Tools used in the U.S. Seasonal Drought Outlook (SDO) included the Climate Prediction Center (CPC) temperature and precipitation outlooks for December (Dec) 2020 and December through February (DJF) 2020-21, various short- and medium-range forecasts and models such as the 7-day quantitative precipitation forecast (QPF) 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, climatology for Dec and DJF, and initial conditions for parameters such as soil moisture. Existing drought areas on the forecast are based on the November 19th U.S. Drought Monitor. Conditions currently reflect the presence of La Niña, with a 95% chance for La Niña to persist through the Northern Hemisphere winter, and forecaster consensus indicates the potential for a strong cold event given the robust ocean-atmospheric coupling already in place.
Mostly dry weather and above-normal temperatures did little to alleviate drought conditions across California, the Great Basin, and the Four Corners region, but recent storminess over the Northwest and northern Rockies brought some relief to ongoing drought conditions during the past few weeks. The short term pattern looks similar, with storm systems bringing additional coastal rain and mountain snows to the Cascades, northern Rockies, and the northern Sierra Nevadas, with accumulations tapering off towards the south. Both the CPC Dec and DJF outlooks reflect climate anomalies associated with La Niña, favoring above-median precipitation for the Northwest and northern Rockies, below-median precipitation for central and southern California through the southern Four Corners region, and equal chances for near-, below-, and above-median precipitation straddling the California Oregon border eastward to southern Idaho and northern Utah. DJF is a very wet time of year for the western U.S., and winter moisture is critical to build mountain snowpacks that feed reservoirs during the dry season months. Therefore, the wet signal favored for the Northwest favors further drought reductions across the Northwest and northern Rockies, with drought elimination possible for Washington state. In contrast, drought persistence is favored for the southern portions of the Western Region, with drought expansion possible across southern California. Above-normal temperatures favored for DJF may exacerbate the drought conditions by keeping mountain snow levels abnormally high.
Forecast confidence is high for the Western region.
Drought changes across the High Plains were chaotic during the past four weeks, with degradations observed across North Dakota, and improvements or degradations fairly localized in other states. The La Niña favored anomalous precipitation dipole is reflected in the seasonal outlook, where above-median precipitation is favored for the Dakotas and northern Wyoming, while below-median precipitation is favored for southern Colorado and most of Kansas and Nebraska. DJF is a dry, cold time of year for the Plains, and any anomalous snowfall during the winter months across eastern Montana and the Dakotas will likely remain above the frozen soils until the spring thaw beyond the DJF period. Therefore, little change in the incipient drought conditions is likely during the outlook period despite the tilt towards above-median precipitation. Further west, some drought improvement is possible across the higher elevations of northwestern Wyoming due to a potential for above-average snowpack building. Further south, despite the dry time of year, a potential for above-normal temperatures and continued dryness favors eastward expansion of drought conditions across Kansas.
Forecast confidence is moderate for the High Plains region.
The most substantial drought development over the past month occurred in Texas due to abnormal dryness and near- to above-normal temperatures. Further east, a patchwork of increasing dryness along the Gulf Coast States and Southeast was interrupted by swaths of heavy rainfall from tropical cyclones, including Hurricane Delta, which impacted the central Gulf Coast, and Tropical Storm Eta, which brought flooding rains to parts of the Florida peninsula. La Niña winters tend to be warm and dry across the south-central and southeastern CONUS, and this is reflected in the probabilistic monthly and seasonal outlooks. Therefore, drought development is likely for the rest of Texas (except extreme eastern Texas), and along a swath extending from southeastern Louisiana eastward across southern Alabama, Georgia, South Carolina, and northern Florida, as these areas have growing precipitation deficits out to 60 days. In contrast, areas that received heavy rainfall from tropical cyclones are favored to begin drying out during DJF, but widespread drought development is unlikely.
Forecast confidence is high for the Southern and Southeastern regions.
Small pockets of drought currently extend across parts of northern Missouri, central Illinois, and northern Indiana, as these areas missed out on heavy precipitation that overspread parts of the Ohio River Valley in association with a strong midlatitude cyclone and the remnants of Hurricane Delta. Additionally, unusually long-lasting drought conditions also persist across central Pennsylvania, upstate New York, and much of New England, though recent storminess brought some relief to these regions. During La Niña winters, the climatological winter storm track shifts to the west, favoring the Ohio Valley through interior New England. This is reflected by enhanced chances for above-median precipitation in the DJF seasonal outlook. Therefore, drought removal is favored for the Corn Belt, and additional drought reductions are the favored outcome across the Northeast. While enhanced chances for above-median precipitation do not extend to coastal New England, climatology favors frequent coastal storms during the winter, and a few such storms should be sufficient to erode the ongoing drought.
Forecast confidence is high for the Midwestern region and moderate for the Northeastern region.
No drought is currently in place or anticipated to develop across Alaska or Puerto Rico. Persistent dryness and above-average temperatures promoted widespread drought development in Hawaii, with nearly three-quarters of the state currently experiencing drought conditions. The start of the wet season in January, as well as enhanced trade winds associated with the La Niña response, favor above-average rainfall, particularly across the leeward sides of the islands. Therefore, drought improvement or removal is favored during DJF.
Forecast confidence is high for Hawaii.
Forecaster: Adam Allgood
Next Seasonal Drought Outlook issued: December 17, 2020 at 8:30 AM EST.