into mid-month in southern parts of the drought region. The October - December forecast favors neither above- nor below-normal precipitation, but the revised October outlook does favor wetness through central and southern Minnesota and a small part of adjacent Wisconsin. Climatologically, the last 3 months of the year are somewhat drier than some other seasons, but with seasonably dropping temperatures, more of the precipitation benefits surface and potentially sub-surface moisture conditions, especially with an increasing proportion of the precipitation falling as snow. These seasonal considerations, along with forecasts favoring wetness through the first two weeks of October, support a forecast for drought improvement by the end of December, though not with much confidence in northern Minnesota where significantly above-normal precipitation during the first two weeks of October is not as likely.
Forecast confidence for northern Minnesota is low. Confidence is moderate through the rest of the upper Mississippi Valley and western Great Lakes region.

Additional relief from the protracted drought affecting much of central and southern Texas appears likely by the end of the year. All forecasts from the start of October through the end of December favor above-normal precipitation, and the anticipated development of moderate El Niño conditions also favors enhanced precipitation during the last 3 months of the year.
Forecast confidence for Texas is high.

Light to moderate precipitation is forecast for the first few days of October in the drought area covering central and eastern Arizona and adjacent sections of the remaining Four Corners states. After that, the official outlooks are essentially noncommittal with respect to above- or below-normal precipitation through the end of the year, although the revised October outlook shows some tilt of the odds toward above-normal precipitation across the northern reaches of the region. Historically, early-autumn precipitation shortfalls have improved more often than not by the end of December throughout the region, and the existence of El Niño conditions seems to make drought relief for this period even more likely. Based on the El Niño considerations, more general climatological trends, and seasonal declines in temperature and systemic water losses, drought is forecast to improve throughout the region.
Forecast confidence for the Four Corners region is moderate.

Above-normal precipitation is anticipated through early October for the areas of northwestern Montana experiencing drought, but all forecasts for the region from mid-October through the end of the year are uncertain at best. Seasonal considerations, including declining temperatures and the increasing potential for precipitation to fall as snow, tilt the outlook toward the expectation of at least limited improvement for the region, but only with considerable uncertainty.
Forecast confidence for northwest Montana is low.

The drought in central and eastern Washington shows no signs of letting up by the end of the year, and some expansion of drought conditions to the east seems likely as well. Precipitation amounts are forecast to be unremarkable through the first half of October, but both the revised October and the October - December forecasts favor below normal precipitation, which agrees with conditions typically experienced during El Niño episodes during the last 3 months of the year. Climatologically, this period tends to be slightly wetter than most other seasons, but even considering this and the fact that temperatures will be dropping, the consistent and fairly confident forecasts for subnormal precipitation make drought persistence and some expansion likely.
Forecast confidence for central and eastern Washington is high.

Some improvement is forecast for the drought in western Washington despite the below-normal precipitation favored by most forecasts during October - December. Climatologically, precipitation increases substantially as the last 3 months of the year progress, and even somewhat below-normal totals would represent a significant amount of moisture for the region. However, the below-normal outlooks introduce some uncertainty to the drought forecast.
Forecast confidence for western Washington is moderate.

For the remainder of the West, drought is expected to persist across western and southern Oregon and northernmost California while some improvement is anticipated in the remainder of California and the drought areas covering Nevada. Little precipitation is anticipated through mid-October, which is not unusual for the region, and neither above- nor below-normal precipitation appears favored for the month as a whole. For the October - December period, there are enhanced chances for subnormal precipitation in northern parts of the region where drought is forecast to persist, with neither above- nor below-normal amounts favored elsewhere. The forecast is based primarily on the revised October and the October - December precipitation outlooks, with some consideration given to climatology, which indicates that October - December is a relatively wet time of year in the northern half of California, across Oregon, and to a lesser extent through northwestern Nevada. In addition, the expected development of moderate El Niño conditions could induce increased precipitation across southern reaches of the region late in the period. However, given the multi-year nature of the drought in most of this area and the uncertainty in late-year precipitation totals for areas south and east of northernmost California, only limited improvement seems most likely by the end of the year for most areas, with persistence expected where October - December dryness is favored.
Forecast confidence for central and southern portions of the West is moderate.

Some improvement is expected for the dry areas covering much of the southeastern half of Hawaii, based primarily on climatological considerations. Neither significantly wet nor dry conditions are favored during October - December, but precipitation typically increases somewhat during this time of year in those parts of the state experiencing drought conditions.
Forecast confidence for southeastern Hawaii is moderate.