U.S. Seasonal Drought Outlook for Sept. 6 to Nov. 30 (Text)
Latest Seasonal Assessment - Drought continues to cover a large portion of the country in early September, but beneficial rains finally brought improvement to a significant part of this area. Heavy, beneficial rains, primarily from the remnants of Hurricane Isaac, fell on much of the Mississippi and lower Ohio Valleys during the last 2 weeks. Parts of Arkansas, Missouri, eastern Kansas, and Illinois recorded over 5 inches of rain, and amounts reached as high as 10 inches in isolated locations. Elsewhere, moderate to heavy rains also fell on portions of the mid-Atlantic, and isolated sites in Arizona received 2 or more inches of rain. The seasonal decline in temperatures over the next 3 months will substantially reduce surface water lost to evaporation and vegetative growth. Furthermore, precipitation tends to fall at a more moderate rate over a longer period of time, which recharges soil moisture efficiently. Snowfall similarly holds moisture that seeps slowly into the soil as it melts. These factors, along with anticipated precipitation patterns at least partially driven by the developing El Nino episode, should bring some relief to drought-affected areas in the East, the Mississippi Valley, the Midwest, and the northern Great Plains. The odds also favor some degree of improvement across a large swath of the Southwest and across southern Texas. Still, there remains a large area covering the central and southern Plains, the central and northern Rockies, the central Intermountain West, and much of California where drought conditions are expected to persist. Most of these areas are moving toward a climatologically drier time of year, and there is no compelling indication that substantially above-normal precipitation will fall during the next 3 months.
Tools used in the U.S. Drought Outlook (USDO) included the official CPC temperature and precipitation outlooks for September 2012 and the long lead forecast for September through November 2012, various medium- and short-range forecasts and models such as the 6-10 day and 8-14 day forecasts, the most recent 384-hour GFS total precipitation amounts, the soil moisture tools based on the Constructed Analog on Soil (CAS) moisture, the Climate Forecast System (CFS, versions 1 and 2), the four-month Palmer drought termination and amelioration probabilities, climatology, and initial conditions. In addition, El Nino conditions are expected to evolve during this time period, so precipitation patterns typically observed during such events were also considered.
Improvement is expected in most drought-affected areas in the East. Climatologically, temperatures progressively decrease during September - November, reducing surface water lost to evaporation and vegetative growth. In addition, when precipitation occurs in autumn, it tends to fall more moderately for longer periods of time, at least in central and northern parts of the region. This more efficiently recharges soil moisture than brief, heavy thunderstorms. For these reasons, drought conditions are expected to ease by the end of November. Relief is expected to be more limited in and near central Georgia, where autumn is climatologically drier than other times of year. The seasonal precipitation forecast favors enhanced precipitation for autumn as a whole in western Georgia, but this is not the case farther east. Forecast confidence for the East is high.
From the High Plains eastward through the Ohio Valley, prospects for improvement generally increase toward eastern and northern sections of the region, although some improvement is also expected in southern Texas. Along the northern tier, there is no compelling reason to expect significantly above-normal precipitation during the period, but the climatological decline in temperatures, as well as increasing chances of snowfall, means that what precipitation does fall should efficiently improve moisture deficits, easing impacts. Only limited improvement is expected in the western Great Lakes region, where some indicators and El Nino considerations favor abnormally low soil moisture at the end of November. Farther east, along the Mississippi and Ohio Valleys, many locations should receive moderate to heavy precipitation during the first five days of the period. There are also some indicators that imply above- normal precipitation for the period as a whole, though not strongly enough to show up in the seasonal forecast except in the lower Mississippi Valley. In contrast, in areas farther west - across the High Plains and the central and southern reaches of the Great Plains - no drought relief is forecast. Climatologically, this region progresses toward their drier time of year during autumn, reducing the prospects for heavy precipitation and resultant drought relief. Also, some dynamic models lean toward near- to below-normal precipitation during this already dry time of year, further reducing chances for relief. Forecast confidence from the Plains eastward through the Ohio Valley is moderate.
Across the large region of drought from the Rockies westward, improvement is expected in the Southwest, extending north and east toward the central Rockies. Official forecasts favor above- normal precipitation in the middle to later parts of September, as well as for the month as a whole. The September - November outlook also shows enhanced chances for above-normal precipitation in central and southern parts of this region. However, no relief is in sight for the other drought-affected areas from the Rockies westward. Normal precipitation declines as autumn progresses in the Rockies, reducing prospects for drought-relieving precipitation. Farther west, this is neither a dry nor wet time of year, but it seems likely that near- to below-normal precipitation will fall during the rest of September, and considering the range of precipitation values observed historically during October and November, it is unlikely that enough precipitation will fall to bring noticeable drought improvement. Forecast confidence from the Rockies westward is high in the Southwest and moderate elsewhere.
In Hawaii, autumn progresses towards the wetter time of year. However, the long-lead forecast indicates enhanced chances for below-normal rainfall for autumn as a whole. As a result, drought is expected to persist in existing areas and slowly expand into other parts of the state, much like the previous Drought Outlook. Still, the fact that the Islands are getting climatologically wetter during the period does imply that the chances for drought-relieving rains improve later in the period, even if only based on climatology. Forecast confidence for Hawaii is low.
SOURCE: National Weather Service
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