September - November 2006

Latest Seasonal Assessment - The first half of August saw a welcomed respite from dry weather across many drought-affected areas of the Great Plains, as well as heavy rains and localized flooding in the Southwest. The Outlook indicates that many of these same areas will see continued drought improvement into autumn, with the best chance for relief extending from the Southwest northeastward across the central Plains. Some improvement is likely in the northern Plains and across the Southeast, with better odds for relief in Florida, the Gulf Coast, and south Texas. Extreme to exceptional drought has persisted over northern Texas and southern Oklahoma and, although some improvement is expected for parts of this area, the drought over northeastern Texas and adjacent portions of Oklahoma and Arkansas may see little improvement. In the Northwest, a tendency toward warm and dry weather through autumn results in the risk for drought development across much of Washington, Oregon, Idaho, western Montana, and northwestern California.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for September-November, 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 Constructed Analogue on Soil moisture.

The general drought outlook calls for improving conditions across large areas of the Great Plains and Southwest, with more limited improvement in the northern Plains, southern Plains, and Southeast. Drought development is a risk for the Northwest. Compared to last month’s Outlook, more improvement is shown for the Southwest and Plains. The area of development in the Northwest is new with this Outlook. Main drivers behind the Outlook include the 2-week soil moisture forecasts and the long-lead September-November precipitation outlook. The drought outlook takes into consideration a 50-50 chance for El Niño development by the end of the year. Here is the thinking behind the Outlook in more detail....

Great Plains… the first half of August saw a break in the extreme heat in the northern and central Plains, and above-normal rainfall from Kansas northward to the Dakotas. To the south, however, drought intensified due to persisting hot, dry weather from southern Oklahoma into southern Texas. The September-November precipitation outlook indicates equal chances for wet or dry over most of the southern Plains, so the short and medium-range outlooks and climatology play a significant role in the drought Outlook. Hot, dry weather early in the forecast period will tend to worsen drought in east Texas and adjacent Oklahoma, while a more normal pattern of temperatures and rainfall are indicated for the 6-10 day period. Climatology, as depicted in the drought probability maps for November, suggest a better chance for improvement closer to the coast, so some improvement is shown for east Texas with persistence farther inland. The large area of improvement shown from the Southwest into Nebraska and South Dakota is driven both by substantial soil moisture improvement expected in the first 2 weeks of the period and, to some extent, a tilt toward wetness for the September-November period from Colorado to North Dakota. The area of some improvement shown extending toward central Montana from North Dakota is driven by heavy showers forecast for the first several days of the Outlook period during and shortly after the August 17 release. These rains may prove to provide only short-term relief, as conditions should turn drier and warmer after the event ends.

The Southwest… abundant rains during the monsoon season, which tend to taper off during September, provided drought relief across the Southwest, especially in New Mexico. Above-normal rains should continue into late August in the Southwest, resulting in additional drought improvement. Although the 3-month outlook shows equal chances for wet or dry in the Southwest, some El Niño composites show a tendency for wetness from southern California into northern Arizona, and this was a factor in extending the area of some improvement into western Arizona.

The Northwest… many short, medium and long-range forecasts point toward a warm and dry pattern for this region. Objective drought index blends already indicate meteorological drought for parts of the region. The area shown as developing drought is governed by current areas of dryness, climatology, and the Constructed Analogues on Soil (CAS) moisture for November. Some thunderstorms are expected to bring localized heavy rains at the start of the Outlook period from Washington and Oregon into Montana, but the impact of the moisture may be fleeting given the dry medium and seasonal range forecasts.

The Mississippi Valley and Southeast… There is much uncertainty for the drought area extending from Missouri into Louisiana, with the seasonal guidance showing a tilt toward dryness from the Great Lakes into northern Arkansas and northeastern Oklahoma, and the last 2 weeks of August showing normal rainfall in the lower Mississippi Valley. Northern Missouri looks wet for at least the first 5 days, but the forecast is more uncertain to the south, where a nearly stationary front may trigger scattered heavy rains during the first week of the period. The drought outlook could have favored persistence or some improvement but, with the 4-month climatological drought probabilities generally favoring improvement, and the 6 to 10 day forecast indicating normal rains and temperatures, the drought outlook sided with some improvement. The monthly and seasonal long-lead outlooks both indicate above-normal rains for Florida, so improvement is shown for the Florida Peninsula, despite recent trends toward drier conditions. The rest of the interior Southeast is uncertain, but climatology and the medium-range forecasts are consistent with at least some improvement. A forecast long-wave trough over the eastern U.S. during the last week of August would support rain for the region. The latest 6-10 day and 8-14 day forecasts, as well as climatology for September-November and some El Niño composites, are consistent with improvement along the Gulf Coast from Mississippi to Florida. No assumptions are made about the impacts of any tropical storms or hurricanes, but such storms could, of course, turn the drought situation around rapidly.

Northeast and mid-Atlantic… High temperatures and lack of rain during July and August rapidly dried topsoils in many areas affected by flooding and excessive rains in June, with an area extending from eastern Kentucky to Virginia and Maryland reporting around one-half of normal rainfall for the 30 days ending August 15. Although this area is a candidate for drought development, a wetter regime toward late August should improve the moisture situation, and probability-weighted El Niño composites indicate a tilt toward wetness for coastal areas from Virginia northward during September-November. As a result, development is not shown at this time for this region.