Tools used in the U.S. Monthly Drought Outlook (MDO) included the updated Climate Prediction Center (CPC) temperature and precipitation outlooks for September, various short- and medium-range forecasts and models such as the 7-day quantitative precipitation forecast (QPF) totals from the Weather Prediction Center (WPC), the 6-10 day and 8-14 day CPC extended-range forecasts (ERFs), Week 3-4 outlooks, CFSv2 monthly forecasts of temperature and precipitation, the 384-hour total precipitation forecasts from several runs of the GFS, climatology for September, and recent precipitation. The August 31 U.S. Drought Monitor (USDM) was used to identify existing drought areas (D1 or drier) in the MDO. For consistency, MDO regional write-ups will be based upon the 7 regions in the U.S. Drought Monitor summary.

During August, temperatures were above-normal across the West, with periods of near-record warm temperatures across the Northwest. The hot weather, in conjunction with persistently below-normal precipitation, caused a continued deterioration of drought and abnormal dryness impacts across Montana, Idaho, Washington, and Oregon. Almost 40 percent of Montana is currently experiencing severe to exceptional drought conditions (D3 or D4), and drought conditions (D1 or worse) cover over 90 percent of the state. Little change to the persistent pattern of heat and dryness is anticipated across the Northwest, particularly during the first half of September. The WPC 7-day QPF depicts little to no precipitation accumulations for the Northwest during Week-1, and the CPC 8-14 day outlook favors below-normal precipitation across Washington, northern Idaho, and Montana. The updated CPC monthly outlook for September depicts a similar pattern of below-normal precipitation, but with lower probabilities due to increased uncertainty later in the month. Based on recent conditions and these outlooks, widespread drought development is anticipated across Washington, Oregon, northern Idaho, and southwestern Montana. In areas where drought is already indicated, persistence or deterioration is expected. Although abnormal dryness (D0) was expanded to the Pacific Northwest coast in the August 31 US Drought Monitor, development to the coast was not indicated in this outlook due to the uncertainty later in the month. Climatologically, any precipitation that falls in late September would be heaviest along the coastal Northwest. Drought persistence is favored for southern California and Arizona, where any late monsoon moisture is unlikely to significantly impact long term drought conditions well ahead of the winter wet season.

Forecast confidence for the Western region is moderate to high.

Widespread areas of precipitation during August promoted some drought relief across much of the central and northern Plains, particularly across North Texas, Oklahoma, Nebraska, the Dakotas, and western Iowa. Hurricane Harvey made landfall over the Texas Gulf coast near Rockport on August 26, and exhibited very slow movement over the next several days while continuing to pull excessive amounts of moisture from the Gulf of Mexico across Texas and Louisiana. Catastrophic and unprecedented flooding is ongoing across these regions, and the small areas of drought and abnormal dryness across eastern Texas have been eliminated. Small pockets of drought remain across far South Texas and along the Rio Grande. During early September, a weak storm system is expected to bring light precipitation across the central and northern Plains and Great Lakes region, followed by a period of below-normal precipitation indicated by the CPC 8-14 day outlook. The updated CPC monthly outlook also indicates increased chances for below-normal precipitation for much of the Plains and upper-Midwest. Due to this tilt towards dryness indicated by the forecast tools, persistence of ongoing drought areas is favored across the High Plains, Southern, and Midwestern climate regions. Low 30-day percent of normal precipitation values indicated on ACIS and AHPS plots reveal areas of northeastern Kansas, northwestern Minnesota, and northeastern North Dakota that are vulnerable to degradation during September. Persistence is also favored across Iowa and the mid-Mississippi Valley, as these regions are west of the current and projected path of Tropical Storm Harvey's remnants. Below-normal temperatures favored during September may serve as a measure of protection from degradation of impacts across the western Corn Belt. Across Texas, equal chances for near-, below-, and above-median precipitation are maintained away from the coast, but tools such as the CFS favor dryness. Therefore, drought persistence is favored near the Rio Grande, while removal is expected for South Texas.

Forecast confidence for the High Plains, Southern, and Midwestern regions is moderate.

Outside of the Tennessee Valley, central Gulf Coast, and mid-Atlantic regions, August was a fairly dry month across the eastern third of the Nation. Other than a pocket of moderate drought (D1) along coastal Maine, no drought conditions are currently indicated for the Southeast and Northeast climate regions, although there are areas of abnormal dryness across New England, parts of the southern Piedmont region, and northeastern Mississippi. Wet weather is forecast early in the period as the remnants of Hurricane Harvey interact with a frontal boundary and move eastward across the Ohio Valley and mid-Atlantic. Accumulations in excess of 1 inch are indicated for much of the southeastern quadrant of the CONUS on the WPC 7-day QPF, with amounts exceeding 8 inches possible over western Tennessee, and 1 to 4 inches possible across much of the mid-Atlantic region. Precipitation is anticipated to increase across New England by the end of the week. The CPC 8-14 day outlook favors a continuation of above-normal precipitation across New England. The updated CPC monthly outlook favors above-normal precipitation across much of the East, although much of this signal is due to the short term forecast. Based on these outlooks, drought development is unlikely for the eastern U.S., and further reduction of drought is favored across eastern Maine.

Forecast confidence for the Northeast and Southeast regions is moderate.

During August, drought conditions expanded across parts of the northwestern Hawaiian islands, and mostly persisted across the Big Island. September is a relatively dry time of year for Hawaii, and persistence of these drought areas is the most favored outcome by the end of the month. There is too much uncertainty at this time to indicate any areas of further drought expansion. A small area of moderate drought (D1) continues to persist across southern Puerto Rico, although recent convection has reduced the eastern extent of this drought area. September is a wet time of year for Puerto Rico, as well as the peak period of Atlantic basin tropical cyclone activity. Therefore, further drought reduction is favored. No drought is currently indicated or expected to develop across Alaska during September.

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

Forecaster: Adam Allgood

Next Monthly Drought Outlook issued: September 30, 2017 at 3pm EDT