September - November 2007

Latest Seasonal Assessment - Forecasts on time scales ranging from 5 days to 3 months call for near- or above-normal precipitation in much of the middle Mississippi Valley, areas near the Gulf and Atlantic Coasts, and the Great Lakes region. As a result, drought improvement is expected by the end of November in these areas. It should be noted, however, that large long-term precipitation deficits exist in parts of the Deep South and the northern Great Lakes region, so the complete elimination of drought conditions seems highly unlikely in these regions. Across the lower Ohio Valley and interior South, indicators are less robust with precipitation totals for the forecast period, but climatological declines in water demand, temperatures, and evaporation rates indicate that some limited improvement is likely for these areas. However, the Drought Monitor continues to indicate exceptional drought for a large part of this region, thus drought elimination or substantial drought improvement seems unlikely. Farther west, above-normal precipitation in the short-term and the enhanced probability of above-normal September-November precipitation shown in the Climate Prediction Center's outlook led to forecasts of improvement in west-central South Dakota, with more limited improvement expected across the rest of western South Dakota, eastern Wyoming, and eastern Montana. Meanwhile, climatology and short-term forecasts argue for improvement from western Montana westward into eastern Washington and northeastern Oregon, but as with some areas farther south and east, entrenched drought conditions make it unlikely that dryness will be completely eliminated, particularly from a hydrologic perspective. Climatology also indicates that some limited improvement can be expected in a strip from north-central Montana southwestward through northern California as this region enters its wetter time of year. Unfortunately, to the south and east, in a large area stretching from interior Montana southward and westward through the central and southern Rockies, Great Basin, most of California, and the desert Southwest, most forecasts on all time scales call for enhanced chances for below-normal precipitation as the monsoon season winds down and the remnants of Hurricane Henriette move primarily east of the drought region. As a result, drought conditions are expected to persist or intensify across this large area. Finally, some drought improvement is expected across affected areas of Hawaii as the dry season winds down.

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, the soil moisture tools based on the GFS model and the Constructed Analogue on Soil moisture, and the CFS monthly precipitation forecasts.

Beyond the next 2 weeks, most forecast tools are rather non-committal through most of the country, so a great reliance was placed on models and official forecasts for the next 2 weeks, and on the official seasonal outlooks from the Climate Prediction Center. Forecast improvement near the Gulf and Atlantic Coasts was based primarily on enhanced chances for above-normal precipitation during the September - November period, with confidence in improvement somewhat enhanced from the Carolinas northward where many models and the official forecasts indicate that a subtropical or tropical system should move along or near enough to the Eastern Seaboard to bring above-normal precipitation through the next 5 days, after which time a frontal system is expected to become pseudo-stationary across this region, leading to an official forecast for above-normal precipitation during the 6 to 10 day time frame. However, it should be noted that 'improvement' doesn't equate to 'elimination' of drought conditions, particularly in parts of southern Alabama and Georgia where significant long-term precipitation deficits are entrenched.

Farther west, outlooks for the next 2 weeks indicate that above-normal rainfall can be expected along the middle Mississippi Valley and across the Great Lakes region, leading to a forecast for improvement. Longer-term outlooks for these regions are neutral, but climatological considerations argue for some continued, limited improvement for the Great Lakes region as autumn progresses. In the drought areas of Arkansas, Missouri, and Illinois, the remnants of Hurricane Henriette may interact with a pseudo-stationary frontal boundary expected to set up across the region, potentially leading to heavy rainfall over the course of the next week or so, enhancing confidence for improvement in this region despite relatively neutral long-term model output and forecasts.

From the Ohio Valley southward across the interior South, precipitation forecasts for the next 2 weeks are less robust than in surrounding areas, and longer-term tools and forecasts are relatively neutral. However, seasonal declines in temperature, water demand, and evaporation led to a forecast for limited improvement across this region. Much of this area is currently experiencing extreme to exceptional drought according to the Drought Monitor, so drought elimination, or even substantial drought improvement, seems unlikely.

It should be noted that La Niña composites would argue for a more pessimistic forecast across much of the Ohio Valley, Southeast, and western Midwest during the September - November time frame. However, given the uncertainty regarding the timing and intensity of La Niña conditions this autumn and/or winter, along with the relative unreliability of La Niña composites in this region during autumn, very little weight was placed on the La Niña composites in the making of this Drought Outlook. But it should be noted that the possibility of a more pessimistic outcome than forecast exists should La Niña conditions increase robustly during the next month or so.

Farther west, short- and long-term forecasts led to forecasts for improvement in west-central South Dakota, and limited improvement to the west of this region. Farther west, seasonal precipitation increases and short-term forecast considerations led to an outlook for improvement from eastern Washington and northeastern Oregon eastward through western Montana. A strip of limited improvement to the south and west of this area was driven primarily by climatological considerations even though part of this region is forecast to receive below-normal precipitation for at least part of the next two weeks. It should again be noted that extreme drought currently prevails in parts of Idaho and adjacent Montana, and improvement in this region does not imply a forecast of drought elimination.

The rest of the drought areas in the West, which encompasses a large area from north-central Montana southward and westward through the Rockies, Great Basin, desert Southwest, and California are expected to persist or intensify. The forecast for the northern Rockies was driven primarily by expected conditions for the next 2 weeks, and climatological considerations thereafter. Farther south, most or all official forecasts for time scales ranging from 6 to 10 days out to 3 months call for below-normal precipitation, making a forecast for drought persistence or intensification relatively obvious.

Drought should show limited improvement along the leeward areas of the Hawaiian Islands as the dry season winds down.