July-September 2005

Latest Seasonal Assessment - A wet spring generally improved the drought situation in the Northwest and northern Rockies, and recent storms have reduced drought in the northern and central Plains. Transitioning into summer, improvement should be more limited in the Northwest and northern Rockies, although a tendency for above-normal rainfall should bring additional improvement to drought areas in parts of Montana. In contrast, a tendency toward above-normal temperatures and below-normal rainfall during July-September should result in little or no improvement over eastern Arizona and northern New Mexico. Storms during the first half of June brought spotty improvement to drought areas in the Great Lakes and Mississippi Valley, but drought is expected to largely persist from southeastern Missouri and southern Illinois into Arkansas, southern Oklahoma, and northeastern Texas. Some improvement is anticipated for northern Illinois and southern Wisconsin, and more substantial alleviation is forecast for Michigan and northern Indiana. Improvement is also on tap for Louisiana and adjacent coastal Texas, but there is enhanced risk for drought expansion across northern and central Texas and westward in Oklahoma. A trend during the last half of June toward hot and dry weather across the Plains is a contributing factor toward the outlook for persisting or developing drought in parts of the southern Plains. Elsewhere, drought over the Big Island of Hawaii and leeward areas of Maui is expected to persist.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for July-September, the drought termination and amelioration probabilities for September, 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 Analogues for the season, and the latest western water supply forecasts.

Statistical and dynamic forecast guidance for the next 3-month period, July-September, once again is fairly consistent in showing above-normal rainfall for the northern Plains and northern Rockies, and the official CPC long-lead outlook depicts an area of slightly enhanced rainfall probabilities extending from Montana southward into Wyoming. The latest seasonal drought outlook, however, is more conservative in showing improvement compared with the two previous outlooks, as climatological considerations argue against much improvement this time of the year, rainfall normally diminishing in this region after June. A second factor is the medium-range outlook for the first 2 weeks of the valid period, with models in general agreement on a large high pressure ridge aloft building across the Plains states. This results in 2-week forecasts based on the GFS model showing sharp reductions in soil moisture from southeastern Montana southward through the Rockies and eastward across the Plains. The result is a scaling back of the green improvement area toward central and western Montana. The improvement shown in Montana is consistent with both short term and long-term rainfall forecasts, and also coincides with areas that have already been undergoing improvement on the U.S. Drought Monitor and are seen as eligible for additional improvement in coming weeks.

Heavy rains are slated for the coasts of Washington and Oregon during the early part of the outlook period, but the heaviest moisture is expected to fall west of the current drought area. Hydrological drought is expected to persist until at least next winter in the Northwest due to the poor snow season during 2004-05, and this is reflected in June 1 streamflow forecasts showing continued below-normal flows during April-September, including 72 percent of average for the Columbia River at the Dalles.

To the south, with the odds favoring warm and dry conditions in the Southwest in July, July-September, and the 6-10 day and 8-14-day periods, the areas of lingering long-term drought in Arizona and New Mexico are expected to persist. As always this time of the year, locally heavy monsoon thunderstorms in these areas cannot be ruled out.

In the Plains, the slight tendency for warmth as shown in the CPC July-September long-lead outlook together with hot and dry conditions forecast for the last half of June result in an enhanced risk for drought development in northern and central Texas as well as western Oklahoma. Drought extending from central and eastern Oklahoma into Arkansas and southeastern Missouri and southern Illinois is expected to persist. This is largely due to the hot, dry weather expected during the last half of June plus indications of warmth from the seasonal forecast tools from Arkansas southward. The latest CAS forecast, based on the most recent soil moisture conditions, shows significant dryness extending from southern Missouri into southern Illinois. Although this forecast tool, along with the 2-week soil moisture forecasts, was factored into the seasonal drought outlook, confidence is low for the central and lower Mississippi Valley due to conflicting messages from the forecast tools, including the CFS model, which shows above-normal rainfall for July-September for Arkansas as well as Louisiana. Mixed messages from the forecast tools resulted in the forecast for only some improvement for the drought in Illinois and southern Wisconsin, with the hot, dry forecasts for the start of the forecast period in June offset to some extent by various models showing a tendency for wetter conditions northward from the Lower Mississippi Valley. Farther north still, there is more agreement from the various forecast tools as well as the Palmer drought probabilities for September that improvement should be evident in Michigan by the end of September, if not much sooner.

The extension of the area of some improvement extending eastward into North Dakota reflects the hydrological drought affecting the Missouri River and associated reservoirs. This drought should be ongoing, despite some good rains in the region.

Near term, an MCS is forecast to bring rain from Oklahoma into Louisiana during the first days of the Outlook period, and it should be kept in mind that organized or disorganized thunderstorms as well as tropical storms and their remnants can wreak havoc with any seasonal drought outlook in the eastern half of the country. The drought outlook merely attempts to depict where the probabilities lie.

In Hawaii, the forecast for development issued last month materialized, although it was limited to the southern islands. For the outlook going through September, the odds tilt toward drought persisting in current drought areas on Maui and The Big Island. This is based on a dry forecast for July and climatological likelihood for persistence into September.