June-August 2005

Latest Seasonal Assessment - More drought improvement may be on tap for the Northwest and northern Rockies this summer, with the greatest chance for improvement extending from eastern Idaho into Montana and western North Dakota. The latest June-August precipitation outlook from the Climate Prediction Center shows the odds favoring above-normal rainfall from the Dakotas to the Pacific Northwest. Across the drought region, recent storms have significantly boosted soil moisture and streamflows, but it is generally too late in the season to eliminate the hydrological drought that is expected to affect water supplies this summer. Forecast additional moisture, however, should benefit grasslands, pastures, and crops. To the south, the remnants of the long-term drought affecting parts of Arizona and New Mexico should persist as we transition into the hottest and driest time of the year. In the southern Plains, short-term dryness has resulted in drought affecting parts of Oklahoma and northern Texas, expanding southeastward. This area should see some improvement in coming months. Elsewhere, an expected tendency toward below-normal rainfall has resulted in an elevated risk of drought development in Hawaii.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for June-August, the drought termination and amelioration probabilities for August, 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 June-August is fairly consistent in showing above-normal rainfall for the northern Plains and northern Rockies, with a few products also spreading the wetness westward into Washington and Oregon. However, the medium-range models, including the 6-10 day and week 2 forecasts, show a tendency for a western high pressure ridge to build up, resulting in a warmer and drier pattern. As a result, there is less confidence in this drought outlook than in the previous outlooks. In addition, forecasting rainfall during the warm season is generally more difficult than during the cold season. The resulting outlook for the northern part of the West is a blend of the short-term forecasts, the June precipitation outlook and the June-September precipitation outlook. The spring-summer streamflow forecast as well as the climatological odds for drought improvement through August are also considered. The improvement area in Montana and vicinity results from the forecast wetness from the June and June-September precipitation outlooks, with more limited improvement shown for surrounding areas due to a drier signal for the next 2 weeks and the June outlook showing weaker odds for wetness outside of Montana. Mid-May mountain snowpack remained below normal from western Montana into Washington and Oregon, consistent with sub-par water supplies forecast in this area and, therefore, more limited drought improvement.

To the south, the area of limited improvement shown last month in Arizona and New Mexico was changed to persisting drought due to the transition to the hotter and drier time of the year, at least until the monsoon kicks in. A forecast tilt toward below-normal rainfall during June-September for parts of the drought region was also a factor.

In the southern Plains, the outcome for the recently developed drought that affects parts of Oklahoma and adjacent areas is very uncertain. Recent medium-range models, such as the GFS, have been showing increasing dryness from Oklahoma southward and eastward, but the most recent model runs have begun to bring more rain into the region. Long-range guidance tends to be relatively wetter for Oklahoma and drier to the southeast near the Gulf. Lack of a strong signal either for drought development or alleviation in the Oklahoma-east Texas-lower Mississippi Valley results in a forecast for limited improvement, since the odds favor a bit more rain overall than seen recently, and the CAS soil moisture forecasts for August do not show drought development.

There is some drought potential for the Southeast, especially from Florida northward, given the summer outlook for below-normal rainfall, but recent storms and 2-week forecasts for cool weather accompanied by normal rainfall argue against showing drought development at this time. The forecast for an active tropical storm season did not play a significant role in this drought outlook, but may be considered in next month’s outlook.

In Hawaii, a few statistical and dynamical models suggest below-normal rainfall either for June or June-August. With recent drying trends already resulting in some areas of abnormal dryness, and the experimental North American Constructed Analogue Soil model showing reduced soil moisture levels for August, an area of drought development is indicated for currently dry areas. This is a low-confidence outlook given that some models show near-normal rainfall for the summer.