October - December 2007

Latest Seasonal Assessment - With the September intensification and expected persistence of La Niña conditions through the end of the calendar year, the current Drought Outlook leaned more heavily on precipitation anomalies that typically occur during La Niña episodes than have the past few Drought Outlooks. In addition, the latest official forecasts through early October, the official October and October-December outlooks from the Climate Prediction Center, climatological considerations, and how firmly entrenched current drought conditions are were all considered. For the large area of drought covering much of the southeastern one-quarter of the country and parts of the Ohio and middle Mississippi Valleys, some limited improvement is expected by the end of December, with a few exceptions. Drought areas along the central Gulf of Mexico Coast and the Florida Gulf Coast should experience more robust improvement, particularly if rain from the semi-tropical storm moving into the Gulf of Mexico falls as expected during September 19-24. Furthermore, near- to above-normal rainfall is expected in these areas during late September and early October, and for the month of October as a whole. Across interior southern Florida, however, the September 19-24 precipitation forecast calls for less rain than along the Florida Gulf Coast, and the odds for wetness during October are also lower, deviating only slightly from climatology. Further considering the exceptionally low levels on Lake Okeechobee and the long-term nature of the dryness in the drainage basin that replenishes it, in addition to the fact that the region is entering its drier time of year, only limited improvement is forecast for this region. Farther north, short- and medium-range forecasts as of September 19 favor below-normal precipitation in much of Maryland, Virginia, and northern North Carolina, and with the region typically experiencing autumn dryness during La Niña episodes, drought in this region is expected to persist or intensify. It should be noted that substantial long-term drought is firmly entrenched across a large part of the South, and with the region entering its drier time of the year, the forecast for some improvement does not imply an end to the current drought. Farther north, above-normal precipitation is forecast through early October, and in some areas through the end of October, which in conjunction with seasonably declining temperatures should allow drought conditions to improve by the end of December. In the High Plains and Western states, forecasts call for relatively robust precipitation through early October across eastern Montana, and above-normal precipitation is favored for all time periods across the rest of Montana, Idaho, western Wyoming, eastern Oregon, northwestern Nevada, and northern California. Thus, drought conditions are expected to improve throughout these areas, although complete drought elimination is highly unlikely in the drier parts of this region. Meanwhile, an area of modest improvement is forecast in a swath from central California eastward throuh northwestern Colorado and the dry areas in the northern High Plains. To the south of this region, tropical moisture could move into parts of southern California during September 19-24, potentially bringing this region its heaviest rain in many months, but given the long-term, firmly-entrenched nature of drought conditions, and the fact that some longer-term indicators favor drier than normal conditions through autumn and at least early winter, drought conditions are forecast to persist across southern sections of California, Nevada, and Utah and throughout the drought areas in Arizona. Finally, drought improvement is expected across affected areas of Hawaii as the dry season winds down, and La Niña conditions favor wetness across the islands late in the forecast period.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for October - December, 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.

With the recent intensification of below-normal sea surface temperatures across the central and eastern equatorial Pacific Ocean, and the expectation that these La Niña conditions should persist through the end of the calendar year, considerably more emphasis was placed on historic La Niña composites (both raw and adjusted toward recent decadal trends) than for the last few issuances of the Drought Outlook. Still, a great reliance was also placed on the official 5-day, 6- to 10-day, 8- to 14-day, October, and October-December outlooks, adjusted for considerations related to how typically wet or dry this season is compared to other times of the year, how firmly entrenched drought conditions are, the seasonal decline in water loss due to declining temperatures, decreased evapotranspirative losses, and reduced public water usage.

In the drought area extending from the middle Mississippi Valley eastward and southward through the Gulf of Mexico, mid-Atlantic, and southern Atlantic coasts, the consensus of sometimes-conflicting indicators pointed toward shallow improvement for most of the region, with a few exceptions. In these areas where some improvement is expected (away from the Gulf Coast and mid-Atlantic regions), forecasts through September 24 call for little or no precipitation while the medium range time periods (through early October) weakly favored above-normal precipitation in western areas, and below-normal precipitation farther east. A few long-range dynamic models called for generally below-normal precipitation through the end of the calendar year, but only modestly so, and La Niña composites were neutral for most of the region, except along the Carolinas coastline, where they weakly indicated dryness. In the final assessment, the areas where the medium-range outlooks favored wetness were areas where La Niña composites weakly favored dryness, and across eastern parts of this region where most time scales favored dryness, the odds were only slightly shifted from equal chances. In conjunction with the facts that the semi-tropical system centered over southern Florida on September 19 was forecast to bring a heavy rain band into the Carolinas coastline during September 19-24, and that the official monthly and seasonal forecasts didn't lean one way or the other with respect to precipitation anomalies, a forecast of some improvement, aided by seasonal evapotranspirative and water demand declines, seemed most prudent.

Along the central Gulf of Mexico Coast and the Gulf Coast of Florida, official forecasts from September 19 through the end of October were more robust with above-normal precipitation probabilities, and these regions appear as though they may receive significant rainfall from the aforementioned semi-tropical system through September 24. As a result, drought improvement was forecast despite La Niña composites that weakly favor below-normal precipitation for the October-December period. However, across interior southern Florida, the short-term precipitation forecast called for less rain than in areas farther west, and the official outlooks through October were not quite as robust with above-normal precipitation probabilities. Furthermore, the water level in Lake Okeechobee has dropped to exceptionally low levels for late in the wet season, and dryness dates back to autumn 2006 near Lake Okeechobee and the areas to the north that drain into the lake, which is longer than for any place else in the current Southeast drought region. Given the additional fact that the region will be transitioning into its climatologically drier time of year, a forecast for only some improvement, similar to most areas across the interior Southeast, seemed most logical.

In contrast, subnormal precipitation is favored through early October in the mid-Atlantic region, which coincides with the region where trend-adjusted La Niña composites more definitively favor dryness for the October-December period. Therefore, despite official October and October-December forecasts that favor neither dryness nor wetness, a forecast of drought persistence or intensification was made for this region based on the consensus of short-term dynamic and long-term statistical indicators.

Improvement is forecast for the drought areas extending from western Minnesota eastward through the northern Great Lakes region and northwestern New York, primarily driven by forecasts for above-normal precipitation through early October from Michigan westward, the official outlook favoring above-normal October precipitation in part of upstate New York, trend-adjusted La Niña composites leaning toward above-normal October-December precipitation, and considerations regarding a typically steep decline in temperature and evapotranspirative moisture losses as the season progresses. In addition, lake-effect snows normally kick in during November and/or December as cold intrusions from Canada push over the Great Lakes which are still relatively mild at that time of year.

Farther west, official forecasts are relatively robust in depicting probabilities for above-normal precipitation through October 3 in central and eastern Montana, and official forecasts for all time scales ranging from 5 days to 3 months out point toward above-normal precipitation across the rest of Montana, Idaho, eastern Oregon, western Wyoming, far northern sections of Utah and eastern Nevada, northwestern Nevada, and northern California. Thus, drought improvement is expected across this large area, bolstered by La Niña composites favoring wetness in much of this region during October-December.

To the southeast of this region, drought improvement should be more modest across central California, central and northeastern Nevada, the northern half of Utah, southern and eastern Wyoming, and the drought areas in the northwestern Great Plains. Short- and medium-range forecasts favoring above-normal precipitation were less robust in areas east of the Sierra Nevada eastward, and official long-term forecasts were non-committal, favoring neither drier nor wetter than normal conditions. For the parts of this region from the Sierra Nevada westward, official forecasts on most time scales favored above-normal precipitation, albeit weakly, but drought conditions here are deeply entrenched and typical seasonal precipitation totals are lower than for areas farther north, thus only modest improvement was forecast.

Across the drought areas in Arizona and southern sections of Utah, Nevada, and California, drought is forecast to persist. Although an influx of tropical moisture may bring some of the heaviest rains in many months to parts of upper southern California by September 24, official forecasts for late September through the end of December favor below-normal precipitation for most of this region, as do La Niña composites for the October-December period. Considering additionally the entrenched, long-term nature of the drought in this area, any notable improvement seemed unlikely by the end of December.

Finally, with Hawaii progressing into its wetter time of the year, and with La Niña composites favoring above-normal precipitation toward the end of the year, improvement was forecast for drought-affected areas across the island chain.