August - October 2009

Outlook Graphic: GIF   PDF 

Latest Seasonal Assessment - As southern Texas continues to struggle with an historic drought, there are indications that the drought may begin to loosen its grip on some areas going into autumn. The beneficial impacts from the ongoing El Niño are usually more pronounced in this region from November on, but long-range forecasts suggest decreasing odds for below-normal rainfall by October. As a result, some improvement is forecast for parts of the Texas drought region over the next 3 months. Significant rains in the near term should contribute to drought improvement for southern Louisiana, southern Mississippi, eastern North Dakota, northwestern Montana, and southeastern Alaska. Some improvement is expected for the long-term drought affecting Wisconsin and parts of Minnesota, with substantial rains forecast during early August. In the Pacific Northwest, some improvement is shown for the recently-developed drought in Washington’s Olympic Peninsula, but the drought should persist over the interior areas. Continued drought is also called for in California and parts of Nevada, while below-normal rainfall during the first half of August is expected to contribute to drought development over southern and eastern Arizona, where conditions have dried out quite sharply during the past several weeks. Below-normal rainfall during the latter part of the summer thunderstorm season is expected to lead to persisting drought for parts of New Mexico. In the Pacific, Hurricane Felicia’s forecast track could result in the storm affecting Hawaii by August 10-11. It is too early to know the impacts, if any, from the storm given that this Outlook was prepared on August 5, but Felicia increases the chances for significant rainfall and some drought relief over the Big Island and Maui.

Tools used in the Drought Outlook included the official CPC precipitation outlook for August 2009 and the long lead forecast for August – October 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 August-October, climatology, and initial conditions.

Parts of the Wisconsin-Minnesota drought area are forecast to see substantial rains during the first week of the forecast period. Nevertheless, the Drought Outlook indicates only some improvement because above-normal temperatures are expected to reduce the benefits of the rain during the next 2 weeks, and the latest (August 5) CPC forecasts show the odds tilting toward below-normal rainfall during the 6-10 day and 8-14 day period. Similarly, the weeks 1 and 2 CPC soil moisture forecasts show the largest increases west and north of the drought area. El Niño composites for August – October and soil moisture forecasts for October based on historic El Niño events indicate variable conditions.
Forecast confidence for Wisconsin and Minnesota is moderate.

Forecast improvement for eastern North Dakota is partly based on the wet outlook for the first 5 days of the period, and also on some seasonal indications for improvement, including the University of Washington 3-month runoff and soil moisture probability forecasts based on El Niño. In contrast, the Extended Range Forecasts from CPC look warm and dry, so some resumption of dry conditions may occur after the first week. El Niño composites for August-October show a tendency for above-normal rainfall in eastern North Dakota.
Forecast confidence for North Dakota is low.

Some improvement is shown for the small drought area in Nebraska based on worsening conditions forecast for the first 2 weeks of the period offset by increased odds for relief later in the season, as indicated by the CPC monthly and seasonal precipitation outlooks. To the south, the lingering drought area in western Oklahoma is forecast to experience mostly hot, dry weather during the next 2 weeks. The odds for relief later in the season appear somewhat less than in areas farther to the north, and drought in this area is expected to persist, a more pessimistic assessment than the previous Outlook.
Forecast confidence for Nebraska and Oklahoma is moderate.

In Texas, the lingering small drought area in north Texas is forecast to experience hot, dry conditions during the first 2 weeks. There are no strong seasonal forecast signals in this area from global forecast models or El Niño composites, although the CFS soil moisture output does indicate increases in moisture by October. This Drought Outlook continues the prior forecast for some improvement, but the odds for persistence may be nearly as high. Southern and south-central Texas continues to endure one of their most intense droughts on record. The odds for significant improvement increase considerably starting in November based on composites from historic El Niño episodes and seasonal forecasts from the global models, such as the CFS, but the outlook into October is not as clear cut. Monthly El Niño composites indicate the dry signal for southern Texas ends after September, while the CFS is depicting above-normal rains for September. The weight of the evidence from the various seasonal models, El Niño soil moisture composites, and forecasts from the short term out to the 3-month time periods suggests that the odds for some improvement increase toward the north and the coast. As a result, persistence is shown for the South Texas area, while some improvement is depicted across the northern and eastern parts of the drought area.
Forecast confidence for north Texas is low and for southern and south-central Texas is moderate.

Subtantial rains during the first 2 weeks of the forecast period should virtually eliminate drought in southern Louisiana and southern Mississippi.
Forecast confidence for Louisiana and Mississippi is high.

The latter part of the Southwest monsoon season has been weak, especially in Arizona, where several areas have quickly dried out over the last few weeks and are nearing drought status. With forecasts out to 2 weeks indicating a continuing lack of significant rain, and with September-October typically drier climatologically, this Outlook presents a more pessimistic picture for the Southwest, showing development in Arizona and persistence in New Mexico.
Forecast confidence in Arizona and New Mexico is moderate.

The Outlook continues to indicate persisting drought in California and Nevada. Although the West Coast typically sees a pickup in rainfall during October, little change in drought conditions is anticipated before December, when much larger increases in precipitation typically occur. El Niño has the potential to bring above-normal precipitation this winter, especially over southern California, as indicated in the CPC long-lead forecasts.
Forecast confidence for California and Nevada is high.

Unseasonably heavy rains during the first 5 days of the forecast period should bring relief to the small drought areas in northwestern Montana. To the west, lower rainfall amounts in the short term, along with the potential for autumn dryness due to El Niño, keep the drought area over interior Washington in the persist category. Coastal rains should provide some relief for the recently-developed drought on the Olympic Peninsula in Washington.
Forecast confidence for Montana is high and for Washington is moderate.

Normal to above-normal rains for southeastern Alaska during the first 2 weeks, and the seasonal trend toward lower temperatures and more precipitation thereafter result in a forecast for improving drought.
Forecast confidence for southeast Alaska is high.

The outlook for Hawaii is more positive with this forecast, in part because the track of Hurricane Felicia toward the islands increases the odds for significant rainfall in the August 10-11 time period. Since this Outlook is prepared on August 5, it is not possible to know in any detail what the impacts, if any, will be from the storm, which is expected to be weakening as it approaches the area. Given the range of possibilities over the next few days, not to mention the ensuing 2 ½ months, the outlook shows some improvement for the Big Island and Maui. The prior forecast depicted persistence for most of both islands. Although the odds for heavy rains appear to be greatest for the southern islands, a more northward track by Felicia could bring substantial drought relief to areas farther north. Looking out into autumn, a number of global numerical seasonal forecast models show increased odds for rainfall near the Big Island during August-October, but not so for the islands to the north, thus the outlook for some improvement over the Big Island considers the seasonal time frame as well.
Forecast confidence for Hawaii is low.