Tools used in the U.S. Monthly Drought Outlook (MDO) included the 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 and 8-14 day CPC extended-range forecasts (ERFs), the Week 3-4 outlook from CPC, dynamical models at the monthly time scale, the 384-hour total precipitation forecasts from several runs of the GFS, climatology, and initial drought conditions (August 28, 2018 U.S. Drought Monitor).
Following a slow expansion of drought conditions during the Summer, rainfall during mid-August brought some relief to parts of northern New York, New Hampshire, and southern Maine, while drought and abnormal dryness continued to linger across northern Vermont and parts of Maine. 14-day average streamflow conditions continue to reflect a sharp gradient between much-above-normal conditions across the mid-Atlantic, southern New York and southern New England, and below-average streamflows in northern New York and Maine. During September, evapotranspiration rates lessen, particularly over northern New England and higher elevations, as vegetative demands and the sun-angle decrease. This generally increases the potential for soil moisture recharge and limits substantial drought development. Forecast guidance, however, strongly favors above-normal temperatures throughout September, with periods of much above-normal temperatures possible. This late season heat will counteract the climatological reduction in moisture loss from the soil, and any periods of dryness during the month may exacerbate drought impacts. QPF forecasts out to 7-days from WPC show generally light accumulations, with a shift towards enhanced chances for above-normal precipitation during Week-2. The CPC Week 3-4 guidance indicates enhanced chances for below-normal precipitation, and the monthly outlook encompassing all of these time scales shows equal chances for below-, near-, and above-normal precipitation. Based on the lack of a strong dry signal in the guidance, there is insufficient confidence to depict any areas of drought development, but the possibility for late season heat reduces the potential for substantial improvements.
Forecast confidence for the Northeast is low.
Along the Gulf Coast, only small areas of moderate drought remain in Texas and Louisiana. After earlier indicators pointed to suppressed convective and tropical cyclone activity, during late August, conditions across the Atlantic basin have become more favorable for development. A tropical wave currently nearing the Bahamas is forecast to reach the Gulf of Mexico during early September. There is considerable uncertainty regarding whether this disturbance will become a tropical cyclone, but regardless of development, there is a potential for widespread heavy rainfall across the Florida peninsula and along much of the Gulf Coast. Due to this anticipated early-month event, removal of drought along the Gulf coast and lower Mississippi Valley is favored.
Forecast for the Texas Gulf Coast and Lower Mississippi Valley is high.
Over the past several days, multiple MCS complexes brought widespread heavy rainfall to eastern Kansas, northern Oklahoma, and Missouri. As this near term moisture works its way into the soils and streams, drought reduction is favored, similar to the improvements that occurred earlier in the month across Arkansas and southeastern Oklahoma. During the first week of September, widespread heavy rainfall is forecast from northern Kansas and Missouri through the Great Lakes region. The QPF from this single week would result in near to above-normal precipitation accumulations for the entire month of September in many locations. Additionally, a generally wet pattern across the Great Plains, Midwest, and Great Lakes is favored to continue in the Day 8-14 period. Based on recent conditions and the wet outlooks for the first two weeks of September (when climatological temperatures are the highest as well), drought reduction is favored for eastern Kansas, Missouri, and the Great Lakes.
Forecast confidence for the Central Great Plains northeastward to the upper Midwest and Great Lakes is high.
During August, precipitation was generally near to below average across New Mexico and Colorado, although spotty monsoonal thunderstorms brought localized relief to ongoing drought areas. Precipitation areas were also hit and miss in Texas, where some parts of central Texas received substantial rainfall while other locations, particularly far western Texas, were drier. During the next 7 days, widespread rainfall is forecast across New Mexico, Colorado and Texas, with the heaviest amounts falling across eastern New Mexico, western and northern Texas, and far southeastern Colorado. This anticipated early-month rainfall shows up in the CPC monthly outlook, with probabilities for above-normal precipitation exceeding 50 percent across eastern New Mexico and North Texas. Following a potential break during the Week-2 period, the CPC Week 3-4 outlook again favors above-normal rainfall for New Mexico and western Texas. Based on these outlooks, primarily in the short term, drought reduction is favored for much of New Mexico, eastern Colorado, and northwestern Texas. Central Texas may receive less precipitation than areas both to the west and east, and given the lack of a clear wet signal in this region, drought persistence is favored.
Forecast confidence for the Central High Plains and Southern Plains is moderate.
The most substantial drought development during August occurred across the Northwest, northern Rockies, and northern Plains, due to a combination of below-normal rainfall and above-normal temperatures. Little additional precipitation is forecast during the first week of September by WPC, and although some tools indicate an increased potential for precipitation during Week-2, the longer range guidance indicates a fairly rapid return to below-normal precipitation. In the absence of a clear wet signal, a continued degradation of drought impacts is anticipated during September, and therefore persistence is favored on this outlook, with additional development favored across the remainder of Washington and Oregon, and along the northern tier of Montana and western North Dakota.
Forecast confidence for the Northwest eastward to the Northern Plains is moderate.
September is a dry time of year across much of California, so little change in the drought depiction is anticipated during the month. Across the Great Basin and Desert Southwest, September is the last chance for substantial monsoon precipitation. Although quite active, most East Pacific tropical cyclone activity during August has been over the western part of the basin, which has not allowed for surges of moisture from the Gulf of California into the southwestern CONUS. Monsoon convection has been spotty, and drought persistence during August was more prevalent than reduction. This pattern is anticipated to continue during September. Although monsoon thunderstorms are expected to continue providing localized relief, the broader long-term drought conditions are anticipated to remain in place.
Forecast confidence for California, the Great Basin, and the Desert Southwest is moderate.
Impacts from Hurricane Lane continue to be assessed in Hawaii, and areas of further drought reduction are possible during September. The East Pacific remains quite active, and a gradual transition towards El Niño conditions makes the potential for westward moving tropical cyclones reaching the central Pacific more likely. Therefore, additional drought expansion is not favored across Hawaii, and drought improvement, particularly across the central islands, is favored. Persistence is maintained across the lee side of the Big Island, which largely missed out on rainfall in association with Hurricane Lane. For Alaska, forecast guidance reflects near to below-normal precipitation across the panhandle region. Therefore, the small drought area across the southern panhandle is favored to persist. Areas of abnormal dryness continue to expand across Puerto Rico, particularly along the southern portions of the island. A base state transitioning towards El Niño conditions typically favors reduced Caribbean convection overall, but September is the peak of hurricane season. Given the uncertainty, no drought development was indicated on this outlook for Puerto Rico.
Forecast confidence is moderate for Hawaii, moderate for Alaska, and low for Puerto Rico.
Forecaster: Adam Allgood
Next Monthly Drought Outlook issued: September 30, 2018 at 3 PM EDT