Drought expanded and intensified across the Pacific Northwest and northern Plains during the past month, while heavy to excessive rainfall resulted in major drought reduction throughout the middle to lower Mississippi Valley and the southern half of the Great Plains. Much of the eastern U.S. remains drought-free except for parts of the Tennessee Valley, New York, and Vermont.
The drought outlook valid from September 20, 2018 through December 31, 2018 is based on 7-day precipitation forecasts, extended range (6-10/8-14 day) precipitation and temperature outlooks, the CPC October through December (OND) precipitation and temperature outlooks, and climatology. Recent rainfall along with 7-day streamflow and soil moisture were also considered.
Long-term drought is likely to persist throughout much of the western U.S. However, an increasingly wet climatology through the fall and early winter favors improvement and removal of drought across the more short-term drought areas of coastal Oregon and Washington. Additional drought improvement and removal are likely across eastern Kansas, Missouri, and the southern Great Plains.
This expected drought improvement extends west to include southern New Mexico and southeast Arizona. Prospects for drought amelioration decrease across the northern Great Plains where persistence is more likely. A majority of the eastern U.S. is forecast to remain drought-free through the end of December.
Drought removal is likely along the Alaska Panhandle due to a wet time of year. Removal is also anticipated for any lingering drought across the Hawaiian Islands. Development is forecast for parts of Puerto Rico, but confidence is tempered due to the expected increase in convection across the Caribbean region during early October.
Forecast confidence is high for coastal Washington and Oregon and moderate for the remainder of the Western Region.
- Drought expanded and intensified this summer across the Pacific Northwest due to above-normal temperatures. Recent rainfall and cooler temperatures resulted in improving conditions across coastal Washington where 7-day streamflows are at or above normal.
- Despite a slight tilt in the odds for below normal precipitation during OND, coastal Washington and Oregon typically receive 40 to 50 percent of their annual precipitation during this three month period. Based on the increasingly wet time of year, removal and improvement of drought is forecast for the coastal Pacific Northwest.
- Drought development is most likely across parts of the northern Rockies where 90-day precipitation deficits are more than 4 inches (based on AHPS), low 7-day streamflows, and the seasonal outlook indicating enhanced odds for below normal precipitation.
- 7-day streamflows across the upper Colorado River Basin that includes Utah are in the lowest 10th percentile, while soil moisture currently ranks in the lowest 30th percentile across much of the western U.S.
- Due to these initial conditions and long-term nature of the drought, broad scale persistence is forecast for California, the interior Pacific Northwest, Great Basin, and parts of Arizona and New Mexico. Prospects for improvement and removal of drought are slightly higher across southeast Arizona and southern New Mexico due to short-term rainfall (local more than 2 inches).
Forecast confidence is moderate for the High Plains Region.
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- Below-normal precipitation was observed across Colorado and Wyoming during the past 30 days. 7-day streamflows are in the lowest 25th percentile across southwest Wyoming and western Colorado.
- Due to these dry initial conditions and lack of a strong wet signal among precipitation tools, persistence is forecast from the central to northern Rockies. Persistence is also most likely across the Dakotas where climatology becomes increasingly dry during OND.
- During the past month, heavy rainfall during the late summer resulted in 1 to 3 class improvements in drought status across the central Great Plains. An additional 1 to 3 inches of rainfall during the next week is likely to result in continued improvement and removal of drought across the central Plains.
Forecast confidence for the Southern Region is high for Louisiana, Oklahoma, and Texas but low for Mississippi and Tennessee.
- Recent, heavy rainfall resulted in 1 to 3 class improvements in drought status across much of Texas and Oklahoma. During the past 30 days, the Texas Gulf Coast and the lower Rio Grande Valley have received as much as 10 to 15 inches of rainfall. A wet pattern is likely during the remainder of September with widespread rainfall amounts of 1 to 5 inches likely.
- Based on these factors along with above normal precipitation favored during OND, Oklahoma and Texas is expected to have a continued large decrease in drought coverage and intensity. The improvement and removal of drought extends east to include northwest Louisiana.
- During the past 60 days, precipitation deficits of 4 to 6 inches were observed across parts of the southwest Tennessee and adjacent areas of northeast Mississippi. Persistence is forecast for these areas in the absence of a strong wet signal among precipitation tools.
- Although drought may expand further across the Tennessee Valley during the next few weeks, it is highly uncertain that drought persists through the end of December since OND is a favorable time of year for soil moisture recharge.
Forecast confidence is high for the Midwest Region.
- Heavy to excessive rainfall occurred throughout most of the Midwest during the late summer with drought coverage and intensity continuing to decline.
- Short-term rainfall is likely to result in additional improvement and removal of drought across Missouri and lower Michigan.
- Persistence is forecast for northwest Minnesota where climatology becomes increasingly dry during the next three months.
Forecast confidence is moderate for the Southeast Region.
- Hurricane Florence made landfall near Wrightsville Beach, North Carolina on Sep 14. Due to weak steering flow, Florence and its remnant brought widespread flooding rainfall to the Carolinas.
- Away from the track of Florence, 30-day precipitation deficits have increased to more than 2 inches across much of Georgia and north Alabama, prompting introduction of small moderate (D1) areas. Since October and November are the two driest months of the year in these D1 areas, persistence is most likely.
- Due to the seasonal precipitation outlook indicating a slight tilt in the odds for above normal precipitation, drought development is not forecast.
Forecast confidence is moderate for the Northeast Region.
- A relatively small area of drought exists across northern parts of New York and Vermont. Based on a favorable time of year for soil moisture recharge and without a dry signal at any time scale, improvement (D2) and removal (D1) of drought is expected by the end of December.
Forecast confidence is high for Alaska.
- Moderate drought continues along the southern Alaska Panhandle. Removal is likely due to typically high precipitation amounts during OND. Also, the seasonal outlook favors above normal precipitation in this region.
Forecast confidence is moderate for Hawaii.
- Moderate to severe drought continues across the leeward sides of the Big Island along with lingering drought in small areas of Maui and Molokai. Since the monthly outlook favors above normal precipitation and OND is an increasingly wet time of year, removal is forecast for the Hawaiian Islands.
Forecast confidence is low for Puerto Rico.
- Abnormal dryness (D0) continues across southern Puerto Rico. Precipitation deficit is more than 2 inches at San Juan from Sep 1 to 19. Development is forecast for the existing D0 area since the NMME model indicates enhanced odds for below normal precipitation during OND. However, forecast confidence is tempered since the predicted evolution of the MJO is expected to result in an increase in convection across the Caribbean region during early October.