Tools used in the U.S. Seasonal Drought Outlook (SDO) included the Climate Prediction Center (CPC) temperature and precipitation outlooks for August through October 2018 (ASO), 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 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 ASO season, and initial conditions.
A month ago, only abnormal dryness (D0) was found in portions of New York and New England. But with the scattered nature of summer convection, combined with above-normal temperatures and increased evapotranspiration, moderate drought developed from Lake Ontario eastward to the southern Maine and northern Massachusetts coasts as 7-, 14-, and 28-day average USGS stream flows had fallen below the 25th percentile. However, yesterday's 1-3 inches of rain (not included in the latest 7/17 DM), a wet 7-day QPF and high probabilities of above-median precipitation for both the 6-10 and 8-14 day ERFs along the East Coast, above-normal Week3-4 precipitation odds, and the possibility of being affected by a tropical cyclone or N'oreaster during ASO pointed toward improvement in the Northeast (and no development in the Southeast). However, given the high climatic variability of the region, no obvious precipitation signal (EC) during August or ASO, and no distinct wet or dry season (e.g. evenly-distributed precipitation climatology), these uncertainty factors tempered the confidence. There was no drought (D1 or drier) in the Southeast, and none was forecast to develop based upon the guidance.
Forecast confidence for the Northeast and Southeast is moderate to high.
The primary drought area in the Midwest remained over northern Missouri and southeastern Iowa (and into eastern Kansas) and worsened. While much of the region surrounding this stubborn drought area received above-normal rainfall during the past 30-days, most of Missouri and extreme southern Iowa (and eastern Kansas and northern Arkansas) recorded less than half of normal precipitation. When combined with 30-day temperatures averaging 2-4 degF above normal, increased evapotranspiration demand due to growing crops/vegetation, and lingering long-term deficits that dated back to last October, areas of D2 and D3 were introduced and expanded into this area (e.g. flash drought) as both agricultural and hydrological impacts were being reported. Father to the northeast, most of Michigan, northern Indiana, and northwestern Ohio saw above-normal temperatures and below-normal precipitation during the past 30-days, which has led to the addition of D0 and some D1. The ASO drought outlook for the Midwest is difficult as the precipitation guidance in the short, medium, and long terms is mixed, with the QPF showing light rain totals (less than 0.5 inches), the ERFs with EC, the Week3-4 below normal odds, and the 1- and 3-month LLFs with EC or slightly below-median chances. Adding to the issue is the fact that the early portion of the ASO season is a time of high water demand for crops as they continue to grow and fill. In addition, it has been wet in areas just to the north, west, and east of Missouri, so there is not much separation between wetness and drought. As a compromise, the drought was left as persistence in northern Missouri and southern Iowa (and eastern Kansas and Arkansas), but development was added to the south (southern Missouri into Arkansas) as the forecasts were somewhat clearer for a dry signal at the longer time periods (1- and 3-month LLFs). In eastern Michigan, nearly all forecasts were EC, so without any definitive guidance, persistence was left there (without any development or improvement).
Forecast confidence for the Midwest is moderate.
In the South (TX, OK, LA, AR, TN, MS), a typical summertime convective pattern of above and below normal rainfall since mid-June has produced marked improvements or deteriorations to many areas of the South. Copious rains from a pair of upper-air disturbances removed drought along the western Gulf Coast while heavy precipitation in the Texas and Oklahoma Panhandles and western Kansas caused 1-3 categories of improvement during the past 4 weeks. In contrast, subnormal rainfall across central Texas, northern Louisiana, southwestern Mississippi, most of Arkansas, and northeastern Oklahoma produced downgrades of 1-2 categories since mid-June. Unfortunately, the drought forecast has trended toward persistence or deterioration, with the 7-day QPF quite dry for much of Texas and Oklahoma, and only light amounts (less than 0.5 inches) for Louisiana, Arkansas, and western Mississippi. The 6-10 and 8-14 day ERFs are mixed, with above-median odds for rain in northern and eastern sections of the South, and EC or below-median chances in the south. Weeks3-4 favored sub-normal preciputation, while both the August and ASO precipitation outlooks tilted slightly below-median. Not surprisingly, the short and long-term temperature forecasts favored above-normal readings. As a result, with the growing season still underway amd crops requiring plenty of water, temperatures expected to be above-normal, and overall precipitation forecasts pointing toward sub-median totals, drought areas were left to persist while existing D0 areas are forecast to develop into drought. An exception was in the Texas and Oklahoma Panhandles where the ERFs depicted high probabilities of above-median rainfall which may lead to some short-term improvement; however, the longer-term outlooks trended drier, meaning that the area may deteriorate after late July - thus this area was left unchanged. And one last wild card in the South is any unforeseen tropical system(s) from the Caribbean or Gulf that could inundate the southern portion of the South and invalidate the development forecast.
Forecast confidence for the South is moderate.
Across the High Plains (CO, KS, NE, WY, SD, ND), much of the region saw ample precipitation during the past 30-days, with much above-normal amounts across most of Wyoming and Nebraska, the western two-thirds of Kansas, northeastern Colorado, western, southern, and eastern South Dakota, and western North Dakota. As a result, dryness or drought were improved or eliminated across much of the High Plains region, with just a few small areas of drought (D1) left in the Dakotas and southwestern Kansas. Where the surplus rains missed, drought conditions remained the same or slightly worsened, and this included most of Colorado (except the northeast) and eastern Kansas. The rainfall forecasts in the short-term (QPF, ERFs) are favorable for the south-central High Plains, with moderate to heavy totals expected in the next 7-days in eastern Colorado and western Kansas, and enhanced odds in the same area for both ERFs. The longer-term forecasts are not as wet, with the Week3-4 with below-normal precipitation chances and the 1- and 3-month LLFs at EC. However, the August and ASO climatology is somewhat wet in eastern Colorado and western Kansas, so above-median precipitation during a typically wet time of year would tend to show some improvement. In contrast, a somewhat drier pattern is forecast for areas to the east at all time periods, and with a drier climatology, eastern Kansas was left unchanged (persist). Farther north, the recent wet pattern and short-term forecasts (QPF and ERFs) in the Dakotas favors continued improvement, thus removal of the D1 areas was made.
Forecast confidence for the High Plains region is low (CO-KS) to moderate (Dakotas).
During the past month, the West Coast saw little or no precipitation, consistent with its dry climatology. In the Southwest, inflow moisture from the remnants of Hurricane Bud brought much needed heavy rainfall to parts of the region in mid-June, and the monsoonal rains also arrived during middle of July. CPC's outlooks predict a robust Southwest monsoon, and combined with the wet climatological ASO season, this should bring general improvement over the very dry regions, at least in the short term, for most portions of the Four Corners region. The various forecasts favor a hot and dry season over the Pacific Northwest, the existing drought areas expected to persist, with development likely over much of Washington, northern Idaho, and northwestern Montana. CPC's forecast suites depict above normal temperatures and close to normal precipitation over southern California and adjacent Nevada, and with the given dry ASO climatological season there, the existing drought is expected to persist.
Forecast confidence for the West is moderate (Southwest) to high (Pacific Northwest).
In Hawaii, most of the rainfall occurred over the windward sides during the past month while drier conditions occurred along the leeward sides of the Islands. CPC's outlooks continue to predict the same rainfall patterns for the next several months, thus drought development or persistence (D1) is favored over those D0 and D1 leeward areas for the Big Island of Hawaii, Maui, Moloka'i and O'ahu. Although showers from the remnants of Hurricane Beryl brought some D0 relief to southern Puerto Rico, the statistical and dynamical models forecast a very quiet (dry) summer tropical season, so drought development is favored for the next several months over southern coast region of Puerto Rico that is currently in D0. Drier conditions also prevailed over the southeastern Alaska Panhandle, and with forecasts favoring above-normal temperatures and below normal rainfall, drought development is likely over the western portion of the Alaskan Panhandle (where D0 exists) and drought should persist across the eastern portion.
Forecast confidence for Hawaii, Puerto Rico and Alaska is moderate.
Forecasters: David Miskus and Yun Fan
Next Seasonal Drought Outlook issued: August 16, 2019 at 8:30 AM EDT