October-December 2005

Latest Seasonal Assessment - Outside of the coastal areas hit by Hurricane Ophelia’s rains, much of the Eastern Seaboard saw increasingly dry conditions develop during September. With prospects dim for significant rains in the near future, if not longer, there is a risk for drought development in a large area extending from the Carolinas northward into Virginia, Maryland and southern Pennsylvania, as well as eastern portions of Kentucky and Tennessee. Prospects look brighter for drought extending from Delaware through New Jersey into southern New England. Elsewhere, tropical rains should benefit the lingering drought area in southeast Texas. In the Arkansas-western Louisiana-east Texas drought area, the official CPC outlook for October-December indicates a tilt of the odds toward above-normal rainfall, so improvement is expected by the end of December. Nevertheless, warm and dry weather forecast over the short-term for parts of the region could result in temporary worsening of drought conditions. To the north, extreme drought persisted across northern and western portions of Illinois and adjacent parts of Iowa and Wisconsin into September. Periodic showers on tap for the Midwest should offer limited relief for this area, but it is extremely unlikely that this region will see enough rain in the next few months to end the drought. Prospects look more favorable to the north, and general improvement is expected in Wisconsin and Michigan, although prospects for ending drought are poor in this region as well. With the Southwest going into a drier time of the year, significant alleviation of long-term drought still affecting parts of this region is unlikely. Drought should largely persist as well in the northern Rockies and the interior Northwest, with some improvement likely in western Washington and western and southern Oregon.

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.

This month’s outlook depicts an area of drought development along the Eastern Seaboard in response to the recent trend toward dryness and forecasts for increasing soil moisture deficits in the next 2 weeks as suggested by the latest 6-10 day and 8-14 day forecasts and the various soil moisture models. The area along the coast from northeastern South Carolina to southeastern Virginia is kept outside of the development region mostly due to the ongoing rains from Hurricane Ophelia, located near the coast of North Carolina as of September 14. There is a fairly high level of confidence that drought or near-drought conditions will develop in the indicated region in the next 2 weeks due to persisting high pressure but, as is often the case, confidence about the duration of any such developing drought beyond the first 2 weeks is not high. The official CPC precipitation outlook for October-December indicates equal chances for wet or dry, so there could very well be some amelioration of the dryness as we go into fall and winter. There is also the wild card of future tropical storms, and there is no way to know at this time where the rains from any such storms will strike. Even during the first 72 hours of the forecast period, there remains uncertainty about rainfall amounts along the coast from Virginia northward due to Ophelia and her interaction with an eastward-moving cold front. Medium-range models show the odds for wetness improving south to north along the East Coast, so confidence for drought relief is higher in New England than in Delaware and southern New Jersey.

A number of cold fronts will be dropping rains on the Great Lakes region in the first 2 weeks of the Outlook period, and this is a factor in showing improvement in Michigan and Wisconsin. More limited improvement is expected to the south in the Illinois, Iowa, Indiana area as the Palmer drought probabilities for December show small odds for drought alleviation. Farther south, in contrast, both the drought probability data and the CPC outlook for wetness during October-December argue for improvement in the Arkansas-Texas-Louisiana region. Nevertheless, the soil moisture forecasts depict worsening dryness over Louisiana, eastern Texas, and southern Arkansas during the first 2 weeks of the Outlook period, so this reduces the confidence in the forecast.

Both climatology and the CPC 2-week soil moisture forecast support improvement in extreme southeastern Texas. With signs the Gulf of Mexico will see additional tropical activity in coming weeks as disturbances track westward beneath the high pressure region to the north, this seems like a reasonable forecast.

To the north, soil moisture models range from slight improvement to slight deterioration in the Nebraska-South Dakota-eastern Wyoming drought areas, with the long-lead seasonal outlook showing equal chances wet or dry. Because a few of the seasonal forecast tools point toward wetness for parts of the region, such as the new CPC consolidated forecast and the CDC multiple regression forecast, some improvement is indicated.

The Southwest is going into its dry season, so significant improvement in the lingering drought areas in Arizona and New Mexico is unlikely, and the areas are shown as persisting. Seasonal considerations also argue against improvement in the northern Rockies, but both medium-range forecasts and the Palmer drought amelioration maps argue for some improvement in western Washington in the case of the medium-range forecasts and western Washington and western and southern Oregon and southwestern Idaho in the case of the 4-month amelioration maps.