October - December 2007

Latest Seasonal Assessment - The updated outlook shows little change from last month’s forecast across the West, with improvement expected in the Northwest and northern Plains, but persisting drought in the arid Southwest. Moderate to heavy rains struck the Northwest region in late September and early October, with snow in the higher elevations, and above-normal precipitation is forecast to continue through the first half of October. Long-range forecasts show wetness for October-December. Medium range and seasonal forecasts look dry for the Southwest. In the Upper Midwest, improvement is expected to continue from Minnesota to Michigan. To the south, however, drought should persist from the Maryland-Delaware area into northern portions of Georgia and Alabama. Tropical rains should lead to improvement along the Gulf and Southeast Coasts, with more limited improvement expected in other portions of the drought region. The drought affecting parts of southern New England may persist, while the drought in upstate New York should improve. Hawaii is likely to see improvement.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for October-December, the updated CPC forecast for October, 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 (CAS) Constructed Analogue on Soil moisture.

Over the South, the updated forecast expands the area of persisting drought from the mid-Atlantic into northern Alabama and Georgia. Negligible rain and above-normal temperatures will worsen drought during the first 5 days of the period, and medium-range forecasts show above-normal temperatures and normal rainfall for this region beyond day 5, as of October 2. At least one model run, the 12Z GFS on October 2, is showing a deep trough over the East Coast and heavy rains in the mid-Atlantic and New England states around October 10. As a result, short-term relief cannot be ruled out. La Niña composites for October-December show below-normal rainfall from southern Pennsylvania into northern Georgia and northeastern Alabama, with the strongest dry signal over the Virginia area during moderate to strong La Niña episodes. Coastal improvement along the Gulf and Southeast is based mostly on tropical moisture expected in October, as shown on the updated October precipitation outlook from CPC.

Medium-range forecasts show continued normal to above-normal rainfall over the Minnesota to Michigan area, supporting continued drought improvement in this region. To the east, the outlook for upstate New York is more uncertain. Climatology suggests good odds for improvement in the Adirondacks, and GFS bias-adjusted rainfall forecasts indicate beneficial rains during week 1 and 2 across the state. In southern New England, prospects are very uncertain. The forecast depicts persisting drought largely based on the dry weather expected during the first half of October, but the outlook would be more positive for relief if the model runs subsequent to the above-mentioned GFS continue to show heavy rains during the second week of October.

In the West, there was little change made to the forecast released on September 20. Short, medium and long range forecasts support the wetness in the Northwest and dryness in the Southwest. In Hawaii, climatology and the La Niña suggest drought improvement.