Tools used in the U.S. Seasonal Drought Outlook (SDO) included the Climate Prediction Center (CPC) temperature and precipitation outlooks for September through November 2018 (SON), 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), the Week 3-4 outlooks and tools from CPC, dynamical models at the monthly and seasonal time scales, the 384-hour total precipitation forecasts from several runs of the GFS and the Canadian Model, climatology for the SON season, and initial conditions. The tropical base state is currently transitioning towards El Nino conditions, with increasing SSTs and a breakdown of the Pacific trade winds regime. Substantial midlatitude atmospheric impacts from ENSO, however, are not anticipated to play a substantial role in the overall pattern until late in the outlook period. Tropical impacts may play more of a role for Hawaii and Puerto Rico.

Long lasting hot and dry conditions under a persistent ridge of high pressure has resulted in substantial expansion of drought impacts across the Northwest, with reports of poor livestock performance and frequent wildfire activity. Severe drought (D2) now covers most of Oregon, and these conditions have expanded rapidly to the Pacific coast. Air quality conditions across the region are unhealthy due to large amounts of smoke from ongoing wildfires. The prospects for substantial relief of the ongoing and worsening drought conditions are low, with little accumulated precipitation forecast over the next 7-days by WPC, and enhanced chances for below-normal precipitation and above-normal temperatures across CPC guidance at all time scales (days 8-14, monthly, and seasonal). Accordingly, persistence is favored for the SON drought outlook, with drought development favored for the remainder of Washington and Oregon, where the probabilities for below-normal precipitation are the highest throughout the Autumn months.

Forecast confidence for the Pacific Northwest through the Northern Rockies is high.

Seasonably dry conditions have continued across California, although persistently hot conditions have promoted record wildfire coverage. East of the Sierra Nevadas, monsoon precipitation eased drought conditions across Arizona and New Mexico, but drought impacts continued to expand across northern Utah and the higher elevations of Colorado. During the next 7 days, locally heavy monsoon precipitation is forecast across southeastern Arizona, much of New Mexico, and the eastern half of Colorado, with moisture feeding into a low-pressure system over the Plains and Midwest. This additional rainfall will help to continue chipping away at the long term drought conditions across the Southwest. A slight tilt towards above-normal precipitation is forecast by CPC for these areas during the 8-14 day period. Additionally, both the monthly and seasonal outlooks favor above-normal monsoon precipitation, which is consistent with dynamical model guidance and a base state favoring enhanced East Pacific tropical cyclone activity. Based on these outlooks, continued drought reduction is forecast for extreme southeastern California, all of Arizona and New Mexico, the Texas Big Bend Country, and southern Utah and Colorado. Prospects for drought improvement wane across the remainder of California, where the bulk of the wet season falls largely beyond the forecast period, and across northern Utah and Colorado, where the advancement of substantial monsoonal moisture is more uncertain.

Forecast confidence for California, the Great Basin, Desert Southwest, and the Central and Southern Rockies is moderate.

Only small areas of drought remain across the Northern Plains and Upper Mississippi Valley. Periods of summer MCS activity eased drought conditions across eastern South Dakota, but drought and abnormal dryness has slowly expanded across northern Montana, North Dakota, and Minnesota, and these areas are primed for further impact degradations in the absence of substantial precipitation. Over the next 7-days, widespread light to moderate (an inch or less) precipitation is forecast across North Dakota and Minnesota. The CPC 8-14 day forecast favors near-normal precipitation, while the monthly and seasonal outlooks maintain equal chances for near-, below-, or above-normal precipitation. While the Autumn months present an opportunity for substantial soil moisture recharge as evapotranspiration rates decline, the absence of a clear wet signal that could ease drought conditions ahead of the climatologically drier cold season favors persistence of the small existing drought areas. Furthermore, given the recent increase in drought impacts over the past few weeks, further drought expansion into areas already abnormally dry across northern North Dakota and Minnesota is possible.

Forecast confidence for the Northern Plains and Upper Mississippi Valley is low to moderate.

Drought conditions deteriorated rapidly across two distinct regions in the south-central U.S.: central and southern Texas, and the mid-Mississippi Valley due to a lack of precipitation. Between these regions, periods of heavy rainfall eased drought across North Texas and the central High Plains. Recently, a slow-moving low-pressure system has been a focus for heavy rainfall extending from central Texas through Kansas. This pattern is expected to continue over the next 7 days, with WPC QPF forecasts indicating a large region of heavy rainfall extending from North Texas through South Dakota and eastward across the Mississippi Valley. The heaviest amounts are anticipated across southern Missouri and northern Arkansas, where accumulations could reach as much as 5 to 7 inches. This precipitation will help ease drought conditions in the short term, and areas receiving multiple inches of rain could see a multiple-category drought reduction. Forecast precipitation tapers off across central and southern Texas. The CPC 8-14 day outlook favors a continuation of wet weather from northern Texas and Oklahoma through the entire Mississippi River Valley. The CPC monthly and seasonal outlook maintain equal chances for near-, below-, and above-normal precipitation, except along the western Gulf Coast, where the monthly outlook favors below-normal precipitation due to a tropical base state unfavorable for Atlantic tropical cyclone activity and a mid-level pattern favoring storm tracks recurving well east of the western Gulf of Mexico. Climatological precipitation is high across the Mississippi Valley during SON, and prospects for continued soil moisture recharge increase as the sun angle decreases and crops reach maturity. Based on the very wet short term forecasts and climatology, drought improvement is favored across North Texas, Oklahoma, Kansas, and the Middle Mississippi Valley. Drought persistence is maintained across Texas, where climatological precipitation is less and short term forecasts indicate lower precipitation amounts. Drought development was considered for parts of the western Gulf Coast, but ultimately not included in this outlook. The driest signal appears to be over the lower Rio Grande Valley, where 120-day percent of normal precipitation values are quite high.

Forecast confidence for the Central and Southern Plains and Mississippi Valley is moderate.

Dry conditions during the Summer led to a gradual expansion of drought conditions across parts of the Great Lakes and Northeast. During the past several weeks, a persistent plume of tropical moisture resulted in heavy rainfall across much of the Eastern Seaboard, including record accumulations over parts of the mid-Atlantic states that led to areas of flooding. This rainfall eased drought conditions across southeastern New England, but drought persisted and expanded across Michigan, northern Ohio, northern New York, and northern Vermont. During the next several days, ample moisture feeding along a stalled frontal boundary is anticipated to bring widespread heavy rainfall (1 to 3 inches) across the eastern Great Lakes region through northern New England. This rainfall is anticipated to ease drought conditions, particularly across southern Michigan and New England. The CPC 8-14 day outlook maintains a similar pattern, with enhanced chances for above-normal precipitation across the Great Lakes and Northeast. The monthly and seasonal outlooks both maintain equal chances for near-, below-, and above-normal precipitation, and climatology favors soil moisture recharge during the Autumn months. Therefore, drought improvement is favored from southern Michigan through northern New England. Persistence is maintained across the northern lower-peninsula of Michigan, where short-term forecasts indicate more modest rainfall accumulations.

Forecast confidence for the Great Lakes and Northeast is moderate.

Moderate drought developed across Alaska's southern Panhandle region. Monthly and seasonal outlooks favor above-normal temperatures and equal chances for near-, below-, and above-normal precipitation. Due to the lack of a clear wet signal in the guidance, drought persistence is favored. Drought conditions have expanded across Hawaii over the past 30 days. Dynamical guidance indicates an enhanced and northward displaced Pacific ITCZ which may bring periods of above-normal precipitation to Hawaii, particularly the Big Island. The primary wet season, however, begins beyond the SON outlook period. Therefore, no additional drought development is indicated in this outlook, but substantial drought reduction is not anticipated. A base state transitioning towards El Nino conditions and reduced Atlantic SSTs both favor large-scale suppressed convection across the Caribbean. Abnormal dryness has been gradually expanding across Puerto Rico, and drought development over large portions of the island is favored in this outlook.

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

Forecaster: Adam Allgood

Next Seasonal Drought Outlook issued: September 20, 2018 at 8:30 AM EDT