Tools used in the U.S. Monthly Drought Outlook (MDO) included the Climate Prediction Center (CPC) updated temperature and precipitation outlooks for December, 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 and tools from CPC, dynamical models at the monthly time scale, the 384-hour total precipitation forecasts from several runs of the GFS, and climatology for December, with the November 24 U.S. Drought Monitor used for current drought conditions. Recent precipitation deficits and surpluses were also considered. La Nina conditions are present in late November, and are likely (~95%) to continue through the Northern Hemisphere winter, and into spring 2021 (~65% during March-May). The MJO is also active over the Maritime Continent, and the strong Indian Ocean event may influence the downstream midlatitude pattern, but dynamical models favor weakening of the signal before the enhanced phase reaches the Pacific.

During November, early season Pacific moisture brought rain and mountain snow across the Northwest, resulting in some modest improvements to drought conditions across Washington, Oregon, northern Idaho, and Montana. A late-month storm is ongoing and bringing additional precipitation to the region, but forecast accumulations based on the WPC 7-day QPF are generally less than 1.5 inches liquid equivalent, even along the coast and at higher elevations. Beyond this near term storminess, a drier pattern is favored for the next two weeks, which will reduce the potential for additional relief during December, even if a stormier pattern sets in during the latter half of the month. Therefore, drought persistence is the favored outcome, but confidence is low due to a wet climatology that tends to be shifted even wetter during La Nina winters. Further south, the updated December outlook forecasts enhanced chances for dry and warm conditions across California and the Great Basin. Given the already drying conditions across southern California, this outlook favors drought development across the remainder of the state and persistence elsewhere.

For the Western Region, forecast confidence is high for the Southwest, and low to moderate for the Northwest.

A narrow band of heavy precipitation during November brought drought relief to southern Kansas, while drought conditions degraded across northern Kansas. Elsewhere, little change to the drought depiction occurred over the previous four weeks. December is a dry time of year for the High Plains, with colder temperatures gradually causing soils and streams to freeze. The updated December outlook indicates enhanced chances for above-normal temperatures and below-median precipitation. Therefore, drought persistence is the most likely outcome. Some small-scale drought degradations are possible for areas that do not freeze over, but confidence is too low to include specific regions on the outlook.

Forecast confidence is high for the High Plains Region.

Over the past four weeks, widespread drought expansion and intensification occurred across Texas, southeastern Oklahoma, and parts of the lower Mississippi River basin. At the end of the month, however, a slow-moving storm system brought widespread copious rainfall to eastern Texas, Louisiana, and the central Gulf Coast, with accumulations topping 5 inches in some locations. Once this moisture absorbs into the soil and runs off into streams and rivers, it will be sufficient to bring immediate relief to the newly developed short term drought conditions. On this outlook, improvements or removal of D1 (moderate drought) are indicated where recent rainfall totals exceeded 2 inches. Additional rainfall is anticipated across the western and central Gulf Coast during Week-1, with conditions favored to dry out by mid-month. Outside of areas that received at least 2 inches of rain, drought persistence is forecast, but no new development is anticipated for the Southern Region or lower Mississippi Valley. Further east, the heavy rainfall largely missed southern Georgia and northern Florida, and drought development is forecast for areas that are already abnormally dry (D0) or have substantial 30-day rainfall deficits. The late-November rainfall may be sufficient to prevent larger-scale drought development further west of this area during December, but given the ongoing La Nina conditions, drought development as indicated on the seasonal outlook may still occur by the end of the winter season.

Forecast confidence is moderate for the Southern and Southeastern Regions.

The same storm system that brought drought relief to southern Kansas overspread southern Missouri, southern Illinois, and southern Indiana, bringing relief to drought and abnormally dry conditions. Little precipitation fell to the north of this area, however, and drought conditions expanded across northeastern Missouri, central Illinois, and northern Indiana. Little additional precipitation is forecast during the first week of December, and the CPC Week-2 outlook slightly favors a continuation of an overall dry pattern. The updated December outlook favors above-average temperatures and below-median precipitation for the Midwest Region, despite the ongoing La Nina conditions. Therefore, drought persistence is indicated on the outlook for existing drought areas, but confidence is somewhat low due to the potential for a shift towards a pattern more in line with a canonical La Nina reponse later in the month.

Forecast confidence is low to moderate for the Midwest Region.

During November, periodic storminess across the Northeast brought gradual drought relief to most areas. In the near term, a potent storm system is bringing heavy rainfall across the mid-Atlantic and coastal New England, with snowfall to the west across Ohio. Additional storminess is forecast later in Week-1, with a potential break by Week-2. The updated December outlook favors above-median precipitation and above-average temperatures, which would promote additional soil moisture recharge. Therefore, continued drought reduction is favored during December.

Forecast confidence is moderate for the Northeast Region.

No drought is currently depicted or anticipated to develop across Alaska and Puerto Rico during December. For Hawaii, La Nina conditions typically result in enhanced trade winds, which increase the potential for drought relief across the leeward sides of the islands. Dynamical model forecasts show the greatest potential for drought relief across the northern Big Island. Given that the wet season begins largely after December, drought persistence is favored for the rest of Hawaii.

Forecast confidence is moderate for Hawaii, and high for Alaska and Puerto Rico.

Forecaster: Adam Allgood

Next Monthly Outlook issued: December 31, 2020 at 3 PM EST