Tools used in the U.S. Seasonal Drought Outlook (SDO) included the Climate Prediction Center (CPC) temperature and precipitation outlooks for September and September through November 2021 (SON), various short- and medium-range forecasts and models such as the 7-day quantitative precipitation forecast (QPF) from the Weather Prediction Center (WPC), the 6-10 day and 8-14 day CPC extended-range forecasts (ERFs), the Weeks 3-4 outlooks and related tools from CPC, dynamical models at the monthly and seasonal time scales, the 384-hour total precipitation forecasts from several runs of the GFS, climatology for the SON season, and initial conditions such as soil moisture. The USDM valid on August 17 was used for initial drought conditions. 

Drought remains firmly entrenched across the western conterminous U.S., with nearly the entire Western Region experiencing drought conditions (D1 or worse) according to the U.S. Drought Monitor, valid on August 17, 2021. Across much of the West Coast, the northern Great Basin, and Northern Rockies, record-setting heat waves in recent weeks to months, coupled with much below-normal precipitation, have resulted in drought expansion and deterioration. Despite the expansion of drought conditions during the Summer months, La Nina is favored to redevelop during the Fall, which typically leads to cooler-than-normal conditions across the Pacific Northwest and Northern Rockies, and acts to increase odds for above-normal precipitation across these same areas, particularly toward the end of the period. Coupled with the fact that November is the climatological start to the rainy season in the Pacific Northwest, drought improvement and removal is favored along the windward slopes of the Cascades, with persistence east of the Cascades.

Farther south over the Four Corners, however, a robust North American Monsoon season has aided in slow drought recovery since July, particularly over parts of Arizona and New Mexico. With the climatological end to the North American Monsoon in September and the favored redevelopment of La Nina conditions in the tropical Pacific Ocean during the Fall season, which typically favors warm and dry conditions over the Southwest, any additional improvements in the Southwest would most-likely be very early in the SON season, with warm and dry conditions setting in for the remainder of the season (October-November). In addition, the U.S. Drought Monitor (as of July 27, 2021) depicts much of Arizona and New Mexico with longer-term (hydrologic) impacts, indicative of the longer-term dryness this region has experienced going back to Summer 2020. Therefore, persistence is forecast across the Desert Southwest and Four Corners, due to the early end (with respect to the SON season) of the monsoon season, coupled with the potential return of La Nina conditions in the Pacific.

Forecast confidence is high for the Four Corners and Great Basin, and moderate elsewhere in the West.

Drought is entrenched across northern and western sections of the High Plains Region (i.e. the Dakotas, Wyoming, and western Colorado). In the Central Plains (eastern Colorado, Nebraska, and Kansas), a wet Spring and active North American Monsoon season has resulted in large improvements, and even drought removal, earlier in 2021. However, drier-than-normal conditions over the past 30 to 90 days have resulted in drought redevelopment and deterioration, particularly across Nebraska and parts of Kansas. As the High Plains Region transitions from a wetter to drier time of year during the SON season, chances for drought improvement will become greatly diminished. Despite an increased potential for heavy precipitation across the Dakotas over the next 7 days (per WPC's 1-7 day QPF) and a lack of temperature and precipitation signals (i.e. equal chances for above and below-normal) during September, warmer and drier conditions are favored for the SON season as a whole across the High Plains. As such, drought persistence is favored for much of the region, with the potential for drought development in the Central Plains.

Forecast confidence is moderate for the High Plains Region.

Since the August-October SDO release, the Southern Region has benefited from a mean frontal boundary that has been draped over the region for the better part of August, causing an active precipitation regime. However, several spots have missed out on the heavier precipitation amounts, particularly across the Tennessee Valley, resulting in some abnormal dryness in addition to deterioration to D1 (moderate drought) conditions in the Smoky Mountains. Meanwhile, in far western Texas, near the Big Bend and Trans-Pecos regions, an active North American Monsoon has helped to nearly eliminate drought altogether from Texas. With above-normal precipitation over the next 7 days and improved precipitation deficits in recent weeks, drought removal is predicted near the Big Bend of Texas. Drought removal is likely in the Tennessee Valley, with equal chances for above-, near-, or below-normal temperatures and above-, near-, or below-normal precipitation during September, despite a likely return to above-normal temperatures later in the Fall. In addition to the equal chances for above-, near-,  or below-normal precipitation, the potential threat for tropical activity also looms, as the climatological peak to the Atlantic Hurricane season will occur toward the beginning part of the SON season (early-to-middle September).

Forecast confidence high for the Southern Region.

Much of the Great Lakes have experienced marked improvements to, and removal of, drought conditions during the past couple of months. The lack of a temperature signal and favored above-normal precipitation across the Great Lakes during September favor drought improvement (D2) and removal (D1) across southern Wisconsin, northern Illinois, and eastern and northern (Upper Peninsula) Michigan. Farther west in Iowa and Minnesota, temperature and precipitation signals are either lacking or lean toward above-normal temperatures and/or below-normal precipitation, indicating drought persistence is likely, despite the increased potential for heavy precipitation across parts of the Upper Midwest over the next 7 days.

Forecast confidence for the Midwest Region is low-to-moderate.

The only drought in the Southeast Region resides along the Blue Ridge Mountains of Virginia and West Virginia, with some locations experiencing 6-12" precipitation deficits since January 1, 2021. Last week's passage of the remnants of Tropical Storm Fred has brought temporary relief, but more will be needed to completely alleviate conditions. However, an active precipitation pattern is favored through much of September, with equal chances of above-, near-, or below-normal for the remainder of the SON season. Additionally, the typical reduction in evapotranspiration rates during the Fall and the threat of potential tropical activity associated with the climatological peak of the Atlantic Hurricane season, removal of drought is favored for areas experiencing D1 (moderate drought) in Virginia.

Forecast confidence is high for the Southeastern Region.

In the Northeast Region, drought is restricted to Cape Cod interior, northern parts of New England, and extreme northeastern New York, with some locations in northern New England experiencing nearly 20" water-year-to-date precipitation deficits (per AHPS) going back to October 1, 2020. Despite favored above-normal temperatures throughout the SON season, equal chances for above-, near-, or below-normal to above-normal precipitation is favored for the Northeast as a whole, with above-normal chances more likely during September. Additionally, with the increased potential during the Fall for coastal lows and/or transitioning tropical cyclones, removal of moderate drought (D1) on Cape Cod is favored, with persistence elsewhere across the Northeast.

Forecast confidence is moderate-to-high for the Northeastern Region.

With improved conditions over the past couple of weeks and near to below-normal temperatures and near to above-normal precipitation favored in the extended range (through Week-2), drought removal is likely in Alaska before the Winter season takes hold. In Hawaii, drought conditions exist mainly on the leeward slopes. Near-normal temperatures are favored during the SON season for Hawaii and, in spite of increased potential for below-normal precipitation, drought persistence is likely, with no additional development. The recent passages of Tropical Storms Fred and Grace has led to improvements in the drought depiction in recent weeks. With the continued threat of an active Atlantic Hurricane season, drought removal is favored for the remaining D1 (moderate drought) areas in Puerto Rico.

Forecast confidence is high for Alaska and Puerto Rico, and moderate in Hawaii.

Forecasters: Adam Allgood, Adam Hartman, and Brad Pugh
 
Next Seasonal Drought Outlook issued: September 16, 2021 at 8:30am EDT