During the past 30-days, western Washington, far southern portions of Arizona, parts of the Rockies, northern Plains, Missouri, northern New York and New England, and parts of the Southeast have experienced at least a one-category improvement in drought conditions, according to the U.S. Drought Monitor. Areas of drought deterioration include northern and central California, and parts of Nevada, Washington, Wyoming, Oklahoma/Arkansas, and Florida.
The Monthly Drought Outlook (MDO) valid for December 2018 is based on 7-day precipitation forecasts, extended range (6-10/8-14 day) precipitation and temperature outlooks, the CPC updated December precipitation and temperature outlooks, dynamical model guidance, climatology, and initial conditions. Recent rainfall and soil moisture forecasts from the GFS model were also considered.
An active southern jet stream and associated storm track is expected to result in widespread areas of drought improvement and/or removal from California east-southeastward across the Southwest, southern Rockies and the Southeast.
For the southern Plains, the situation is less clear, since most precipitation outlooks out to one month in advance do not predict high enough precipitation amounts to justify drought improvement or removal. Accordingly, drought persistence is considered to be the best bet for this region. For remaining drought areas, such as the Pacific Northwest, northern half of the Intermountain region, portions of the Rockies, and the Dakotas, drought persistence is favored.
Long-term hydrologic drought is expected to persist across the southern portion of the Alaska Panhandle, despite short-term storm activity during the month of December. The Hawaiian Islands and Puerto Rico are forecast to remain drought-free through the end of December.
Forecast confidence is moderate to high for approximately the southern two-thirds of the West, and moderate for the northern third of the West.
- Percent of Normal Precipitation (PNP) during the past 30-days (from the Advanced Hydrologic Prediction Service) indicates values of 25-75 percent for Washington, Oregon, central and southern Idaho, northern Utah, much of northern Nevada, and the northern half of California. For southern California, southern Nevada, southern Utah, and most of Arizona, 30-day PHP values lie within the lowest quartile of the historical distribution.
- SNOTEL Snow Water Content (SWC) from the Western Regional Climate Center reveals basin average SWC values of 125-200 percent (or more) of average for the Sierra Range in California, 25-50 percent of average for western Washington and western Oregon, with remaining areas of the West depicting values of 50-125 percent of average.
- Although the rainy season is now in full-swing across the western contiguous U.S., PNPs and SWCs for many areas still remain below average for this time of year.
- Looking forward, precipitation outlooks for practically all time ranges out to a month in advance favor above normal precipitation for California (offsetting short-term drought effects), the southern half of the Intermountain region, and the southern Rockies. Accordingly, improvement and/or removal of drought is favored for these areas. This region generally coincides with the 40 percent (or greater) contour indicated on CPC’s updated precipitation outlook for December.
- For remaining portions of the West (Pacific Northwest, northern half of the Intermountain Region, and central Rockies), drought persistence is favored.
Forecast confidence for the High Plains region is moderate.
- Most of the High Plains region is drought free at this time. With the exception of a tiny residual area of moderate drought (D1) in eastern Kansas (which is forecast to be removed during December, all other areas are favored to experience drought persistence.
Forecast confidence for the South is low to moderate.
- As was the case with the High Plains region, the South is also mostly drought free at this time. A handful of tiny drought areas linger in northeastern Oklahoma and extreme northwestern Arkansas, the Texas Panhandle, and far western Texas.
- Precipitation outlooks for most time ranges raise doubts as to whether or not these areas will receive enough precipitation to justify drought improvement or removal.
Forecast confidence for the Midwest region is high.
- There is no drought over the Midwestern states at this time, and none is anticipated to develop during the month of December.
Forecast confidence is moderate to high from northern Florida to South Carolina; but confidence is low across south-central Florida.
- Across the Southeast region, only a few small-scale drought areas remain, along the southern Atlantic Coast and south-central Florida.
- Climatologically, this is a dry time of year for this region. However, with the predicted mean storm track across the southern states during December, these lingering drought areas are more likely to be removed.
- In the coastal region between northern Florida and South Carolina, above normal precipitation is favored for practically all time ranges out to a month in advance, which supports drought removal.
- For south-central Florida, however, confidence is significantly lower as predicted precipitation amounts are less than those farther north.
Forecast confidence is moderate to high for the Northeast Region.
- At present, there is no drought over the Northeast, due to surplus precipitation (1-5 inches), stream flows within the highest quarter of the distribution for this time of year, and surplus soil moisture values (1-6 inches), all within the past 30-days.
- Though far northern New York and New England have received the lowest amount of precipitation compared to the rest of the Northeast region, drought is still unlikely to develop at this time of year.
Forecast confidence is moderate for Alaska.
- Moderate to severe drought continues along the southern Alaska Panhandle. As this is primarily a long-term hydrologic drought, it is thought the drought will continue through the month of December.
Forecast confidence is moderate to high for Hawaii and Puerto Rico.
- The Hawaiian Islands and Puerto Rico are forecast to remain drought-free.